UNIVERSITY OF CALIFORNIA 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 CIRCULAR 330 
 December, 1933 
 
 THE ROOT-KNOT NEMATODE 
 
 JOCELYN TYLEE^ 
 
 Root-knot, a disease that occurs on the roots of a large number of plants 
 of all kinds, is caused by the invasion of a microscopic nematode worm, 
 Eeterodera marioni (Cornu),^ popularly called eelworm or garden 
 nematode. Thercsare other species of nematodes (eel worms) which infest 
 plants, among the more important of which are the sugar-beet nema- 
 tode, Eeterodera schachtii Schmidt, and the bulb or stem nematode, 
 Anguillulina dipsaci (Kiihn).^ Soil or water may also contain countless 
 species of free-living nematodes, which are not injurious to plants. 
 There are other species of nematodes found as parasites in animals and 
 in man. 
 
 The root-knot nematode came originally from the tropics, but it has 
 been spread to nearly every country in the world and flourishes under 
 the favoring conditions of cultivation. In colder regions, where it is not 
 a serious field pest, it may become destructive in greenhouses. 
 
 FIELD DETERMINATION OF NEMATODE INFESTATION 
 
 Injuries and Symptoms. — Root-knot is responsible for the crop losses 
 in many cases where climatic conditions are blamed, and in some regions 
 the reduced yield is considered normal. The infestation may thus become 
 serious before its presence is even suspected. In parts of California, it 
 has become the limiting factor in raising many truck and field crops, 
 and it may seriously reduce orchard and vineyard yields. It also in- 
 creases the susceptibility of plants to other diseases, such as cotton wilt, 
 black shank of tobacco, and rhizoctonia disease of peanuts. 
 
 1 Eesearch Assistant, Division of Entomology and Parasitology. 
 
 2 Formerly known as Eeterodera radicicola (Greeff ) . Caconema radicicola (Greeff ) 
 is also a synonym. 
 
 3 Formerly known as Tylenchus dipsaci (Kiihn). 
 
2 University of California — Experiment Station 
 
 Heavily infested plants are stunted and off-color, and wilt readily. 
 /The roots are beaded with galls (figs. 1 and 2), and all the energy of the 
 plant is used up in producing new lateral rootlets, which are promptly 
 attacked in their turn. 
 
 The galls are round or elongated. Their size and shape vary with the 
 host plant, and also with temperature conditions. There may be little or 
 no swelling on the roots of such plants as strawberry, iris, freesia, and 
 cyclamen, but on most plants the galls are conspicuous and occasionally, 
 even grow to a diameter of an inch or more. They are distinguished from^ 
 the beneficial nodules of the nitrogen-fixing bacteria, which occur on 
 leguminous plants, by the fact that the latter are only loosely attached 
 at the sides of the roots, while nematode galls involve all the root tissues 
 and cannot be separated from them. These galls are most often found on 
 ' the more tender feeding rootlets, though in older infestations they may 
 be large and woody. They may perhaps be confused with phylloxera 
 galls, which occur on grape rootlets in heavy soils, or with the early 
 stages of crown gall, which may start with small irregular nodules on 
 roots or stems, although it characteristically causes large tumors on the 
 main roots. Clubroot, on plants of the cabbage family, causes large swell- 
 ings, which are yellowish inside and less knot-like than nematode galls. 
 
 Whether or not the nematode secretes a toxic substance, its presence is 
 an irritation which stimulates abnormal growth of the plant cells and 
 causes distortion of the sap-conducting vessels. The gall is characterized 
 by a knot of these gnarled and broken vessels, surrounded by fleshy 
 tissue, which may be discolored and furrowed. It is the first part of the 
 root to die, and large galls (fig. 3) are usually more or less decayed. 
 Finally the infested rootlets are unable to transfer water and mineral 
 nutrients from the soil to the plant, and all the vital functions of the 
 plant are seriously affected. 
 
 Nematodes may also infest fleshy roots or tubers. In a potato (fig. 4), 
 their presence is recognized by a pimply surface and by a ring of small 
 brown spots Vs of ^n inch under the skin, each spot containing one or 
 more females and egg masses. 
 
 Examination of Indicator Plants. — The easiest and also the surest 
 way to determine the presence of nematodes in any soil is to examine the 
 roots of susceptible plants which have been growing there for at least 
 three weeks, in warm weather and while the soil is moist. A rough esti- 
 mate of the nematode population can be obtained from the abundance 
 and size of galls, from the diseased or healthy condition of the roots, and 
 from the amount of growth. 
 
 Roots of the weeds and crop plants listed in table 1 (page 10) are the 
 
ClR. 330] 
 
 The Koot-Knot Nematode 
 
 
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 jjK^F^'^A 
 
 1 ^H 
 
 
 
 Jr 
 
 
 
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 ^: 
 
 Fig. 1. — Nematode galls on tobacco root. 
 
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 ^-p— V 
 
 
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 V 
 
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 I 
 
 Fig. 2. — Hoot-knot on sugar beet. The inset is an enlargement of one 
 small gall, showing the protruding egg mass. 
 
4 University of California — Experiment Station 
 
 })ost indicators of nematode infestation. In some cases it may be ad- 
 visable to plant watermelon, tomato, or a susceptible variety of cowpea 
 for examination. The soil should be carefully loosened before pulling the 
 plant; otherwise rootlets with galls may break off and remain buried. 
 Roots should be taken from many parts of a field, and they must be 
 examined before they become dry. The final test is to find nematodes in 
 the galls. This is done most easily if a gall is not cut but broken open. 
 The most conspicuous form is the adult female, described in the next 
 section. 
 
 BIOLOGICAL CHARACTERISTICS OF THE ROOT-KNOT NEMATODE 
 
 Life History. — The larger galls usually contain nematodes in all 
 stages of development. A mature female (fig. 5 D and F) is shiny white, 
 gourd-shaped, and somewhat larger than a pinhole. With the female 
 there is often an egg mass as large as her own body. The egg mass may 
 be entirely inside the root tissue, or it may have broken out into the soil 
 and be seen as a reddish-brown or dark-brown spot on the outside of the 
 gall (fig. 2, inset). Individual eggs and young larvae (fig. 5A) are the 
 size of the smallest dust particles. They are not ordinarily seen by the 
 naked eye, unless they are present in great numbers. 
 
 The newly hatched larvae may work their way to a fresh location in 
 the original root, or they may escape into the soil and live in the film of 
 water which surrounds the soil particles. They do not feed except in 
 living plant tissue. Hence they cannot grow or develop while free in the 
 soil, but they can survive for months without food if they are not too 
 active. They are supposed to have very delicate sense organs and to 
 recognize from a considerable distance the presence of a host plant, to 
 which they will travel even against the drainage current. Whether or 
 not this is the case, many of them find and enter roots; many others 
 never succeed in doing so, but the numbers of eggs and larvae are so 
 great that the soil population is always increasing when susceptible roots 
 are available, in spite of the loss of many individuals. 
 
 The larva usually attacks a rootlet near its growing tip. If many larvae 
 are present, 20 or more of them may enter a root near the same point, 
 and j)enetration is more rapid than when larvae are scattered. Once 
 located inside, they give up all activity except feeding and growing. 
 The females become enormously swollen and filled with fat globules. A 
 yellowish jelly-like secretion is extruded, and the small white eggs are 
 deposited into it. One female may lay from 500 to 1,000 eggs, and fertili- 
 zation ])y a male is not necessary for their development and hatching. 
 
CiR. 330 
 
 The Root-Knot Nematode 
 
 Fig. 3. — Old and decayed nematode galls on watermelon root. 
 
 Fig. 4. — Potatoes infested with the root-knot nematode. 
 
6 University of California — Experiment Station 
 
 The egg mass darkens to reddish brown, and may later become almost 
 black. When the larvae hatch, nearly all of them leave the gall and 
 migrate within the root, or else through the soil in search of fresh 
 rootlets. 
 
 Contrary to statements which are sometimes found in popular bul- 
 letins, the root-knot nematode has no true cyst stage, as has the sugar- 
 
 Fig. 5. — Stages in the development of the root-knot nematode. A, Eggs and 
 larvae: a, egg; b, larva. B and C, Partly grown females; D, mature female; E, ma- 
 ture male ; F, gall from tobacco root, partially dissected : a, egg mass ; c, female. 
 
 beet nematode, but dormancy may occur. The gall tissue, the secretion 
 around the eggs, and tlie egg: shell all give a certain amount of protection 
 to the embryo; but this is not like a cyst, which is extremely resistant to 
 drying as well as to other adverse conditions. 
 
 The majority of the nematodes developing in healthy roots are 
 females. They require ample nourishment for producing eggs. The males 
 (fig. 5E) require less food, and develop more rapidly. After a period of 
 growth and metamorphosis, a male may emerge again from the gall as a 
 free-traveling worm, more than three times as long as the larva. Some- 
 
Cm. 330] The Root-Knot Nematode 7 
 
 times he finds a female, but as has already been mentioned, mating is not 
 always necessary for the reproduction of this species. The males are 
 short-lived, and few of them are found in the field unless they are espe- 
 cially sought, with a knowledge of the conditions of their life history. 
 
 Influence of Temperature on Nematodes. — Development of this nema- 
 tode has no seasonal limits, but progresses slowly or rapidly at any tem- 
 perature between 50° and 90° Fahrenheit.lAll stages of the life cycle can 
 therefore be found at almost any time. ' 
 
 Most rapid development occurs around 81° Fahrenheit. At this tem- 
 perature, a nematode may grow from a free-living larva to a mature 
 egg-laying female in a minimum of 16 daj^s. A week or 10 days more are 
 necessary for the development and hatching of the eggs. Development 
 is progressively slower at lower temperatures, until at 58° the same 
 amount of growth may require 80 days instead of 16, plus 5 or 6 weeks 
 for hatching of eggs. 
 
 The rate of development must be borne in mind in connection with 
 control by fallow, where conditions which might permit e^^ formation 
 must not occur. The roots of some plants will remain alive for several 
 days, even after the tops are killed, and nematodes maj^ continue to 
 develop for a few days after that. In this way it is possible that in hot 
 weather some females may take enough nourishment from plants grow- 
 ing only 10 or 12 days to enable them to mature and to form viable eggs. 
 Weeds should therefore be destroyed every 10 days in midsummer, but 
 they may be left in the field for a longer time in cool weather. 
 
