UNIVERSITY OF CALIFORNIA ■ COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CIRCULAR 330 December, 1933 Revised July, 1944 UNIVERSITY O't /o n r AUG 4 ^4 v^j u LIBRARY THE ROOT-KNOT NEMATODE 1 JOCELYN TYLER 2 Root knot, a disease occurring on the roots of numerous plants of nearly all families, is caused by a microscopic nematode worm, Heterodera marioni (Cornu), 3 popularly called eelworm or garden nematode. Some other species of nematodes (eelworms) infect plants; among the more important are the sugar-beet nematode, Heterodera schachtii Schmidt, the meadow nematode, Pratylenchus pratensis (de Man), and the bulb, or stem, nematode, Ditylen- chus dipsaci (Kiihn).* Soil or water may contain many species of free-living nematodes, which are not injurious to plants. Still other nematodes are found as parasites in animals and in man. The root-knot nematode probably developed originally in the tropics. It nourishes under the conditions of field cultivation and has been spread to nearly every country in the world. 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. — Injuries caused by root knot are often insidious. Climate may be blamed for crop losses actually caused by this disease ; yield and quality often suifer more than the grower realizes ; in some regions the re- duction in yield is even considered normal. An infestation may thus become serious before its presence is suspected. In parts of California, root knot has become the limiting factor in raising many truck and field crops ; it may also seriously reduce orchard and vineyard yields. It increases the susceptibility of plants to other diseases, such as cotton wilt, tomato wilt, black shank of to- bacco, and rhizoctonia disease of peanuts. Heavily infected plants are stunted and off-color, and wilt readily. The roots are beaded with galls (figs. 1, 2, and 7) , and the energy of the plant is used up in producing new lateral rootlets, which are promptly attacked in their turn. The galls are rounded or elongated. Their size and shape vary with the host plant and also with the temperature. There may be little or no swelling on the 1 The discussion and lists of susceptible and resistant plants were checked and brought up to date by W. W. Mackie, Agronomist in the Experiment Station, retired July 24, 1943 ; and by Leonard H. Day, Associate Pomologist in the Experiment Station. 2 Research Assistant, Division of Entomology and Parasitology; resigned June 30, 1933. 3 Formerly known as Heterodera radicicola (Greeff). Caconema radicicola (Greeff ) is also a synonym. 4 Formerly known as Tylenchus dipsaci (Kiihn), and as Anguillulina dipsaci (Kiihn). [11 2 University of California — Experiment Station roots of such plants as strawberry, iris, f reesia, and cyclamen. On most plants, however, the galls are evident to the naked eye, and may be conspicuous be- cause of their size and abundance or because of the excessive growth of root branches sometimes associated with this infection. Galls are most often found on the tender feeding rootlets, though in older infections they may be large and woody, sometimes an inch or more in diameter. Various other kinds of galls and nodules, also found on roots, may be mis- taken for root knot. The beneficial nodules of the nitrogen-fixing bacteria occur normally on leguminous plants, but are only loosely attached at the Fig. 1. — Nematode galls on tobacco root. sides of the roots, whereas nematode galls involve all the root tissues and can- not be separated from them. Phylloxera galls occur on grape rootlets in heavy soils. Crown gall may start with small irregular nodules on roots or stems, though it characteristically causes large tumors on main roots. Clubroot, on plants of the cabbage family, causes large swellings, yellowish inside, with less distortion of vessels than in nematode galls. Woolly apple aphid sometimes produces a knotted condition on the roots of apple or other plants. Whether or not the nematode secretes a toxic substance, its presence is an irritation that stimulates abnormal growth of the plant cells and distorts 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 infected rootlets are unable to trans- port water and mineral nutrients effectively, and all vital functions of the plant are seriously affected. Root knot may also infect fleshy roots or tubers. In a potato (fig. 4) its [Cm. 330 The Root-Knot Nematode presence is recognized by a pimply surface and an irregular ring of small brown spots, less than % of an inch under the skin, each spot containing one or more female nematodes and their egg masses. Examination of Indicator Plants. — The easiest, surest way to detect the presence of nematodes in any soil is to examine the roots of susceptible plants that have been growing there for at least 3 weeks, while the weather is warm and the soil moist. The nematode population can be roughly estimated from the abundance and size of galls, from the diseased or healthy condition of the roots, and from the amount of top growth. Fig. 2. — Root knot on sugar beet. The inset is an enlargement of one small gall, showing the protruding egg mass. Roots of the weeds and crop plants listed in table 1 are the best indicators of field infestation. Sometimes it may be advisable to plant watermelon, tomato, or a susceptible variety of cowpea for examination. One should loosen the soil carefully before pulling the plant ; otherwise rootlets with galls may break off and remain buried. Roots should be taken from many parts of the field, and they must not be allowed to dry before examination. The final test is to find nematodes in the galls. The most conspicuous form is the adult female, de- scribed in the next section. BIOLOGICAL CHARACTERISTICS OF THE ROOT-KNOT NEMATODE Life History. — Small galls contain one or more nematodes; 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. Near 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 tan, reddish, or brown spot on the outside of the 4 University of California — Experiment Station gall (fig. 2, inset) . Individual eggs and young larvae (fig. 5, A) are the size of very small 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 may escape into the soil and travel in the film of water that surrounds the soil particles. Since they feed only in living plant tissues, they cannot grow or develop while free in the soil. They can, however, survive for months without food if they are not too active. Supposedly they have delicate sense organs and can detect from a considerable distance the presence of a host plant, to which they will travel even against the drainage current. Fig. 3.- — Old and decayed nematode galls on watermelon root. Whether or not this is true, many of them find and enter roots. Others never succeed in doing so ; but the eggs and larvae are so numerous that, despite the loss of many individuals, the soil population is always increasing when sus- ceptible roots are available. The larva usually attacks a rootlet near the growing tip. If many larvae are present, twenty or more may enter a root near the same point and penetrate more rapidly than if they were scattered. When once established in a root the larvae do not leave this location, but feed there from large nectarial cells formed in the galls. The females become enormously swollen and filled with fat globules. A yellowish jellylike secretion is extruded, and the small white eggs are deposited into it. One female may lay 500 to 3,000 eggs, and fertilization by a male is not necessary for their development and hatching. The jellylike covering darkens, especially if exposed outside the root. After the larvae hatch, they leave their original position in the gall and migrate either within the root or through the soil in search of fresh rootlets. [Cm. 330 The Root-Knot Nematode Contrary to statements sometimes found in popular bulletins, the root-knot nematode has no true cyst stage, like that of the sugar-beet nematode; but dormancy may occur. The gall tissue, the secretion around the eggs, and the eggshell all protect the embryo somewhat ; but this is not like a cyst, which has a definite structure and is usually resistant to drying as well as to other adverse conditions. Most of the nematodes developing in vigorous roots are females, which re- quire ample nourishment for producing eggs. The males (fig. 5, E) require less food and develop more rapidly. After growth and metamorphosis, a male may emerge again from the gall as a free-traveling worm, more than three Fig. 4. — Potatoes infected by the root-knot nematode. times as long as the larva. Sometimes he finds a female ; but, as mentioned, mat- ing is not always necessary for the reproduction of this species. The males are short-lived, and few are found in the field unless they are especially sought, with a knowledge of their life history. Influence of Temperature on Nematodes. — Development of this nematode has no seasonal limits in a moderate climate, but progresses slowly or rapidly at any temperature between 50° and 90° F. All activities are much retarded by cold. Most rapid development occurs around 81° F. At this temperature a nema- tode may grow from a free-living larva to a mature egg-laying female in a minimum of 16 days. A week or 10 days more is necessary for development and hatching of the eggs. At lower temperatures development is progressively slower until at 58° the same amount of growth may require 80 days instead of 16, plus 5 or 6 weeks for hatching of eggs. 6 University of California — Experiment Station The rate of development is important in connection with nematode control in the field by means of fallow, where conditions that permit egg formation must not occur. The roots of some plants will remain alive for several days even after the tops are killed, and nematodes may continue to develop for a few days after that. Thus in hot weather some females may take enough nour- ishment from plants growing only 10 or 12 days to enable them to mature and to form viable eggs. Weeds should therefore be destroyed within 10 days after 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, mature male ; F, gall from tobacco root, partially dissected : a, egg mass ; c, female. they show above the ground in midsummer, but may be left in the field for a longer time in cool weather. In cold weather, fewer nematodes are able to reach and penetrate the roots, and susceptible plants may thus escape injury even in infested ground. Such crops as cabbage, carrots, celery, lettuce, potatoes, radish, spinach, and vetch, which can be grown at soil temperatures around 55° F, will not suffer, al- though 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 infection when nematodes become active with the advancing season. In the southern states, nematodes multiply at the rate of five to ten genera- tions a year and are also very active in finding and attacking plants. In more northerly states they may be a less important outdoor pest, because of the [Cm. 330] The Root-Knot Nematode 7 short growing season. However, as soil populations become established, such infestations may eventually become serious. On the other hand, in regions where intense cold penetrates deep in the soil, there may be little more than a single season's increase in nematode population to deal with. Repeated freez- ing at 32° F does not kill eggs or larvae, but -4° kills all stages in 2 hours. Doubtless many individuals are killed by somewhat less severe cold, if ex- posure is prolonged. Influence of Moisture on Nematodes. — The root-knot nematode can tolerate almost any condition of moisture. Although it is killed by complete drying, it may live in a soil that is apparently dry if, as commonly happens, the soil atmosphere below 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, and plants may be seriously attacked by nematodes after a heavy irrigation. Water cannot be withheld entirely, however, because infected plants may die during a dry spell that would not kill healthier ones. Soils Favorable to Nematodes. — Porous sandy or loam