 In cold weather, fewer nematodes are able to penetrate the roots, and 
 susceptible plants may thus escape injury even in infested land. Such 
 crops as carrots, potato, and vetch, which can be grown at soil tempera- 
 tures around 55° Fahrenheit, will not suffer, although they may harbor 
 a few nematodes and carry over the infestation. If a plant can be started 
 in early spring, its roots will be better able to outgrow the infestation 
 when nematodes become active with the advancing season. 
 
 In the southern states, nematodes multiply at the rate of 5 to 10 
 generations a year, and they are also most active in finding and attack- 
 ing plants. In more northerly states, they may be a less important out- 
 door pest because their development is slow at low temperatures. As 
 populations become established, such infestations may eventually be- 
 come serious. However, in those parts of the northern states w^here 
 intense cold penetrates deep in the soil, there is only a single season's 
 growth in nematode population to deal with. Ordinary freezing at 32° 
 Fahrenheit does not kill these larvae, but 0° will kill them in only 2 
 
8 University of California — Experiment Station 
 
 hours, and doubtless many of tliem will succumb to temperatures be- 
 tween these limits. 
 
 Influence of Moisture on Nematodes. — The root-knot nematode is able 
 to tolerate almost any condition of moisture. Although it is killed by 
 desiccation, it may live in a soil which is apparently dry, if, as is com- 
 monly the case, the soil atmosphere l)elow the dry surface layer is 
 saturated with water vapor. This condition favors the long-continued 
 survival of the larvae, because they are not so active as to deplete their 
 store of reserve energy. 
 
 Larvae are more active in the wetter soils. It has been observed that 
 plants may be seriously injured by nematodes after a heavy irrigation; 
 yet water cannot be withheld entirely, because infested plants may die 
 during a dry spell which would not kill more healthy i)lants. 
 
 Soils Favorable to Nematodes. — Porous sandy or loam soils are most 
 favorable for the increase and spread of the root-knot nematode, while 
 infestations in heavy soils are usually less serious. The effect of a heavy 
 soil is to make it more difficult for the larvae to travel in search of a host, 
 and this greater exertion may perhaps exhaust them more readily. 
 There is nothing to prevent their development once a root has been 
 reached, and as a population becomes established it is bound to spread. 
 In comparison with other factors, soil texture is of minor importance 
 and cannot be relied on for more than a temporary check of nematode 
 activities. There are reports of severe infestations in heavy soils. 
 
 Depth of Nematode Populations. — Nematodes are generally abundant 
 in the soil as deep as host roots are growing and as deep as plowing may 
 have turned under and mixed any infested soil. Their greatest numbers 
 are usually in the root zone from 3 to 10 inches below the surface. Their 
 depth distribution, however, varies widely with conditions; it is of 
 course limited by a layer of hardpan or by a high water table. In sandy 
 soil, galls have been found on peach and tig roots 6 and 8 feet deep. 
 Sometimes a root will escape heavy infestation if it is set deep in the 
 soil, but nematodes will eventually find and attack it and become estab- 
 lished that much deeper. 
 
 Conditions of Survival in the Soil. — With even tlie lightest and most 
 recent infestation, there are larvae in the soil, and usually in a wider 
 area than is supposed. They are well adapted to the conditions of their 
 life there. It has been reported that they can survive for more than 15 
 months without feeding, living on the nourishment which they have 
 conserved from the eggs, in the form of granular material or fat. While 
 it is undoubtedly true that some members of a population survive a 
 prolonged fallow, it may be that part of the time is passed in the egg 
 
CiR. 330] The Root-Knot Nematode 9 
 
 stage. Controlled experiments have proved, however, that many indi- 
 viduals are capable of surviving as larvae for at least 40 weeks in a com- 
 pletely fallow soil. 
 
 The larvae are most active under the conditions of cultivation, when 
 the soil is warm and moist. They can then normally find host plants and 
 multiply abundantly. The presence of growing roots is probably an- 
 other stimulus to their activity. On the other hand, when the soil is cold, 
 or fairly dry, and no crops are being grown, the larvae are equally well 
 adapted to conserve their energy and live over, in a more or less 
 quiescent state, until plants are again available. Under natural condi- 
 tions they survive in either case. However, if they are very active and 
 there is no host root available — an artificial situation — they will ulti- 
 mately use up their reserve food and die of exhaustion and starvation. 
 
 The eggs also may remain dormant until warmth and moisture stimu- 
 late them to hatch. Dormant eggs are found in old galls and in potato 
 tubers, and it may be also that they survive under some conditions free 
 in the soil — to hatch later and infest the field with a fresh and vigorous 
 new population. 
 
 Spread of an Infestation. — By its own activity, a nematode larva 
 might possibly travel a foot and a half a day, in a light soil with favor- 
 able conditions of temperature and moisture. Evidence on this point is 
 scarce and conflicting. It may certainly be questioned how far any indi- 
 vidual would travel in a straight line away from the infested area. If it 
 finds and enters a root, the next generation of nematodes may continue 
 the migration, but this makes for a very slow spread. 
 
 Nematodes are carried greater distances by surface drainage from 
 infested areas, and thej^ are often worst in low spots where the water 
 settles. When they are once introduced in a field, even in a small area, 
 they are carried farther with each tillage. They may even be scattered 
 by walking over muddy or plowed land, or by stray animals. In view of 
 the possibilities of multiplication, every precaution against spreading 
 an infestation must be taken seriously. 
 
 Host Plants. — Nearly 900 plants have been reported as hosts of the 
 root-knot nematode. They do not all suffer ecpially. A few of them even 
 react against the invaders and may eventually kill them, but unfor- 
 tunately not before the nematodes have laid eggs and the plant itself 
 has suffered considerable injury. 
 
 Almost all of the truck crops and many trees and vines are subject to 
 loss or failure in heavily infested soils. Plants to be avoided especially 
 in such land are listed in table 1. None of them will yield at all profitably, 
 and they will onlj^ serve to feed a new nematode population. There are 
 
10 
 
 University of California — Experiment Station 
 
 many host plants not listed specifically, including most of the members 
 of the cabbage, composite, gourd, nightshade, parsley, and pea families. 
 Perennial host plants are constantly subject to reattack by larvae 
 which have been bred in their own roots. Even a minor infestation is 
 bound to increase each year. Complete destruction of a tree or vine 
 sometimes occurs suddenly. 
 
 TABLE 1 
 Most Important Hosts of the Root-Knot Nematode 
 
 TRUCK CROPS 
 
 FRUIT AND NUT CROPS 
 
 elm 
 
 bean 
 
 banana 
 
 gardenia 
 
 beet 
 
 fig 
 
 hibiscus 
 
 carrot 
 
 grape 
 
 hollyhock 
 
 cucumber 
 
 papaya 
 
 lobelia 
 
 dasheen 
 
 peach 
 
 mulberry 
 
 eggplant 
 
 pecan 
 
 pansy 
 
 endive 
 
 pineapple 
 
 peony 
 
 lentil 
 
 plum, myrobalan 
 
 rose 
 
 lettuce 
 
 strawberry 
 
 snapdragon 
 
 lima bean, except Hopi 
 
 walnut, black and 
 
 tuberose 
 
 variety 
 
 English 
 
 violet 
 
 melons 
 
 
 weeping willow 
 
 okra 
 
 SPECIAL CROPS 
 
 
 coffee 
 
 
 pea 
 
 flax 
 
 WEEDS 
 
 peppers 
 
 ginseng 
 peppermint 
 sugar cane 
 
 amaranth, or careless 
 
 potato, Irish 
 
 pumpkin 
 
 squash 
 
 weed 
 cheeseweed 
 dandelion {Taraxacum) 
 
 sweet potato, many 
 
 SHADE TREES AND ORNA- 
 
 dock 
 
 varieties 
 
 MENTALS 
 
 dog fennel 
 
 tomato 
 
 begonia 
 
 fennel, sweet 
 
 
 buddleia 
 
 fenugreek 
 
 
 calendula, most varieties 
 
 groundsel (Senecio) 
 
 FIELD AND FORAGE CROPS 
 
 carnation 
 
 Jimson weed, or stra- 
 
 bean 
 
 catalpa 
 
 monium 
 
 clover 
 
 chrysanthemum 
 
 lamb's-quarters, or pig- 
 
 cowpea, most varieties 
 
 cineraria 
 
 weed 
 
 soybean, most varieties 
 
 clematis 
 
 nightshade 
 
 sugar beet 
 
 coleus 
 
 purslane 
 
 sunflower 
 
 cyclamen 
 
 smartweed 
 
 tobacco 
 
 dahlia 
 
 sow-thistle 
 
 vetch 
 
 echeveria 
 
 tansy 
 
 Weeds as host plants are also to be reckoned with. They may not show 
 serious sym]:)toms of disease, but they feed nematodes and allow them to 
 multiply and keep the soil stocked. 
 
 Plants Resistant to Nematodes. — Grains and grasses, with a few ex- 
 ceptions, are the group of plants most resistant to root-knot. The list of 
 nonhosts presented in table 2 has suffered many revisions and deletions, 
 because a number of plants which some observers have reported as 
 immune have been found under other conditions to be hosts of more or 
 less importance. For this reason, lists of resistant plants can have no 
 absolute authority. 
 
<-'iR- 330] The Root-Knot Nematode 11 
 
 TABLE 2 
 Plants Eeported Kesistant to the Root-Knot Nematode 
 
 TRUCK CROPS FIELD AND FORAGE CROPS {continued) 
 
 sweet potato: Big Stem Jersey, Gold oats 
 
 Skin Jersey, Old Long Red, Red rice 
 
 Jersey, Yellow Jersey, and Yel- lye 
 
 low Jersey Vineless varieties sorghums 
 
 soybean: Laredo variety 
 
 FIELD AND FORAGE CROPS velvet bean: Florida and Mauritius 
 barley varieties 
 
 cowpea : Iron K890-3* and Victor va- wheat 
 
 ^^^^^^^ FRUIT AND NUT CROPS 
 
 Crotalaria spectaMlis; C. juncea apricot, most varieties 
 
 (sunn hemp) avocado 
 
 grasses: para grass, redtop grass, eitrusf 
 
 rescue grass or Schrader's brome date 
 
 grass, perennial rye grass, teo- feijoa 
 
 sinte, timothy, and most other jujube 
 
 kinds are highly resistant; mea- peach roots: Bokhara and Shalil va- 
 
 dow fescue. Natal grass, orchard rieties 
 
 grass (Dactylis glomerata) , sheep's pistachio 
 
 fescue, tall fescue, and tall meadow plum: Marianna 
 
 oat grass are resistant but some- 
 times harbor nematodes ornamentals 
 lespedeza, or Japan clover Eustachys petraea 
 
 • 11 i 11 1 • T 1. XI. gaiJlardia 
 
 millet: all kinds except those men- °_ . , , 
 
 ,. 1 • . 1 1 o mangold 
 
 ^^^^^^^ ^^ table 3 ^.^^J^ 
 
 * This is a selection by the late P. B. Kennedy from a variety established for nematode resist- 
 ance some thirty years ago by H. J. Webber, at that time connected with the United States Depart- 
 ment of Agriculture, but now with the Citrus Experiment Station of the University of California. 
 It has fixed resistance to nematodes, fusarium wilt, and charcoal rot. It is adapted to the Cali- 
 fornia interior climate and to all types of soil. Approved seed can be obtained from J. J. Anderson, 
 Route 1, Box 163, Turlock, California. It may eventually be carried by seedsmen. 
 
 t Citrus roots are attacked by the root-knot nematode in very small numbers if at all. They are 
 more heavily parasitized by the citinis-root nematode, Tylevchulus sewipenetrans Cobb. The 
 injuries caused by the latter are generally considered negligible, but doubtless depend on the 
 amount of infestation. 
 