soils especially favor the increase and spread of the root-knot nematode, whereas infestations in heavy soils are usually less serious. In a heavy soil it is 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, however, once a root has been reached ; and as a population becomes estab- lished 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. — In most infestations, nematodes are abundant as deep as host roots grow and as deep as plowing may have turned under and mixed any infested soil. Usually their greatest numbers are in the root zone 3 to 10 inches below the surface. Their depth distribution, however, varies widely with conditions; it is of course limited by hardpan or a high water table. In sandy soil, galls have been found on peach and fig roots 6 and 8 feet deep. Sometimes a root will escape heavy infection if it is set deep; but nematodes will eventually find and attack it and become established that much deeper. Conditions of Survival in the Soil. — With even the lightest and most recent infestation, there are always larvae in the soil — usually in a wider area than is supposed. They are well adapted to the conditions of their life there, for they have nourishment stored from the eggs, in the form of granular material and fat. Some members of a population have survived a fallow of more than 15 months ; a part of that time may have been passed in the egg stage. In con- trolled experiments, however, many individuals have survived as larvae for at least 40 weeks in a completely 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 another 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 8 University of California — Experiment Station energy and live over, in a more or less quiescent state, until plants are again available. Under natural conditions they survive in either case. If, however, they are very active and find no host root available — an artificial situation — they will ultimately use up their reserve food and die of exhaustion and starva- tion. The eggs also may remain dormant until warmth and moisture stimulate them to hatch. Dormant eggs are found in old galls and in potato tubers, and TABLE 1 Most Important Hosts of the Boot- Knot Nematode truck crops Bean* Beet Cabbage Carrot Cucumber Dasheen Eggplant Endive Lentil Lettuce Melons Okra Parsley- Pea Pepper Potato, Irish Pumpkin Squash Sweet potato* Tomato FIELD AND FORAGE CROPS Alfalfa Bean Clover Cotton* Cowpea* Soybean* Sugar beet Sunflower Tobacco Vetch FRUIT AND NUT CROPS SHADE TREES AND ORNAMENTALS Fig Begonia Grape* Buddleia Olive Calendula Papaya Candytuft Peach* Carnation Pecan Catalpa Pineapple Chrysanthemum Plum* Cineraria Strawberry Clematis Walnut : black and English Coleus Cyclamen SPECIAL CROPS Dahlia Coffee Delphinium Ginseng Echeveria Peppermint Elm Sugar cane Euphorbia Gardenia florida WEEDS Gerbera Amaranth, or Gourd pigweed Hibiscus Dandelion Hollyhock Fennel Lobelia Fenugreek Mulberry Groundsel Pansy Jimsonweed Peony Lamb's-quarters Petunia Morning-glory Rose* Mustard Snapdragon Nightshade Tuberose Purslane Violet Smartweed Willow, weeping Sow thistle Zinnia * Eesistant varieties of this highly susceptible plant have been developed. See tables 2 and 3. it may be also that under some conditions they survive 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 1% feet a day, in a light soil with favorable temperature and moisture. Evidence on this point is scarce and conflicting. How far any indi- vidual would travel in a straight line away from the infested area is uncertain. If it finds and enters a root, the next generation of nematodes may continue the migration; but this makes for a very slow spread. [Cm. 330] The Root-Knot Nematode 9 Nematodes are carried greater distances by surface drainage from infested areas, and are often worst in low spots where the water settles. Once intro- duced into a field, even in a small area, they are carried farther with each tillage. They may even be scattered by men or animals that walk over muddy or plowed land. Considering the possibilities of multiplication, one must take every precaution against spreading an infestation. HOST PLANTS More than 1,400 plants have been reported as hosts of the root-knot nema- tode. Not all suffer equally. A few even react against the invaders and may eventually kill them, but unfortunately not before the nematodes have laid eggs and the plant itself has suffered considerable injury. Almost all 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. Except during cool seasons, few of them will yield at all profitably; but they will greatly increase the nematode population. A few, notably mulberry, seem to thrive despite heavy infection. Alfalfa, sunflower, and zinnia may also be found more or less tolerant. Tomato may sometimes appear so ; but considerable decrease in yield has been found in field experi- ments, and wilt resistance is lowered by root-knot infection. There are many host plants not listed here, including most of the members of the cabbage, gourd (cucurbit), nightshade, parsley, pea (legume), and sunflower (com- posite) families. Perennial host plants are constantly subject to reattack by larvae that have been bred in their roots. Even a minor infection is bound to increase each year. Complete destruction of a tree or vine sometimes occurs suddenly. Weeds as host plants are also to be reckoned with. Even when they do not show serious symptoms of disease, nematodes feed and multiply on them, and thus the soil is kept populated with infective larvae. PLANTS RESISTANT TO ROOT KNOT Grains and grasses, with a few exceptions, are the group of plants most resistant to root knot. A list of nonhosts is given in table 2. This list has been revised many times, because certain plants, usually considered immune, have sometimes been found more or less seriously infected. For this reason, lists of resistant plants can have no absolute authority. The breeding of resistant plants is one of the most important ways of attacking the nematode problem. It is a long and