 Resistance to nematodes is in some cases related to the vigor of growth. 
 If a highly resistant plant is growing under adverse conditions of soil 
 or of climate, it may be heavily attacked by nematodes, or it may show 
 serious injury from a relatively light infestation. 
 
 The root-knot nematode is much less specialized in its selection of 
 hosts than are other species of plant nematodes. There are, however, 
 observations which suggest that if there is a choice of hosts, a population 
 in a given field tends to prefer the kind of roots which have been grown 
 most frequently or most recenth^ on that land. It is therefore advisable 
 not to plant the same crop two successive years in any infested field, even 
 a crop which is considered resistant. Perhaps no plant is entirely im- 
 mune from attack. A few may be less attractive to the nematodes or less 
 easily located, and certain others may have thicker-walled roots which 
 are more difficult to penetrate; but when no other host plant is available, 
 the nematodes may be forced by starvation to attack the less favored 
 root. 
 
12 
 
 University of California — Experiment Station 
 
 TABLE 3 
 
 Plants Keported as Giving a Profitable Orop Even When Infested 
 BY the Root-Knot Nematode 
 
 TRUCK crops 
 
 artichoke, globe 
 
 asparagus* 
 
 cabbage* 
 
 cauliflower* 
 
 celery* 
 
 corn, sweet 
 
 horseradish 
 
 Jerusalem artichoke* 
 
 lima bean: Hopi 155t and Hopi 2,000 
 
 onion 
 
 parsnip* 
 
 radish* 
 
 rhubarb 
 
 spinach* 
 
 sweet potato : the Calif ornian, Pump- 
 kin Spanish, Southern Queen, and 
 Triumph varieties are usually only 
 moderately infested; the Creola, 
 Dixie Yam, Early Carolina, Enor- 
 mous, Golden Porto Rico, Improved 
 Big Stem Jersey, Japan Brown 
 Sweet, Little Stem Jersey, New 
 Gem, and Porto Rico varieties are 
 only lightly infested 
 
 turnip 
 
 field and forage crops 
 
 alfalfa 
 
 berseem, or Egyptian clover 
 
 buckwheat 
 
 carob, or St. John's bread 
 
 chufa {Cyperus esculentus) 
 
 corn 
 
 cotton : Acala variety 
 
 cowpea: Brabham, Iron, Monetta, 
 and Wood's Virginia Blaekeye are 
 less susceptible than other varie- 
 ties, but are sometimes severely in- 
 fested 
 
 Crotalaria striata 
 
 guar 
 
 millet: light infestations have been 
 reported on barnyard millet (Echi- 
 nocJiloa crus-galli) , on foxtail mil- 
 let (Setaria italica), and on ragi 
 millet (Eleusine coracana) 
 
 peanut: nematodes are occasionally 
 serious, and they increase the dam- 
 age by rhizoctonia 
 
 pigeon pea 
 
 soybean: Biloxi and 0-too-tan varie- 
 ties 
 
 FRUIT AND NUT CROPS 
 
 grape: Vitis champini ; roots of the 
 Dog Ridge and Barnes varieties 
 have shown some resistance to 
 nematodes 
 
 V. doaniana, Salt Creek variety, is 
 more resistant but less satisfac- 
 tory as a rootstock 
 V. riparia X (F. cordi folia X V. 
 
 rupestris), No. 106-8$ 
 F. riparia X V. rupestris, No. 101- 
 
 14$ 
 V. riparia X V. rupestris, 
 
 No. 120A$ 
 Solonis X Othello, No. 1613$ 
 Solonis X F. riparia, No. 1616$ 
 
 and possibly also 
 7'. herlanclieri X F. riparia, 
 No. 420-A$ 
 loquat 
 olive* 
 persimmon 
 pomegranate 
 
 * Somewhat doubtful: infestation may be serious in some cases. 
 
 t This variety was bred by W. W. Mackie, of the Division of Agronomy of the University of 
 California. It is highly resistant to nematodes, and also to charcoal rot, fusarium wilt, and heat. 
 It can be counted on to give a good crop in nematode-infested soil, but since the nematode resist- 
 ance is not yet fixed, the variety is not a good one to use in a starvation program: some vines 
 may revert to the susceptible type and even be killed by nematodes under unfavorable growth 
 conditions. Approved seed may be obtained from the Lima Bean Growers' Association, Oxnard, 
 California. Hopi 2,000 is another variety bred by W. W. Mackie. This small lima is excellent in 
 home gardens, but it is not a commercial variety, and it is somewhat less resistant to nematodes 
 than is Hopi 155. Seed may be obtained from growers in the San Fernando Valley. 
 
 X The roots of hybrids listed have shown apparent resistance to nematodes under favorable 
 growth conditions. The commonly grown phylloxera-resistant variety, Yitis rupestris St. George, 
 is not sufficiently resistant to nematodes. Hybrid stocks are being propagated in the California 
 experimental vineyards of the United States Department of Agriculture, and are being tested for 
 resistance to nematodes as well as to phylloxera. The resistance of a stock depends to a large 
 degree on its adaptation to local soil and climatic conditions, and on the congeniality of stock 
 and scion. 
 
Cm. 330] 
 
 The Root-Knot Nematode 
 
 Table 3 — Continued 
 
 Plants Reported as Giving a Profitable Crop Even When Infested 
 BY the Eoot-Knot Nematode 
 
 ORNAMENTALS 
 
 agave 
 
 aloe 
 
 alyssum 
 
 arctotis 
 
 aster 
 
 calendula: Eldorado and Vaughn's 
 
 Mammoth Mixture varieties 
 calliopsis 
 candytuft 
 Centaurea imperialis, or Royal sweet 
 
 sultan 
 cereus 
 cosmos 
 
 didiscus 
 
 geranium 
 
 gladiolus 
 
 honeysuckle, Japanese 
 
 larkspur 
 
 marguerite 
 
 mesembryanthemiim 
 
 mirabilis 
 
 nasturtium 
 
 Nepeta hederacea, or ground ivy 
 
 opuntia 
 
 Phlox drummondi 
 
 sweet William 
 
 vinca, or periwinkle 
 
 Table 3 is a list of plants that are usually able to tolerate their 
 nematode parasites, and may reasonably be expected to give a profitable 
 crop in infested land. However, they often harbor many nematodes and 
 must not under any circumstances be used in a starvation control pro- 
 gram. It should also be remembered that nematode injury is insidious 
 and that the loss is often greater than one realizes. 
 
 TABLE 4 
 
 Host Plants Which Are Not Reported as Seriously Injured by the 
 Root-Knot Nematode 
 
 truck crops 
 
 FRUIT and nut crops 
 
 ornamentals 
 
 broccoli 
 
 apple 
 
 calla lily 
 
 Brussels sprouts 
 
 blackberry 
 
 fern 
 
 chives 
 
 butternut 
 
 iris 
 
 cress 
 
 chestnut 
 
 lily 
 
 garlic 
 
 currant 
 
 narcissus 
 
 leek 
 
 dewberry 
 
 nolana 
 
 rutabaga 
 
 gooseberry 
 
 poinsettia 
 
 
 guava 
 
 privet, California 
 
 
 loganberry 
 
 tulip 
 
 
 mango 
 
 wisteria 
 
 
 pear 
 
 
 
 quince 
 
 
 
 raspberry 
 
 
 Other plants, about which there is no particular information avail- 
 able, are listed in table 4. Most of these plants are hosts, and should not 
 be used unless tables 2 and 3 offer nothing suitable; but injury to them 
 is probably not severe. 
 
 The breeding of resistant plants is a promising way of attacking the 
 nematode problem. It is, however, a long and difficult undertaking, and 
 few results are as yet available for publication. 
 
14 University of California — Experiment Station 
 
 INTRODUCTION OF NEMATODES 
 
 Means of Introduction. — Human agencies are most often responsible 
 for the introduction of nematodes into clean fields. The following list 
 indicates some common sources of infestation : 
 
 Infested seed potatoes 
 
 Young plants set out from greenhouses or from infested nurseries 
 Soil brought in from outside, which may contain nematodes 
 Irrigation water coming from infested land, or from shallow wells 
 
 where drainage may have brought eggs or larvae 
 Manure containing fresh plant debris that might carry nematodes 
 Bean straw from infested fields 
 Muddy implements used in infested soil 
 Culls or peelings of infested potatoes or other vegetables, fed to hogs 
 
 or chickens or buried to fertilize the garden 
 
 Nematodes are not brought into clean soil by green or farm manure, 
 except from infested land, nor by dry seed. Growers have sometimes 
 supposed that an infestation was started by some particular crop. How- 
 ever, if the precautions discussed in the next section are observed, there 
 is little danger of introducing nematodes by growing any new crop on 
 clean land. Of course, if there is already a small and unobserved infesta- 
 tion in a field, the cultivation of susceptible plants will show up its 
 presence, and also provide food for the multiplication of the nematodes. 
 
 Precautions Against Introducing Nematodes. — In dealing with root- 
 knot, prevention is the measure most to be urged but least often con- 
 sidered. Growers are frequently unaware of an infestation until it has 
 reached the destructive stage. It is necessary to be constantly on the 
 watch against the introduction of such a persistent pest into clean or 
 treated land. It must be remembered that a single larva may enter a 
 root, develop, and lay several hundred eggs, which need not even be 
 fertilized by a male. Three or four generations of nematodes may mature 
 during the growth of one crop, and each female may lay more hundreds 
 of eggs. The first nematode must not be introduced, for in one season it 
 may literally populate the soil with several million larvae. 
 
 There are a number of precautions that may be taken by anyone who 
 is anxious to keep his land free of nematodes : 
 
 Seed potatoes should not be planted without making certain that they 
 are uninf ested. 
 
 All seedlings to be set out into clean land should be most carefully in- 
 spected, because hotbeds are very frequently infested with nematodes, 
 which are carried out into the field with the young plants. The source of 
 all transplanted roots, from nurseries, etc., should also be checked up. 
 This is important even if there are no conspicuous galls, for some roots 
 
CiR. 330] The Root-Knot Nematode 15 
 
 may be fairly resistant and yet carry nematodes in the adliering soil ; 
 or originally clean stock may have become recently contaminated by 
 heeling-in or by other handling. Suspected roots may be set out in quar- 
 antine boxes for a month, to avoid the risk of introducing a field infes- 
 tation. They should be examined again before transplanting, and indi- 
 cator roots growing in the same box should also be examined. Whenever 
 possible, plants should be propagated by seeds, cuttings, or runners that 
 have had no contact with infested soil. 
 
 The source of any soil brought in for garden or greenhouse should be 
 rigidly checked by the examination of indicator plants. Even virgin soil 
 may have become contaminated either by drainage water or else by 
 shovels, wheelbarrows, or trucks that had previously been used with 
 infested soil. 
 
 The course of irrigation water coming from fields which might be in- 
 fested should be watched with suspicion. Drainage water that might 
 carry nematodes should be diverted by ditches. 
 
 Bean straw should be thoroughly dried before being spread on clean 
 land, if it comes from infested or suspected fields. 
 
 Field implements should be washed or scraped and thoroughly dried 
 each time they are used. Also, the cleaning of implements which may be 
 taken from the greenhouse to the field is exceedingly important. 
 
 Potatoes to be fed to the hogs should be boiled for a few minutes if 
 there is any suspicion of their carrying nematodes. Uncooked garbage 
 should not be thrown out, nor buried in the garden without some ade- 
 quate treatment. 
 
 Nematodes can be eradicated from infested soil or roots only in certain 
 special cases. Most of the treatments which claim to make such material 
 "practically free" are inadequate. No one who has clean soil can afford 
 to take a chance on even a single nematode. 
 
 NEMATODE CONTROL BY CULTURAL METHODS 
 
 In combating any organism, it is essential to understand its life his- 
 tory and habits. The foregoing discussions of "Biological Character- 
 istics" are therefore of more than theoretical interest. Familiarity with 
 this material is an important preliminary to an intelligent reading and 
 application of the sections which follow. 
 
 The control measures of rotation, fallow, and flooding are all based on 
 starving the larvae in the soil. The problem is not only to limit or destroy 
 available host plants, but at the same time to provide conditions most 
 favorable for keeping the larvae active, in order to hasten their exhaus- 
 
16 University of California — Experiment Station 
 
 tioii. It is still more important to encourage the development and hatch- 
 ing of the eggs, which would otherwise provide a dormant reserve popu- 
 lation. Fallow and flooding are thus of relatively little value in winter, 
 when low temperatures prevent any considerable activity on the part of 
 the nematodes. The success of any starvation program depends also to 
 a large extent on the thoroughness of weeding, since weeds are reservoirs 
 of infestation. 
 
 Crop Rotation with Resistant Plants. — Rotation with resistant plants 
 is the principal recommendation for field control of root-knot. It has the 
 advantages that the land is productive and that cultivation provides the 
 best conditions for hatching of eggs and activity of larvae, while limiting 
 their food plants so that most of them must starve. As already stated, 
 there is practically no crop that will not harbor a few nematodes and 
 allow some reproduction. 
 
 If a rotation program is carried out faithfully, it should be possible 
 to harvest one susceptible crop every two or three years, though of course 
 the infestation will remain in the soil. It may be feasible to rotate re- 
 sistant crops in summer with whatever susceptible crops can be grown 
 in winter. Warning must, however, be given that since winter crops are 
 growing in the soil during warmer days in fall and spring, they will 
 somewhat increase the numbers of nematodes. 
 
 In order to reduce an infestation, crops for rotation should be selected 
 from table 2 only. An alternation of grains and resistant legumes is 
 recommended for cases where it is at all practical. Control will be more 
 complete with highly resistant crops planted in rows so that weeds can 
 be destroyed and the soil can be frequently cultivated. On the other 
 hand a good smother crop, such as Iron cowpea or a good stand of mixed 
 grasses, requires less care and may be almost as effective. 
 
 Fallow. — The one rule for controlling nematodes by fallow is that 
 there shall be no root of crop plant or weed left growing in the soil long 
 enough to allow any worms to form eggs. This is exceedingly important, 
 because the new eggs and larvae propagated in these roots have a fresh 
 reserve of energy, and it will take many additional months before they 
 can be starved or exhausted. In hot weather, when nematodes mature 
 most rapidly, weeds must be destroyed every 10 days, but the interval 
 between plowings may be graduallj^ lengthened to a month or two in 
 cold weather. Each i)l owing pa3^s for itself by shortening the time re- 
 quired for control. 
 
 There is a different measure, described later, which makes use of weeds 
 to remove moisture from the deeper layers of the soil. Tlie two methods 
 sliould not be confused; the value and applica])ility of each should be 
 
Cm. 330] The Root-Knot Nematode 17 
 
 considered in relation to local conditions. A moist clean fallow probably 
 exhausts the strength of the nematodes more rapidly than a dry fallow, 
 whereas complete drying, if it is practicable, kills them directly. 
 
 The conditions of fallow are found in the chicken yard and in the pig 
 pen, unless infested vegetables have been used as feed. Hogs or poultry 
 might thus be made a part of a rotation program, and be confined on the 
 most heavily infested areas. 
 
 Even a short period of fallow is a great benefit, because the root-knot 
 nematode can multiply only in living plants, and as soon as such repro- 
 duction ceases its numbers begin to decline. It has been stated that 
 nematodes can even be eradicated by two years of strict starvation. 
 However, a less extreme fallow will not ordinarily accomplish eradica- 
 tion. The survival of the remaining fraction of the population is favored 
 by the usual conditions of fallow, which allow the larvae to be so inactive 
 that they can conserve their energy for a long time before they starve. 
 The eggs that are not killed by drying probably remain more or less 
 dormant also for some time. When the starvation program is considered 
 completed, and the field is again planted, they may hatch and provide a 
 fresh population. It is therefore advisable to continue an attempted 
 starvation program by rotation with the most resistant crop available. 
 
 Flooding. — Flooding is a useful measure for hastening the starvation 
 process during fallow. The theory is that old galls decay under water, 
 and eggs are stimulated to hatch. The increased activit}^ in water will 
 especially hasten the exhaustion of larvae. In field experiments in Cali- 
 fornia, flooding for 6 months killed most of the nematodes, but there 
 were still a few survivors even after a year under water.* 
 
 Where water is available, submerging the land for even a month will 
 be of benefit, if the weather is warm, provided that flooding is followed 
 up by fallow^ or by a highly resistant crop, or better, by a rotation of two 
 or three resistant crops before a host crop is again planted. 
 
 Desiccation. — All stages of the root-knot nematode are killed imme- 
 diately hy dryness. Small amounts of soil can be spread out in a thin 
 layer in a warm greenhouse and dried in a few hours; but it will take a 
 couple of weeks, with the most careful attention, to desiccate the galls, 
 which are never all removed from any soil. The danger of attempting 
 this method of control in the field is that the time of survival of eggs 
 and larvae is actually prolonged in a fairly dry soil which is not com- 
 XJletely desiccated. 
 
 Sun-drying, however, is very valuable where it is practicable. In the 
 
 4 Unpublished data of L. N. Brown, of the United States Department of Agricul- 
 ture, Bureau of Agricultural Engineering. 
 
18 University of California — Experiment Station 
 
 liot valleys, a dry fallow during the summer months will greatly reduce 
 the nematode population of a field. Weeds use up much of the moisture 
 from the deeper soil, and thus considerably hasten the drying process; 
 but even after weeds are killed by drought, the soil may hold enough 
 moisture to keep nematodes alive, and the weeds will have served for the 
 propagation of new eggs and larvae. At this time, therefore, it is essen- 
 tial to further the drying process by turning up the deeper soil. When 
 weeds are not allowed to grow, frequent plowing is advisable, especially 
 before and during a hot dry spell, to expose the deeper layers of soil, 
 where the majority of the nematodes are protected, directly to sun and 
 wind. More soil will be exposed by plowing across the previous furrows. 
 Of course this should not be practiced unless the infestation is general 
 and there is no danger of spreading nematodes to clean land. 
 
 Dry fallow is probably less harmful to the soil than has generally been 
 believed. Indeed, a gradual and complete desiccation may increase the 
 fertility of land by partial sterilization, on the same principle as that 
 explained in connection Avith heat treatments (see page 21) . 
 
 The fact that larvae cannot travel through dry soil can also be applied. 
 A strip of dry ground, such as a road or walk, may be a barrier prevent- 
 ing migration of nematodes from an infested to a clean field. Of course 
 this does not prevent soil from being carried across the road on imple- 
 ments or on feet. 
 
 Mulchmg. — It is reported that nematodes do not thrive in orchards 
 where a mulch of straw, weeds, or other plant material is piled a foot 
 deep around the trees. The action of a mulch is not well understood. It 
 may be only to increase the vigor of the trees, or to inhibit nematode 
 activity by reduced aeration; yet it is also possible that the gases formed 
 by decomposition may even be fatal to nematodes as they are to many 
 insects in the soil. 
 
 Decay. — A heavy stand of a covercrop or green-manure crop may be 
 plowed under and allowed to decay under conditions of warmth and 
 moisture. The heat and gases formed by the decomposition of this mate- 
 rial have been found in experimental tests to kill from 5 to 22 per cent 
 of the population of the sugar-beet nematode, which is more resistant to 
 unfavorable conditions than is the root-knot nematode. 
 