difficult undertaking, how- ever, and the best strains now becoming available have required many years of breeding and testing for their development. Resistance to nematodes is sometimes related to the vigor of growth. If a highly resistant plant is grow- ing under adverse conditions of soil or climate, it may be heavily attacked by nematodes, or may show serious injury from a relatively light infection. The root-knot nematode is much less specialized in its selection of hosts than are other species of plant nematodes. Observations suggest, however, that if there is a choice of hosts, a population in a given field tends to prefer the kinds of roots grown most frequently or most recently on that land. It is therefore 1 University of California — Experiment Station TABLE 2 Plants Reported Resistant to the Root-Knot Nematode field and forage crops Cowpea: Blackeye nos. 1, 5, 7, and 7711 (hybrids* with Iron) ; Iron, pure strain Crotalaria retusa; C. spectdbilis Grasses: para grass (Panicum purpurascens) , redtop grass, rescue grass or Schrader's bromegrass, perennial ryegrass, teosinte, timothy, and many other kinds are highly resistant; meadow fescue, orchard grass (Dactylis glomerata), sheep fescue, and tall oatgrass are resistant but sometimes harbor nematodes Guar (Cyamopsis tetragonoldba) Millet : all kinds except those listed in table 3 Oats Pigeonpea : California Station strains Rye Sorghums Velvet beans : especially the Bush and Florida varieties Vigna marina TRUCK CROPS Sweet potato : Big Stem Jersey, Gold Skin, Old Long Red, Red Jersey, Yellow Jersey, and Yellow Jersey Vineless varieties FRUIT AND NUT CROPS Apricot Avocado Cherimoya Cherry: morello and other sour cherries; mazzard Citrusf Currant Feijoa Gooseberry Grape : Dog Ridge and Salt Creek varieties Jujube Nectarine : Quetta and Traveller varieties usually resistant Peach roots: Bokhara, Shalil, and Yunnan selections; P.I. No. 61302 (Bolivian Cling peach x Quetta nectarine) ; P.I. No. 41395, dwarf peach; selected "ornamental" or flowering peaches. Resistance usual in the varieties listed Pistache Plum : chickasaw, hortulan, wildgoose ; selected strains of damson, myrobalan, and a few hybrids ; Marianna,$ and some native varieties in Florida and Georgia Quince : California Station selections ORNAMENTALS Amaryllis Azalea Camellia Day-lily Dusty miller (silver cineraria) Ferns Lantana Marigold, African Narcissus Rhododendron Tulip * These varieties were bred by W. W. Mackie, Agronomist in the Experiment Station, Emeritus. t Citrus roots are attacked by the root-knot nematode in very small numbers if at all. They are more heavily parasitized by the citrus-root nematode, Tylenchulus semi-penetrans Cobb. t Vigorous seedlings of Marianna plum have been selected by the California Station. These propagate readily by cuttings. [Cm. 330] The Root-Knot Nematode 11 TABLE 3 Plants Reported Lightly Infected with Root Knot or Tolerant of Heavy Infection truck crops Artichoke, globe Bean, pole : Alabama Station selections Corn, sweet* Lima bean: Hopi nos. 12, 13, 14, 15, 2000, and 5989; Rico, Westan (baby limas) ; lea (a large lima)f Onion, but seedlings may be injured Sweet potato: Creola, Dixie Yam, Enormous, Golden Porto Rico, Japan Brown, New Gem, and Porto Rico* varieties are usually only lightly infected Tomato : Lycopersicon peruvianum, a possible breeding stock Turnip* field and forage crops Alfalfa Barley* Beggarweed, Florida Berseem, or Egyptian clover Buckwheat Carob, or St. John's-bread Chufa {Cypvrus esculentus) Corn* Cotton : most upland varieties, except in dry soils Cowpea : Brabham, Conch selections, Iron hybrids, Monetta, and Victor are less suscep- tible than other varieties, bat are sometimes severely infected Crotalaria juncea (sunn-hemp), C. striata, and other species Grasses are still considered more or less resistant; but light infections have been reported on Bermuda grass, bluegrass, carpet grass, goosegrass, Napier grass, Natal grass, Rhodes grass, Sudan grass, and Vasey grass, and some heavier infections on bull grass, crabgrass, and wild oat. In the San Joaquin Valley all grasses have been found too heavily infected for use in control rotations Lespedeza* Millet: light infections have been reported on barnyard millet (EchinocMoa crus-galli), foxtail millet (Setaria italica), and ragi millet (Eleusine coracana) Peanut: root knot is occasionally serious, and it increases the damage by rhizoctonia; peanut crops should be harvested Rice* Sesbania Soybean : Laredo variety* Sugar cane: Cayana and Japanese varieties; hybrids P.O.J. 213, Co. 290, and C. P. 29/116 Sweet clover Wheat fruit and nut crops Almond : California Station selection Apple Blackberry : Hall's Lawton and other early varieties Grape: Barnes variety of Vitis Champinii; hybrids No. 1613 (Solonis x Othello) and No. 1616 (Solonis x V. vulpina) X * Somewhat doubtful : infection may be serious in some cases. f These varieties were bred by W. W. Mackie, Agronomist in the Experiment Station, Emeritus. No. 2000, somewhat less resistant than other selections, is excellent in home gardens but is not a commercial variety. t The commonly grown phylloxera-resistant variety, Vitis 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 con geniality of stock and scion. 12 University of California — Experiment Station Gardenia Thuribergia rootstock Geranium Honeysuckle Mesembryanthemum Michaelmas daisy Oak Opuntia Quihou privet, Ligustrum Quihoui Rose : var. Mme. Plantier ; roots of hybrid Rosa multifloraxB. blanda Eudbeckia TABLE 3— (Continued) fruit and nut crops — continued Loquat Mulberry Pear : Beurre Hardy, Easter Beurre, and P. Barry varieties Persimmon, especially on native roots Pomegranate SHADE TREES AND ORNAMENTALS Agave Aloe Alyssum Aster Calliopsis* Cereus Chloris petraea Conifers Cosmos* Evening primrose Four-o'clock Gaillardia * Somewhat doubtful : infection may be serious in some cases. advisable not to plant the same crop two successive years in any infested field, even a crop that is considered resistant. Perhaps no plant is entirely immune. A few may be less attractive to the nematodes or less easily located, and certain others may have thicker-walled roots, 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 and may even develop a population especially adapted to the new host. The plants listed in table 3 may be grown in infested soil, but never during a nematode-starving rotation. On most of them the infection may be relatively light ; but others, especially mulberry, become heavily infected and stand up only because of their vigorous growth. Alfalfa, upland cotton, and the succu- lents listed are tolerant of infection rather than actually resistant. INTRODUCTION OF NEMATODES Means of Introduction. — Human agencies are most often responsible for introducing nematodes into clean fields. The following list indicates some common sources of infestation : Infected 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 into which drainage may have brought eggs or larvae Manure containing fresh plant debris that might carry nematodes Bean straw from infested fields Implements caked with mud from infested soil Culls or peelings of infected potatoes or other vegetables, fed to hogs or chickens or composted too short a time Root knot is not brought into clean soil by green or farm manure except from infested land, or by dry seed. Growers have sometimes supposed that an infestation was started by some particular crop. If, however, the precau- [Oir. 330] The Root-Knot Nematode 13 tions discussed in the next section are observed, there is little danger of intro- ducing nematodes by growing any new crop on clean land. Of course, if there is alreadjr a small and unobserved infestation in a field, the cultivation of susceptible plants will reveal its presence, and also provide food for the multi- plication of the nematodes. Precautions against Introducing Nematodes. — In dealing with root knot, prevention is the measure most to be urged but least often considered. Fre- quently growers are unaware of an infestation until it has reached the destruc- tive stage. One must guard constantly against the introduction of such a persistent pest into clean or treated land. 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 a long-season crop, and each female may lay more hundreds of eggs. The first nematode must not be introduced, for in one season it may populate the soil with several million larvae. The following precautions may be taken by anyone who is anxious to keep his land free of nematodes : Seed potatoes should not be planted unless they are certainly not infected. All seedlings to be set out into clean land should be carefully inspected, because hotbeds are often infested with nematodes, which are carried out into the field with the young plants. Nurseries and other sources of transplanted roots should be checked. Even if there are no conspicuous galls, as on a resist- ant root, there may be nematodes in the adhering soil ; or originally clean stock may have become recently contaminated by heeling-in or by other handling. Suspected roots may be set out in quarantine boxes for a month, to avoid the risk of contaminating a field. They should be examined again before being transplanted, and indicator roots growing in the same box should also be exam- ined. 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 by drainage water or by shovels, wheelbarrows, or trucks previously used with infested soil. Irrigation water coming from fields that might be infested should be viewed with suspicion. Drainage water that might carry nematodes should be diverted by ditches. Bean straw from infested or suspected fields should be thoroughly dried before being spread on clean land. Field implements should be washed or scraped and thoroughly dried each time they are used. The cleaning of implements that may be taken from the greenhouse to the field is exceedingly important also. Potatoes to be fed to the hogs should be boiled a few minutes if there is any possibility of their carrying nematodes. Uncooked garbage should not be thrown out or buried in the garden without some adequate treatment. Nematodes can rarely be eradicated from contaminated soil or roots. Most of the treatments that 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. 14 University of California — Experiment Station NEMATODE CONTROL BY CULTURAL METHODS To combat any organism, one must understand its life history and habits. The section on "Biological Characteristics of the Root-Knot Nematode" is of more than theoretical interest. Familiarity with this material is an important preliminary to an intelligent reading and application of the sections that follow. The control measures of rotation, fallow, and flooding are all based on starv- ing the larvae in the soil. The problem is not only to limit or destroy available host plants, but simultaneously to provide conditions most favorable for keep- ing the larvae active, in order to hasten their exhaustion. It is still more important to encourage the development and hatching of the eggs, which would otherwise provide a dormant reserve population. Fallow and flooding are thus of relatively little value in winter, when low temperatures restrict nematode activity. The effectiveness of fallow is reduced also when the ground is dry and caked. Crop Rotation with Resistant Plants. — Rotation with resistant plants is the chief recommendation for field control of root knot. It has certain advantages : the land remains productive ; and cultivation provides the best conditions for hatching of eggs and activity of larvae, while their food plants are limited so that most of them must starve. As already stated, however, almost any crop may harbor a few nematodes and allow some reproduction. If a rotation program is carried out faithfully one susceptible crop can usually be harvested every two or three years, though of course some infesta- tion will remain in the soil. It may be feasible to rotate resistant crops in summer with whatever susceptible