 MEASURES WHICH CANNOT BE RECOMMENDED WITHOUT WARNINGS 
 
 Hot-Water Treatments for Infested Roots. — Two papers recently 
 published recommend a hot-water treatment for infested peony roots, 
 similar to that now used for ])idl)s. This is a very promising measure, 
 and it is being tried experimentally for various roots and corms, both by 
 
CiR. 330] The Root-Knot Nematode 19 
 
 the United States Department of Agriculture and by the California 
 State Department of Agriculture. However, because time and tempera- 
 ture must be balanced with the greatest precision, in order to insure kill- 
 ing of all nematodes without injury to the plants, the treatments are not 
 yet standardized for general use. Until their success has been thor- 
 oughly proved, there is always the danger that planting stock unofficially 
 treated may introduce nematodes into clean land. 
 
 Trap Crops. — The trap-crop method is not to be generally recom- 
 mended for control of nematodes, although in theory it gives much 
 promise. If some very susceptible and rapidly growing plant, such as 
 squash, rape, or a susceptible cowpea, is sown thickly in a field and later 
 plowed under before any nematodes have time to reproduce, the half- 
 grown worms in the roots will die. It is true that, if this is repeated two 
 or three times in succession, many larvae will be removed from the soil. 
 However, in practice, it is a dangerous method. If the plants are left 
 growing even a day too long, some new eggs will almost inevitably get 
 back into the soil and defeat the purpose of the program by prolonging 
 considerably the time required to starve out the pests. 
 
 Egg formation can be prevented by plowing 10 days after the sprouts 
 appear. This time limit need not be observed strictly except in the hot- 
 test summer weather. In a cooler season, it may be safe to wait as long as 
 3 weeks, but unless a time margin is allowed, only expert supervision can 
 ensure the destruction of the plants before egg formation. Because of its 
 dangers and the care and expense involved, the trap-crop method is not 
 used in this country, though with proper supervision it could be applied 
 to advantage in some cases. 
 
 Over fertilizing. — The value of overfertilizing is still in dispute. It is 
 not good practice to use excess fertilizers to force a crop, hoping that it 
 may outgrow the nematodes. In some cases this treatment will increase 
 the yield of one crop or perhaps postpone complete failure, and it has 
 been recommended for this immediate advantage. Of course, a normal 
 amount of fertilizer is necessary, since infested plants suffer more in 
 poor soils than in fertile ones. Overfertilizing, however, can only result 
 in feeding the parasites on more succulent roots, so that they are well 
 nourished and reproduce in greater numbers. 
 
 A possible exception to the rule may be the use of a potassium fer- 
 tilizer. It has been claimed that this protects plants to some extent from 
 nematode injury. Different explanations are given for the point. The 
 treatment probably has little value except in soils deficient in potassium. 
 
 Molasses is sometimes used in sugar-cane fields to correct growth fail- 
 ure, aggravated by root-knot and by invasions of other nematodes. 
 
20 University of California — Experiment Station 
 
 The result is apparently not the destruction of the nematodes, but only a 
 stimulation of root growth. Molasses alone causes the removal of nitrates 
 from the soil ; but if fertilizers containing nitrogen and phosphorus are 
 applied at the same time or earlier, the ultimate value of the latter is 
 increased by the molasses. These processes depend on the activities of 
 soil bacteria. Fermentation is, however, avoided. The measure is men- 
 tioned here only to explain its action ; its application is probably limited 
 to cane fields, where molasses is a by-product. 
 
 Biological Control. — Predacious nematodes, mostly of the genus Mo- 
 nonchns, have been suggested as a possible means of controlling root- 
 knot. It is true that mononchs do destroy some plant nematodes, but 
 these pests are not necessarily the main diet of the predators in the field. 
 Even though mononchs might be very valuable for control, it is difficult 
 to establish a p()i)u]ation of theiu Avhere it is needed. They recjuire a moist 
 soil, and they liave their own parasitic diseases. 
 
 Plant nematodes may also be destroyed under certain conditions by 
 fungi, or by ants or other enemies, which may serve to check an other- 
 wise excessive multiplication but are not of economic importance. 
 
 SPECIAL SOIL TREATMENTS 
 
 Partial Sterilization of Soil by Heat. — The use of heat in any form, 
 wherever it is feasible, is the first recommendation for soil treatment. 
 A temperature as low as 110° Fahrenheit is sufficient to kill the root- 
 knot nematode, if maintained for 2 hours through the entire volume of 
 the soil; 120° kills in less than 10 minutes; and 135° will kill all stages 
 instantly. Heat, depending of course on the degree, also destroys weed 
 seeds, insects, and the bacteria and fungi which cause plant diseases, 
 more generally than does any other single method of soil sterilization. 
 A crop in heated soil may show a slight setback at first, but it soon grows 
 with added vigor. 
 
 Steam treatment is now a regular practice in greenhouses. Nematodes 
 can be eradicated from limited volumes of soil by a careful and thorough 
 steaming, or controlled for one season by a less complete treatment. 
 References to literature on the installation of steam equipment may be 
 found at the end of this circular. The relative merits of the inverted-pan 
 and buried-tile methods are still open to dispute. Since the soil should 
 be well spaded before any heat treatment, the labor of laying deep tiles 
 serves a double purpose. Drenching with boiling water condensed from 
 steam pipes is probably not completely effective, but it is used to advan- 
 tage for the ground beds in some greenhouses. It may also be used for 
 seed beds or for flower borders. A serious objection is that puddling, 
 
Cm. 330] The Root-Knot Nematode 21 
 
 which is apt to occur unless the drainage is excellent, may prevent pene- 
 tration of the heat. For all these treatments, the soil should be warm, 
 dry, and loose, and should be covered during and after heating. 
 
 A steam bin is effective for treating small volumes of soil for green- 
 house use. The pipes must be close enough together to insure an even 
 distribution of heat. Small volumes of soil can also be baked in an oven 
 or in open pans. For best results, the soil should be neither too wet nor 
 too dry before baking. 
 
 The application of heat in the field is not practicable on a large scale. 
 However, if brush is to be burned, this should be done over spots where 
 nematodes are abundant. Nematodes will not be reached unless the soil 
 is plowed before burning, because they are not at the surface, and heat 
 will penetrate only a few inches. Any type of reflector oven that can be 
 devised will increase the efficiency of heating. An oil-burning blowtorch 
 is used in Holland for killing the bulb nematode in soil. 
 
 There is a general prejudice against the heating of soil, on the ground 
 that it "burns out the humus." This would be true if a very high tem- 
 perature were reached, as might happen in oven-baking. Possible in- 
 jury to the soil by any treatment considered must of course be weighed 
 against the effectiveness of the treatment and the necessity for nematode 
 control. In the case of heat, however, it is now accepted that the fertility 
 of a soil is increased by heating it to 140° or 150° Fahrenheit, while the 
 root-knot nematode is killed between 110° and 135°. Steaming is per- 
 haps less destructive of the normal soil properties than is dry heat; yet 
 even oven-baking and surface fires have been recommended by authori- 
 ties, after field and laboratory experimentation, for stimulating crop 
 growth quite apart from nematode control. 
 
 The most conspicuous result of heating a soil is a change in the rela- 
 tive proportions of the soil microorganisms, which then cause by their 
 activities an immediate increase of ammonia and an ultimate increase of 
 nitrates. In other words, after partial sterilization the remaining bac- 
 teria are able to make the organic matter of the soil more available for 
 crop growth. The amount of nitrogen thus released depends of course on 
 the amount of organic matter present. A temporary excess of ammonia 
 may inhibit germination for a time and make the soil appear toxic. 
 Moisture is said to reduce this danger. 
 
 Heat may also lower the water-holding capacity of a soil by changing 
 the physical state of the colloidal particles, both organic and inorganic. 
 With moderate heat, this change is only temporary. It is least marked 
 in sandy soils, while very heavy soils may profit by })eing made more 
 friable. Desiccation has practically the same effect. 
 
22 University of California — Experiment Station 
 
 Heat treatment sliould be sufficiently tlibrougli to penetrate all parts 
 of the soil with killing temperatures, but it should not be carried farther 
 than this, because many of the injuries caused by heating are due to un- 
 necessarily prolonged high temperatures. 
 
 Chemical Treatments of Soil. — Chemical treatments are reasonably 
 effective in very sandy soils, bnt are practically wasted in clay or organic 
 soils. No chemical has ever completely eradicated nematodes in the field, 
 and the few eggs or larvae which survive soon start a new population. 
 The expense of the treatments is seldom warranted for the temporary 
 control obtained, and so their use is not recommended except in very 
 special cases. In greenhouses, however, a minimum dosage is sometimes 
 applied, knowing that it must be repeated for each crop. Again, in the 
 treatment of small and valuable beds, an extra effort can be made to mix 
 the chemicals thoroughly into all parts of the soil to a considerable depth, 
 •and heavy dosages will not add so greatly to the expense. Care should 
 be exercised in selecting the treatment most suitable for the particular 
 conditions. 
 
 There are three reasons for the failure of chemicals in the field : ( 1 ) 
 Chemicals do not penetrate readily into the deeper layers of soil, where 
 nematodes are still numerous though less abundant than in the first foot; 
 they do not penetrate into clods; and gases may settle in pockets. (2) An 
 appreciable portion of the chemicals applied to clay or organic soils is 
 never used for fumigation, but is lost by combining with proteins or by 
 adsorption on the surface of the minute colloidal particles. (3) Many of 
 the chemicals which will kill nematode larvae in soil are without effect 
 on the egg masses, which are more resistant, especially if they are pro- 
 tected in galls. 
 
 Some chemicals are advertised for their fertilizing as well as for their 
 fumigating value, but it does not always follow that the particular ele- 
 ments supplied are those needed by the soil which is to be treated. Carbon 
 disulfide and chlorpicrin, the most volatile of the fumigants, may in- 
 crease the fertility of a soil by partial sterilization, as do heating and 
 drying. Chemical residues, e.g., those left by cyanide, may actually de- 
 crease fertility by inhibiting the essential bacterial activities of decom- 
 position and nitrification. 
 
 The most promising of the many chemicals which have been recom- 
 mended for nematode control seem at present to be carbon disulfide and 
 chlorpicrin. Liquid carbon disulfide is highly explosive, but it is con- 
 sidered effective against nematodes in the field. It also kills weeds, and 
 must not be used near crop plants. Carbon disulfide emulsion^ is less 
 
 ^ In California, carbon disulfide emulsion can be obtained from the Wheeler, 
 Eeynolds, and Stauffer Chemical Co., San Francisco. 
 
^iR- 330 J The Root-Knot Nematode 23 
 
 danj^eroiis to handle, although its Yai)or has been known to explode. It 
 is frequently less effective in the field than in greenhouses, where the 
 soil is warm, loose, and moist. 
 
 A grower may mix his own emulsion by the following formula." 
 