crops can be grown in winter. Warning must be given, however, that susceptible winter crops will carry over some nematodes, especially if their growth period includes warm days in fall and spring. To reduce an infestation, crops for rotation should be selected from table 2 only. An alternation of grains and resistant legumes is recommended 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 smother crop, such as Iron cowpea or a good stand of mixed grasses, requires less care and may be almost as effective. Fallow. — The value of fallow has been demonstrated by the Oregon Agricul- tural Experiment Station on a test farm in the Klamath Valley, where the owner now voluntarily continues to fallow several fields each summer. A rotary rod weeder is used for cultivation. Winter grains are grown for green- manure before and after the fallow. A splendid crop of potatoes is grown the following summer. Root knot is no longer a problem on this farm. The one rule for controlling nematodes by fallow is to leave no root of crop plant or weed growing in the soil long enough for the worms to form eggs. This is exceedingly important, because the new eggs and larvae propagated in these roots have a fresh reserve of energy. In hot weather, when nematodes mature most rapidly, weeds must be destroyed within 10 days after they appear; but the interval may be gradually lengthened in colder weather. Each cultivation pays for itself by shortening the time required for control. [Cm. 330] The Root-Knot Nematode 15 A different measure, described later, makes use of weeds to remove moisture from the deeper layers of the soil. The two methods should not be confused ; the value and applicability of each should be considered in relation to local conditions. A moist clean fallow probably exhausts the strength of the nema- todes more rapidly than a dry fallow, whereas complete drying, if it is prac- ticable, kills them directly. The conditions of fallow are found in the chicken yard and in the pig pen, unless uncooked infected 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 of great benefit, for the root-knot nematode can multiply only in living plants; as soon as such reproduction ceases, its numbers are reduced by enemies, exhaustion, and various adverse conditions. It has been stated that nematodes can even be eradicated by two or three years of strict starvation. A less extreme fallow, however, will not ordinarily accom- plish eradication. Part of the population may survive under the usual con- ditions of fallow, which allow the larvae to remain inactive and to conserve their energy for a long time. Eggs not killed by drying probably remain more or less dormant also. 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 irrigate and cultivate the fallow fields, and to con- tinue the attempted starvation program by planting the most resistant crop available. Flooding. — Flooding helps to hasten the starvation process during fallow. The theory is that weeds will be killed under water, old galls decay, and eggs be stimulated to hatch. The increased activity of larvae in water will especially hasten their exhaustion. In field experiments in California, flooding for six months killed most of the nematodes ; but there were still a few survivors after a full year under water. Where water is available, submerging the land for even a month will be of benefit if the weather is warm, provided flooding is followed 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. Drying. — All stages of the root-knot nematode are killed immediately by thorough air-drying. Small amounts of soil can be spread out in a thin layer in a warm greenhouse and dried in a few hours ; but a couple of weeks, with the most careful attention, is required to desiccate the galls, which are never all removed from any soil. The danger of attempting this control method in the field is that the survival of eggs and larvae is actually prolonged in a soil which is fairly but not completely desiccated. Sun-drying, however, is effective where practicable. In the hot valleys, a dry fallow during the summer will greatly reduce the nematode population of a field. Grains or weeds use up much of the moisture from the deeper soil, and thus considerably hasten the drying; but even after all plants 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, one must further the drying process by turning up the deeper soil. When weeds are not allowed to grow, frequent plowing is advisable, 16 University of California — Experiment Station especially before and during a hot dry spell, to expose to sun and wind the deeper layers of soil where most of the nematodes are protected. More soil will be exposed by plowing across the previous furrows. Of course this is not suit- able unless the infestation is general and there is no danger of spreading nematodes to clean land. In many parts of California where nematodes injure crops, especially in bean areas, an early, thickly sown crop of barley, harvested early as either hay or grain, reduces the population. If the field is promptly plowed and kept free of weeds during the summer following, two or three years may be required to build up the nematode population sufficiently to injure bean crops. Dry fallow is probably less harmful to the soil than has generally been believed. Indeed, a gradual and complete drying may increase the fertility of land by partial sterilization, on the same principle as that explained later under "Partial Sterilization of Soil by Heat." 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 preventing migra- tion of nematodes from an infested to a clean field. Of course it will not prevent soil from being carried across the road on implements or on shoes. Mulching. — It has been 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. Possibly, however, the gases formed by decomposition may be fatal to nematodes as they are to many insects in the soil. Decay. — A heavy stand of a covercrop, or green-manure, may be plowed under and allowed to decay under conditions of warmth and moisture. In experimental tests in Utah this measure destroyed 5 to 22 per cent of sugar- beet-nematode infestations. This partial control may have been caused by decomposition products, to which some nematodes succumb readily ; or it may have been caused by the increase of nematode enemies. (See "Biological Con- trol.") CONTROL MEASURES THAT CANNOT BE RECOMMENDED WITHOUT WARNINGS Hot-Water Treatments for Infected Roots. — Egg masses of the root-knot nematode have been killed in 10 minutes at a temperature of 118° F. Some living, dormant roots will tolerate this degree of heat — or more — in a water bath ; but a treatment of at least 30 minutes must be given to planting material in order to penetrate the gall tissue and make certain that every nematode egg receives an adequate treatment. The time and temperature of exposure vary somewhat according to the thickness and tolerance of different roots; one must balance them with the greatest precision in order to kill all nematodes without injuring the plants. It is better to kill a few plants, however, than to introduce root knot into clean land ; unless an exact recommendation is known for jthe particular plants to be treated, the exposure should be higher than the minimum given above. Roots should not be too tightly bunched. The volume of water in the tank should be at least five times that of the roots treated. Its temperature may be a little above 118° before the roots are immersed, and should again rise to 118° before the timing of the treatment is begun. The [Cm. 330] The Root-Knot Nematode 17 water should be stirred constantly, and its temperature controlled by an accurate thermostat. For small home treatments the tank may be stirred and regulated by hand ; but this work takes practice and careful watching. Trap Crops. — The trap-crop method for control of nematodes is not recom- mended, though in theory it gives much promise. If some very susceptible and rapidly growing plant, such as a susceptible cowpea, squash, or sunflower, is sown thickly in a field and later plowed under before any nematodes can repro- duce, the half-grown worms in the roots will die. If this procedure is repeated two or three times in succession, many larvae will be removed from the soil . In practice, however, such measures are dangerous. 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 the time required to starve out the nematodes. It therefore becomes necessary to de- stroy the trap plants before any extensive root growth has taken place; and thus the trapping value of the roots becomes too limited for best results. The formation of eggs in trap-crop roots can be prevented by plowing under the entire crop 10 days after the sprouts appear. This time limit need not be observed strictly except in the hottest summer weather; in a cooler season 3 weeks or longer may be safe. The trap-crop method is not used in this country, because of its dangers and the care and expense involved, though with proper supervision it could doubtless sometimes be applied advantageously. Overfertilizing. — The value of overf ertilizing is still in dispute. Using excess fertilizers to force a crop in the hope that the plants may outgrow the nema- todes is not good practice. Sometimes this treatment will increase the yield of one crop or perhaps postpone complete failure ; it has been recommended for this immediate advantage. The use of organic manures may increase nematode enemies, as explained under "Biological Control." Overfertilizing with inor- ganic materials, however, usually ends by feeding the parasites on more succu- lent roots, so that they are well nourished and reproduce in greater numbers. Of course a normal amount of fertilizer is necessary, since infected plants suffer more in poor than in fertile soils. A potassium fertilizer has been claimed helpful for protecting plants from nematode injury. Various explanations have been given. The treatment prob- ably has little value except for soils deficient in potassium. Biological Control. — Natural enemies of soil nematodes — plant parasites and free-living nematodes — include fungi, predaceous nematodes (species that capture and feed on living animals) , mites, and ants. They probably serve to check an otherwise excessive multiplication of nematodes. In Hawaii some soil populations of the root-knot nematode have been appreciably reduced by plowing under huge quantities of fresh plant material. According to detailed studies there, free-living nematodes in the soil multiply rapidly in response to this rich food supply. Fungi and other enemies of nematodes then increase greatly and destroy some plant-parasitic nematodes, which have not multi- plied during this treatment, along with many free-living nematodes. Though predaceous nematodes have also been suggested as a possible aid in controlling root knot, their value is doubtful. They do destroy some plant-parasitic nema- todes ; but the latter are not necessarily their main diet under natural condi- tions. It would, furthermore, be far from easy to transfer and establish a 18 University of California — Experiment Station population of predaceous nematodes where needed. They have strict require- ments for moisture and other soil conditions, and are also subject to parasitic SPECIAL SOIL TREATMENTS Partial Sterilization of Soil oy Heat. — The use of heat in any form, wher- ever feasible, is the first recommendation for soil treatment. A temperature as low as 110° F will 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 140° will kill all stages instantly. Heat, if high enough, also destroys weed seeds, insects, and the bacteria and fungi that cause plant diseases, more gen- erally than does any other single method of soil sterilization. A crop in heated soil may show a slight setback at first, but 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 buried-tile method is considered more thorough than the inverted-pan method. Since the soil should be well spaded before any heat