 Volume, in W^eight, in 
 
 Material gallons l)()un(ls 
 
 Carbon disulfide 68 680 
 
 Rosin fisli-oil soap 6 54 
 
 Water 26 208 
 
 The soap and water may be mixed together in a wooden barrel, and 
 stirred vigorously with a long-handled shovel. The carbon disulfide 
 should be added last, while stirring. This gives a 68 per cent, or concen- 
 trated emulsion, which should be diluted for use, usually 1 :50 in water. 
 Homemade emulsions are much less expensive than those commercially 
 prepared, but they are also less satisfactory. In addition to the fire risk, 
 the product is not evenly mixed, and its application will therefore be 
 irregular and inefficient. 
 
 Chlorpicrin,' or tear gas, has not j^et been generally applied in this 
 country for soil fumigations. However, in experiments in Hawaii, it has 
 given as high as 90 per cent control of nematodes in loam soil. The first 
 crop after treatment shows enormous improvement. This is not due 
 solely to its freedom from nematodes, because increased plant growth 
 follows any partial sterilization of soil. While working with chlorpicrin, 
 a gas mask should be worn, because a person may be injured by the 
 cumulative effects of small doses, especiallj^ if exposed indoors. Larger 
 doses cause nausea, and skin burns may be serious. The fumes will kill 
 plants in a greenhouse. 
 
 Various other treatments have given promising results under opti- 
 mum conditions, but have proved less successful in other cases. All chem- 
 icals show this inconsistency of action in different soils and under differ- 
 ent conditions, so that no standardization of their effectiveness is pos- 
 sible. Table 5 gives a summary of the more important chemicals which 
 have been recommended by various investigators. The minimum dosage 
 gave a fair control under the conditions of the experiment. A much 
 heavier application would not give 100 per cent kill, except in small 
 volumes of soil which could be fumigated in air-tight compartments. 
 
 6 Formula quoted from: Guba, E. F. Carbon disulfide emulsion for the control of 
 the root-knot nematode. Massachusetts Agr. Exp. Sta. Bui. 292:1-16. 8 tables. 
 3 figs. 1932. 
 
 7 Chlorpicrin is distributed by the California Spray Chemical Co., as well as by 
 several eastern firms. A Vermorel injector is very valuable for measuring the 
 doses and drilling the liquid into the soil. This implement is manufactured by 
 Vilmorin-Andrieux et Cie., Paris, and is not at present sold in America. 
 
24 
 
 University of California — Experiment Station 
 
 TABLE 5 
 
 Chemicals Which Have Been Eecommbnded for Control of the 
 
 Root-Knot Nematode 
 
 Material 
 
 Amount 
 
 Application 
 
 Remarks 
 
 EfiFectiveness 
 
 Carbon 
 
 100 to 303 gals, per 
 
 Bury in holes 6 to 9 
 
 Gas spreads only in 
 
 Gives fair to good 
 
 disulfide 
 
 acre, or % to 2 
 
 inches deep, 18 inches 
 
 light dry soil; plot 
 
 control, for one 
 
 
 fluid oz. per hole 
 
 apart each way, in 
 staggered rows; cover 
 
 must be aired a week 
 or more before plant- 
 ing 
 
 crop 
 
 Carbon 
 
 Dilute 1 part of the 
 
 Pour evenly over sur- 
 
 If a fungicide is needed, 
 
 Most effective in 
 
 disulfide 
 
 concentrated (68 
 
 face; soil should be 
 
 formaldehyde may 
 
 greenhouses 
 
 emulsion 
 
 per cent) emul- 
 sion with 49 parts 
 of water; use 1 
 gal. per sq. ft. 
 
 moist and loose 
 
 be added, H gal. to 50 
 gals, of the diluted 
 emulsion 
 
 
 Chlorpicrin 
 
 253 to 380 lbs. per 
 
 Bury in holes 6 inches 
 
 Plant after odor has 
 
 Good temporary 
 
 
 acre, or ^ to ^ 
 
 deep, 18 inches apart 
 
 disappeared from soil 
 
 control in sand 
 
 
 fluid oz. per hole 
 
 each way, in stag- 
 gered rows; cover 
 
 
 or loam 
 
 Sodium 
 
 600 to 1,200 lbs. 
 
 Pour aqueous solution 
 
 Ammonium sulfate is 
 
 Most effective 
 
 cyanide 
 
 per acre 
 
 of cyanide on newly 
 
 needed to liberate the 
 
 form of cyanide; 
 
 
 1 
 
 plowed land, irrigate 
 
 cyanide gas; both 
 
 results vary 
 
 Ammonium 
 
 900 to 1,800 lbs. 
 
 heavily; pour on am- 
 
 compounds have fer- 
 
 with different 
 
 sulfate 
 
 per acre 
 
 monium sulfate solu- 
 
 tilizer value; air soil 
 
 soils and clima- 
 
 
 
 tion the same day. 
 
 at least 2 weeks 
 
 tic conditions; 
 
 
 
 irrigate again lightly; 
 
 
 little value in 
 
 
 
 cover 
 
 
 clay 
 
 Cyanogas, or 
 
 600 to 1,200 lbs. per 
 
 Spread dry over sur- 
 
 The various commer- 
 
 Variable 
 
 crude calcium 
 
 acre per year, in 1 
 
 face, disk in, irrigate 
 
 cial Hake and dust 
 
 
 cyanide 
 
 or 2 applications 
 
 
 forms contain be- 
 tween 17 and 50 per 
 cent of calcium cya- 
 nide; the gas is readi- 
 ly liberated by water 
 
 
 Calcium 
 
 1,600 to 2,000 lbs. 
 
 Mix well into soil, irri- 
 
 Air 6 to 8 weeks; old or 
 
 Less than the cya- 
 
 cyanamide 
 
 per acre 
 
 gate 
 
 caked material is tox- 
 ic to plants 
 
 nides 
 
 Potassium 
 
 300 lbs. per acre 
 
 Mix dry salts, spread 
 
 Xanthate contains car- 
 
 Fair control, vary- 
 
 xanthate 
 
 
 immediately, irrigate 
 
 bon disulfide, which 
 
 ing with soil con- 
 
 
 
 the same day; this is 
 
 is liberated by the 
 
 ditions; does not 
 
 Super- 
 
 300 lbs. per acre ■ 
 
 a minimum dosage 
 
 acids of the super- 
 
 kill eggs in galls 
 
 phosphate 
 
 
 
 phosphate and sul- 
 fur 
 
 
 Sulfur 
 
 50 lbs. per acre J 
 
 
 
 
 Formalde- 
 
 Dilute with water 
 
 Sprinkle surface; cover 
 
 Good fungicide; none 
 
 Reports contra- 
 
 hyde 
 
 1 : 50; use 1 to 8 
 
 
 lost in soil; toxic to 
 
 dictory: prob- 
 
 
 qts. per sq. ft. 
 
 
 plants 
 
 ably little value 
 against nema- 
 todes 
 
 Sulfur, 
 
 300 to 1,000 lbs. per 
 
 Broadcast between 
 
 Acidity must later be 
 
 Reports contra- 
 
 ground 
 
 acre 
 
 plowing and harrow- 
 ing 
 
 neutralized with lime 
 
 dictory 
 
CiR. 330] 
 
 The Koot-Knot Nematode 
 
 TABLE 5— (Concluded) 
 
 25 
 
 Material 
 
 Amount 
 
 Application 
 
 Remarks 
 
 Effectiveness 
 
 Lime or 
 
 1,500 lbs. to 2 tons 
 
 Spread and plow 
 
 Value depends on soil 
 
 Reports contra- 
 
 quicklime 
 
 per acre per year, 
 in 2 or 3 applica- 
 tions 
 
 
 conditions 
 
 dictory; not a 
 disinfectant 
 
 Ammonia 
 
 Not determined 
 
 Drench soil with 1 : 400 
 
 Fertilizer, but may be 
 
 Effective on a 
 
 
 
 dilution in water; 
 
 toxic to plants if im- 
 
 small scale; too 
 
 
 
 cover 
 
 pure 
 
 expensive for 
 field use; not 
 tested in this 
 country 
 
 Soap flakes 
 
 No recommendation 
 
 Spread, plow, irrigate 
 
 May injure soil 
 
 No evidence 
 
 Crude naph- 
 
 850 lbs. per acre 
 
 Spread and plow 
 
 Air 2 weeks or longer 
 
 
 thalene or 
 
 
 
 
 
 creosote salts* 
 
 
 
 
 A recent line of in- 
 vestigation ; 
 
 Cresylic acid, 
 
 1 : 40in water; 9 to 
 
 Irrigate freely 
 
 Soil must be aired a 
 
 value and safety 
 
 called liquid 
 
 36 gals, per sq. 
 
 
 full month 
 
 not yet demon- 
 
 carbolic acid 
 
 yd. 
 
 
 
 strated for gen- 
 eral application 
 
 Cyanogas 
 
 1,000 to 1,740 lbs. ~ 
 per acre 
 
 Spread and irrigate 
 
 Fair control under 
 greenhouse condi- 
 tions 
 
 
 Naphthalene 
 
 1,000 to 1,740 lbs. 
 
 
 
 
 flakes 
 
 per acre 
 
 
 
 
 Paradi- 
 
 300 to 600 lbs. per 
 
 Broadcast, or sow in 
 
 Small doses may be 
 
 Doubtful 
 
 chlorbenzene 
 
 acre 
 
 furrows; cover with 
 soil, tamp; in fallow 
 land, wet the surface 
 
 used in orchards, if 
 trees are more than 6 
 years old; must not 
 touch bark. In gar- 
 dens, long airing is 
 required before 
 planting 
 
 
 Tobacco 
 
 Spread G inches 
 
 Rake, water heavily; 
 
 Old recommendation 
 
 Temporary reduc- 
 
 stems, cut or 
 
 deep 
 
 allow to soak in for 2 
 
 
 tion claimed; 
 
 ground 
 
 
 or 3 weeks 
 
 
 may kill larvae 
 but not eggs 
 
 * Vaporite is a mild form of these salts. 
 
 Other chemicals are heard of from time to time, but many of them are 
 recommended without sufficient evidence of their practical value. Mer- 
 cury compounds are an example. They are not especially effective against 
 nematodes, and the residues may injure future crop growth. Moreover, 
 much of the material applied is lost by combination with proteins in the 
 upper soil. The inadequacy of chemical treatment of soil, as explained 
 above, is the reason for being skeptical of all panaceas unless they are 
 well demonstrated. At the time of writing, there is no information which 
 promises success with treatments not mentioned in this circular. 
 
26 University of California — Experiment Station 
 
 Rules to Be Observed in Connection with Soil Treatments. — Chemical 
 as well as cultural control measures will be most successful in warm 
 weather when the nematodes are active and probably most sensitive to 
 external conditions, though there are always larvae in the soil, in addi- 
 tion to the egg masses in whatever roots may be present. 
 
 The large roots and as many as possible of the galls should be removed 
 from the soil before fumigating, because egg masses contained in galls 
 are more difficult to reach and kill with treatment than any other stage 
 in the life cycle. In the field, where this is not practicable, decay of the 
 old galls during two or three weeks of fallow may be fairly effective, if 
 the soil is warm and moist. 
 
 For most treatments the soil should be reasonably dry, although some 
 chemicals need to be dissolved and washed into the soil. Cyanide gas is 
 soluble in water, but neither carbon disulfide nor chlorpicrin, unless they 
 are emulsified, will penetrate in wet soils. 
 
 For effective fumigation, tlie surface of tlie soil should be immediately 
 packed, sprinkled, or covered with paper or wet canvas, to minimize the 
 escape of gases and to ])rolong their action, though it will only partially 
 prevent loss. Wetting the surface of the soil after applying chemicals 
 will hold gases even better than a cover. This measure is so successful 
 that it must not be tried in orchard fumigations, for an otherwise harm- 
 less treatment may thus become fatal to the trees. 
 
 Residues from chemical treatments are of course toxic to vegetation. 
 Carbon disulfide or chlorpicrin will be dissipated from drj^ soils in a few 
 days after removing the cover. Formaldehyde remains a longer time. 
 Use of the cyanides requires that the soil be aired for several weeks be- 
 fore planting, though the time may be shortened when ammonium sul- 
 fate is used to hasten the liberation of cyanide gas. 
 
 Heat treatments also require a fairly dry soil; otherwise a large part 
 of the heat energy is wasted in a limited area, in raising the temperature 
 of the excess water. Steam does not spread through wet soil, but an ex- 
 ceedingly dry soil may be injured more by the heating. Whether or not 
 to sow immediately after steaming is a matter of opinion. It may be wise 
 to wait 2 or 3 weeks to air out the excess ammonia, but the benefits in 
 increased fertility (see page 21), are lost with longer delay. 
 
 The soil management for several months after partial sterilization, 
 especially by dry heat, should provide for frequent irrigations, using 
 less than the usual amount of water at one time because water cannot 
 now be taken uj) and held by the soil. If fertilizers are still needed in the 
 poorer soils or in sand, they should be in soluble form, but ammonia 
 should be avoided at this time. A later application of green or farm 
 
Cm. 330J The Root-Knot Nematode 27 
 
 manure or crop residues may be beneficial to some soils. No general rules 
 can be given, because each soil presents a different problem, with many 
 phases. 
 
 APPLICATIONS OF THE PRECEDING RECOMMENDATIONS 
 
 Getting Along ivith Nematodes. — Control measures are not usually 
 intended to eradicate nematodes, but only to make the best of a bad bar- 
 gain. It is indeed doubtful if eradication is possible under field condi- 
 tions. However, in most cases a light infestation will not affect the yield. 
 Many of the treatments, both chemical and cultural, as ordinarily ap- 
 plied, give only sufficient reduction of the soil population to assure the 
 success of a single susceptible crop. When eradication is not possible, 
 this may be satisfactory from the standpoint of immediate returns. For 
 economy of materials, volatile chemicals are sometimes used in the crop 
 rows, just before planting, leaving infested soil between the rows. Or 
 again, a few rows may be treated with boiling water. Whether or not a 
 measure pays depends on the value of the crop grown as well as on the 
 expense of the treatment used. 
 
 The grower is entitled to a return for the effort and expense involved 
 in any control program, and when the population of a field has been re- 
 duced to the point where a susceptible crop can be raised, the temptation 
 is to plant that crop and take the profit. The result is a fresh population 
 of nematodes bred in one season. On the other hand, a properly chosen 
 rotation might double or treble the value of the control already obtained. 
 At the beginning of any season also, a nematode population may have de- 
 creased somewhat from natural causes alone. This is another case where 
 it is important to weigh the advantages of further control against the 
 prospect of multiplication. If the nematodes are first reduced to a mini- 
 mum, the infestation and therefore the increase on crop roots will be 
 materially less the first season, but of course the pest will eventually 
 come back. With suitable measures, it is possible to reduce an infestation 
 to the point where it can be kept under control. However, since the men- 
 ace of a serious new infestation always remains, only the cheapest and 
 most practical measures should be attempted. The grower must be con- 
 stantly aware of the danger, and he should apply one or another of the 
 cultural control measures whenever it is at all practical to do so. He 
 should use the most varied rotation possible, of crops and of plots, even 
 if he does not consider the infestation serious, instead of waiting until 
 nematodes have ruined a crop and taken possession of his land. 
 
 In infested soil, plants can sometimes be grown under conditions less 
 favorable to the nematodes than to the plants, such as low temperatures 
 
28 University of California — Experiment Stat 
 
 ION 
 
 or lieavy soil. There is probably also a degree of dryness wliich would 
 favor tlie nematodes somewhat less than the plants, 1)iit tliis mnst he 
 determined for each case by practical experience. If a plant can once 
 get a good root system established under such conditions of reduced 
 nematode activity, the later infestations of new rootlets may be more 
 successfully tolerated. 
 
 Eradication of Nematodes. — As just stated, eradication of nematodes 
 in the field is almost impossible. Control of an infestation means fighting 
 it continually, even though the actual population may, with care, be 
 kept below the danger point. Eradication means the killing of the very 
 last nematode, so that the land is entirely free of the pest unless it is re- 
 introduced in one of the many possible ways. In any case where eradica- 
 tion is possible, no half-way control should be accepted. There are situa- 
 tions where it would pay to take heroic measures in order to be rid of 
 the menace of a growing infestation. 
 
 Greenhouses and nurseries are the most obvious situations where 
 eradication should be both feasible and profitable. There is another case 
 where it may be possible, and is very important for preventing future 
 losses. This is in the field or garden, when nematodes are discovered in 
 an isolated spot, or sometimes even in a wider area if the soil is shallow 
 or the land is valuable. Since it is impossible to prevent the spread of the 
 pest when land is cultivated or irrigated, the infested area must be sepa- 
 rated from the clean land by a fence which will prevent all communica- 
 tion. Any possibility of drainage from the infested spot must be removed, 
 if necessary by digging a ditch inside the fence. The extent of the in- 
 festation must first be determined, as it is probably wider than is sus- 
 pected. Many indicator roots must be examined with the greatest care, 
 and the fence must be 6 to 10 feet beyond the farthest point where any 
 gall can be found. Shoes must be changed at the gate, or scraped and 
 disinfected in sheep dip. Implements must not be removed from the plot 
 until they are thoroughly dried or disinfected. Roots should be turned 
 under and allowed to decay. If they are dug, they should be burned 
 inside the fence. 
 
 Tlien finally a series of control measures must be faithfully carried 
 out. Frequent deep plowing, burning of brush over freshly plowed 
 soil, sun-drying, and chicken-raising are recommended. Other measures 
 discussed in this circular may perhaps apply to the particular situation. 
 If these measures are too difficult, a prolonged clean fallow should be 
 enforced. 
 
 Above all, every precaution against spread must be taken. Plants out- 
 side the fence, and from all parts of the field, must be frequently ex- 
 
CiR. 330] The Root-Knot Nematode 29 
 
 amined for nematode galls. The fence must not be removed until very 
 careful examinations of indicator roots grown in the soil for 3 months 
 have proved that the infestation is actually eradicated. Even then, 
 watchfulness must not be relaxed. 
 
 These points are not overemphasized. The writer has observed re- 
 grettable negligence, in the field and in greenhouses, on the part of 
 persons who should know better but wlio consider precautions either use- 
 less or else too troublesome. If the expense of a program for eradication 
 is undertaken, it is worth some extra pains to accomplish that end. In no 
 case should any reasonable measure for avoiding spread be ignored. It 
 should not be necessary to remind any grower that the menace of nema- 
 todes must be taken seriously. 
 
 Comhinations of Practices for Field Control. — A well-planned com- 
 bination of practices will go much farther toward control of nematodes 
 than any of the recommended treatments alone. The value and perman- 
 ence of any chemical or cultural treatment will be increased if it is fol- 
 lowed up by a wet fallow, or by a resistant crop, with particular atten- 
 tion to the control of weeds. 
 
 Since heated soils are subject to sudden drying, field control by desic- 
 cation will be considerably more effective if preceded by even a super- 
 ficial burning of weeds and brush. It will be an advantage to start by 
 plowing up infested roots so that they will be burned and the deeper 
 layers of soil, which are more heavily infested than the surface, exposed. 
 Liming also may hasten desiccation. 
 
 Although fallow is an important control measure, its effectiveness in 
 causing exhaustion of nematodes is limited by its own conditions. The 
 presence of growing plants is probably one of the principal factors in 
 stimulating eggs to hatch and larvae to be active. Since this factor is 
 absent during fallow, it is suggested that a fallow be broken into two 
 periods, separated by one or two very brief trap crops for the purpose of 
 activating the nematodes and hastening their exhaustion. Of course the 
 danger of reproduction in new galls on the trap roots must be carefully 
 guarded against. For the purposes of general field control, however, the 
 weeds which grow for two or three weeks must perform this function. 
 
 For an attempted eradication, the following program is suggested : 
 
 1. Burning, two or three times, if possible, each preceded by spading 
 or plowing 
 
 2. Chicken-raising, or dry fallow with frequent plowing 
 
 3. One or two well-irrigated trap crops, completely destroyed 2 or 3 
 weeks after sprouting 
 
 4. Moist fallow in warm weather, without Aveeds 
 
 5. Resistant crops in rotation, kept free of weeds 
 
 6. Repetition of 4 and 5 
 
30 University op California — Experiment Station 
 
 Special Measures for Greenhouses and Nurseries. — It is essential that 
 nurseries and seed beds be free of nematodes, both because of the loss 
 from rejections and because of the dano^er to clean land in transplanting. 
 A minor infestation which may be unobserved by the purchaser is more 
 treacherous than a conspicuously heavy infestation. In greenhouses, 
 also, the warmth and cultural conditions are favorable for a very rapid 
 increase of any nematodes that may be present. All of the measures sug- 
 gested in this circular should be thoroughly understood; the remarks in 
 the section on "Precautions" (page 14) api)ly to everyone. It is so difficult 
 to eradicate nematodes from ground beds that every precaution must be 
 taken against introducing even a single larva. If there are any nematodes 
 in the vicinity, it may be advisable to dip the shoes in sheep dip before 
 walking over the plots. No suspected fertilizers should be used, and 
 transplanted seedlings, from whatever source, should be watched with 
 the greatest care. 
 
 For complete eradication of nematodes after they are once introduced, 
 more than one of the s])ecial measures may l)e needed. Cliemicals are 
 thought of first; steam or sun-drying may be even more effective. New 
 greenhouse beds should be equi]iped with a deep tile system for steam 
 treatments, which may be needed periodically. Clean fallow, even, is 
 recommended for infested nursery land, wasteful as it may seem. Its 
 value depends on the thoroughness of weeding and on the duration of the 
 treatment. 
 
 Benches should be raised completely oft' the ground, or else insulated 
 by concrete construction,^ with a 6-inch layer of cinders in the bottom. 
 If drain tiles are used, they will also serve for steam treatment in case of 
 a later infestation. 
 
 Drying is a convenient method of destroying nematodes on imple- 
 ments, frames, and benches. Flats and other containers should be dried 
 as a routine practice every time they are emptied, and should be refilled 
 only with uninfested soil. Boiling water in drenching quantities, or 
 strong chemical solutions, such as lye, sheep dip, or fresh hot whitewash, 
 may be used for various cleaning purposes. Sheep dip used in benches 
 might be toxic to plants. It should be applied as a spray, and later 
 washed off with the hose. 
 
 Suggestions for Truck Gardeners. — The truck gardener will find little 
 assistance in the suggestions for crop rotations, but his land may be 
 worth the care entailed in some of the other measures. If he has enough 
 land, he should divide it into three plots, on only one of which he grows 
 
 8 Steel forms and instructions for building concrete benches may be obtained 
 from the Advance Co., Kichmond, Indiana. Tlie cost of the benches, including 
 materials and labor, is estimated by this company at 50 cents per lineal foot. 
 
CiR. 330J The Root-Knot Nematode 31 
 
 a susceptible crop. Another plot should grow a resistant crop, and the 
 third should be treated for control, probably by fallow. The three prac- 
 tices should be rotated in each plot. All three plots should be weeded and 
 well cultivated. 
 
 As a resistant crop, either for rotation or to follow up some more direct 
 treatment, the varieties of sweet potato listed in table 2 may be used. 
 Hopi lima beans and plants of the onion group are usually fairly free 
 of nematodes. Corn is considered resistant, but may be rather heavily 
 attacked under unfavorable growth conditions. A good yield may be ex- 
 pected even with the doubtful plants listed in tables 3 and 4, if the soil 
 is not too heavilj^ infested. The increase of the nematode population on 
 these intermediate hosts may not be so great as it would be on the more 
 favored hosts. 
 
 In truck gardens particularly^, the soil should be broken whenever 
 there is no crop growing, as the numbers of nematodes are somewhat re- 
 duced by clean fallow in even a short time; while if any weeds are al- 
 lowed to grow, even along the edges of the field, they serve as breeding 
 places for more nematodes. 
 
 Partial Control in Orchards. — When an orchard is infested it is im- 
 possible to eradicate the nematodes, but it is most important to control 
 their multiplication. If only a few trees show infestation, they should 
 be removed together with the soil around them, and the adjacent area 
 should be subjected to drastic treatments, to reduce the danger of 
 spreading. 
 
 If nematodes are already established, the ground between the trees 
 must be given particular attention. Any susceptible plants growing there 
 will foster the nematodes. A covercrop may be used, but it should be 
 chosen for its high resistance. The varieties of cowpea listed in table 2 
 are suggested, but it is wise to rotate also with other resistant plants. 
 Indicator roots should be examined frequently, in order to keep track 
 of the extent of the infestation. Clean cultivation may sometimes be 
 necessary. Always, between crops, the deep soil should be turned u]) for a 
 thorough sun-drying, and prunings should be burned on the worst spots. 
 
 It is sometimes reported safe to use cyanide between the trees, but the 
 small doses which could be used would not give an adequate control. 
 Treating while the trees are dormant is supposed to reduce the danger 
 of injury, but the value of fumigations also is correspondingly less in 
 cold weather. It is possible that mulching would kill as many nematodes 
 as would any weak chemicals, and with less danger to the trees and soil. 
 
 When trees once show serious nematode injury, it may not be profit- 
 
32 University of California — Experiment Station 
 
 able to save them. If they are worth keeping, however, tliey should be 
 given as good a chance as possible to escape farther infestation. 
 
 Questions are often raised as to the advisability of setting out an or- 
 chard in infested land. Because of the enormous increase each yesiv in 
 numbers of nematodes and in injuries, it is very unwise to use any sus- 
 ceptible rootstock in land that is even lightly infested. Chemical or cul- 
 tural treatments cannot be depended on to eradicate the infestation, 
 even before planting. Of course, if the roots are well established before 
 nematodes become abundant, the tree has a fair chance of keeping ahead 
 of its parasites. 
 
 There are possibilities in the grafting of desired scions onto resistant 
 roots. Apricot roots are usually resistant to nematodes, and are used 
 in some sections for plum and even for peach tops, although grafts of 
 other species on apricot stock are not always congenial. Marianna plum 
 is used in the East instead of myrobalan, but is less satisfactory in Cali- 
 fornia. The California Agricultural Experiment Station is now work- 
 ing with peach seedlings of the Bokhara and Shalil varieties, which show 
 promise as resistant rootstocks. 
 
 SUMMARY 
 
 The root-knot nematode causes the formation of galls on the roots of 
 most wild and cultivated plants, interrupting the flow of sap so that the 
 plants grow pale, wilt, and sometimes die. It spreads most rapidly in 
 sandy soils, but may also become a pest in clay. It tolerates a wide range 
 of moisture conditions. It is carried by irrigation or drainage water from 
 infested land, and may be introduced in seed potatoes, or in transplanted 
 roots, or in soil adhering to implements. Precautions against introduc- 
 ing nematodes into clean land are urged. 
 
 Many individuals survive winter freezing, but all are killed at 0° 
 Fahrenheit. The larvae may live for a year or more in the soil, but they 
 feed and grow only in living plants. Development of all stages takes 
 place at temperatures between 50° and 90°. At the optimum tempera- 
 ture, which is around 81°, a larva may develop into an egg-laying female 
 in 16 days. One female may lay from 500 to 1,000 eggs, which can hatch 
 without being fertilized by a male. 
 
 Susceptible perennials, particularly orchard trees, should be started 
 only in nematode-free land. Susceptible annuals should not be planted 
 where an infestation is at all serious. 
 
 It is possible to get along with nematodes by raising resistant crops in 
 rotation, with careful control of weeds. One susceptible crop may be 
 grown every two or three years, and host plants may also be grown dur- 
 
CiR. 330] The Root-Knot Nematode 33 
 
 ing the winter. The infestation will remain in the soil indefinitely, 
 althongh starvation and other natural canses of death destroy many 
 individual nematodes when host plants are not available. 
 
 Control by starvation may be effected in the field by clean fallow, and 
 more rapidly by flooding. Trap crops may either reduce or increase an 
 infestation, according to field conditions. Dryness kills all stages of the 
 root-knot nematode, but incomplete desiccation may favor survival in 
 the field. 
 
 A temperature of 135° Fahrenheit kills all stages instantly, 120° kills 
 in 10 minutes, and 110° kills in 2 hours. A standardized hot-water treat- 
 ment for killing nematodes in roots and tubers may be expected soon, but 
 premature recommendations are not reliable. Either steaming or baking 
 ma3^ succeed in eradicating nematodes from greenhouse soil. Surface 
 fires and boiling water have a limited application for ground beds. 
 
 Chemicals are not recommended for general field treatments, since 
 they give only a temporary reduction, and never eradicate the pest. 
 Carbon disulfide and chlorpicrin, however, give promise of reasonably 
 effective control, and cyanide is sometimes helpful in sandy soils. 
 
 In the field, a combination of measures gives much better control of 
 nematodes than any one method alone. 
 
34 University op California — Experiment Station 
 
 LIST OF SELECTED REFERENCES FOR FURTHER READING 
 
 Bessey, E. a. 
 
 1911. Root-knot and its control. U. S. Dept. Agr. Bur. Plant Industry Bui. 
 217:1-89. 4 tables. 3 pis. 3 text figs. (Out of print.)* 
 
 Brown, H. I)., I. L. Baldwin, and S. D. Conner. 
 
 1922. Greenhouse soil sterilization. Indiana Agr. Exp. Sta. Bui. 266:1-27. 8 
 tables. 11 figs. 
 
 Childs, L. 
 
 1913. Root-knot — cause and control, including a list of susceptible host plants. 
 California State Comm. Horticulture Monthly Bui. 2(12) : 737-756. 8 figs. 
 (Out of print.)" 
 
 Godfrey, G. H. 
 
 1923. Root-knot: its cause and control. U. S. Dept. Agr. Farmers' Bui. 1345:1- 
 26. 26 figs. 
 
 Johnson, J. 
 
 1930. Steam sterilization of soil for tobacco and other crops. U. S. Dept. Agr. 
 Farmers' Bui. 1629:1-13. 7 figs. 
 
 MiLBBATH, D.G. 
 
 1929. Treatment of soil for prevention of plant diseases. California State Dept. 
 Agr. Monthly Bui. 18(1): 7-15. 
 
 Newhall, a. G., and C. Chupp. 
 
 1931. Soil treatments for the control of diseases in the greenhouse and the seed- 
 bed. New York Agr. Col. [Cornell] Ext. Bui. 217:1-59. 3 tables. 26 figs. 
 
 Sackett, W. G. 
 
 1927. Soil sterilization for seedbeds and greenhouses. Colorado Agr. Exp. Sta. 
 Bui. 321:1-24. 8 tables. 16 figs. 
 
 Watson, J. R. 
 
 1921. Control of root-knot, II. Florida Agr. Exp. Sta. Bui. 159:29-44. 1 fig. 
 
 ZiMMERLY, H. H., AND H. SpENCER,. 
 
 1923. Hot water treatment for nematode control. Virginia Truck Exp. Sta. Bui. 
 43:267-278. 1 table. 6 figs. 
 
 * Publications that are out of print may in many cases be found on file in libraries. 
 
 lOm-2,'34