treatment, the labor of laying deep tiles serves a double purpose. In greenhouses the installa- tion can be made permanent. Drenching with boiling water condensed from steam pipes is used to advantage for the ground beds in some greenhouses, though probably not 100 per cent effective. It may also be used for seedbeds or flower borders. Unless the drainage is excellent, puddling is apt to occur, and may prevent penetration 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 greenhouse use. The pipes must be close enough together to insure an even distribution of heat. Small volumes of soil can be baked in an oven. For best results the soil should be neither too wet nor too dry before baking. The application of heat in the field may become practical as machines are developed for the purpose. Meanwhile, if brush is to be burned, this should be done over spots where nematodes are abundant. Not many nematodes will be reached unless the soil is plowed before burning, because they are not at the surface, and heat penetrates only a few inches in soil. Any type of reflector oven that can be devised will increase the efficiency of heating. In Holland an oil-burning blowtorch is used for killing the bulb nematode in soil. There is a general prejudice against heating soil, the argument being that it "burns out the humus." This would be true if a very high temperature were reached, as in oven-baking. Possible injury to the soil by any treatment consid- ered must of course be weighed against the effectiveness of the treatment and the necessity for nematode control. The fertility of a soil may be increased, however, by heating it to 140° or 150° F, whereas the root-knot nematode is killed between 110° and 140°. Steaming is perhaps less destructive of the nor- mal soil properties than is dry heat ; yet even oven-baking and surface fires have been recommended by authorities, after field and laboratory experi- mentation, for stimulating crop growth quite apart from nematode control. The most conspicuous result of heating a soil is a change in the relative proportions of the soil microorganisms, which then cause by their activities an [Cm. 330] The Root-Knot Nematode 19 immediate increase of ammonia and an ultimate increase of nitrates. In other words, the bacteria remaining 1 after partial sterilization are able to make the organic matter of the soil more available for crop growth. The quantity of nitrates thus released depends of course on the amount of organic matter pres- ent. 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; and very heavy soils may profit by being made more friable. Thorough air- drying has practically the same effect. Heat treatment should be thorough enough to penetrate all parts of the soil with killing temperatures; but it should not be carried beyond this point, because many injuries caused by heating are due to unnecessarily prolonged high temperatures. Chemical Treatments of Soil. — Chemical treatments are reasonably effective in very sandy soils, but are relatively ineffective in clay or organic soils. No chemical has ever completely eradicated nematodes in the field, because it can- not reach them all. The few eggs or larvae that survive soon start a new popula- tion, which increases rapidly because the chemical has destroyed many enemies of the nematodes. Since the expense of soil treatments is seldom warranted for the temporary control obtained, their use is not recommended except in very special cases. In greenhouses, however, a minimum dosage is sometimes ap- plied, to be repeated for each crop. Again, in treating small and valuable beds, one can mix the chemicals thoroughly into all parts of the soil to a considerable depth ; and heavy dosages will not add greatly to the expense. The treatment chosen should suit the particular conditions. There are three reasons for the failure of chemicals in the field : (1) Chemi- cals 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 or escape into the air. (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 adsorp- tion on the surface of the minute colloidal particles. (3) Many of the chemicals that will kill nematode larvae in soil are without effect on the egg masses, which are protected by a rather impervious, jellylike material in addition to the gall tissue. Though some chemicals are advertised for their fertilizing as well as their fumigating value, the particular elements supplied may not happen to be those needed. Carbon disulfide and chlorpicrin, the most volatile of the fumi- gants, may increase the fertility of a soil by partial sterilization, as do heating and drying. On the other hand, chemical residues, for example those left by cyanide, may actually decrease fertility by inhibiting the essential bacterial activities of decomposition and nitrification. The most promising chemicals for nematode control seem at present to be the volatile organic fumigants. Carbon disulfide and chlorpicrin have been used for years, with fairly good results. Liquid carbon disulfide is highly explosive, but is considered effective against nematodes in the field. It also kills 20 University of California — Experiment Station weeds, and must not be used near crop plants. Carbon disulfide emulsion 5 is less dangerous to handle but it is much less effective than undiluted carbon disulfide. A grower may mix his own emulsion by the following formula : fi Volume, in Weight, in Material gallons pounds Carbon disulfide 68 680 Rosin fish-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, during stirring. This gives a 68 per cent, or concentrated, emulsion, which should be diluted for use, usually 1 : 50 in water. Homemade emulsions, though much less expensive than those commercially prepared, 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 7 has given as high as 90 per cent control of root knot in loam soil, in experiments in Hawaii. Good results have been obtained with it in the southern states also, and in greenhouses, but only in treating warm soils. It cannot, however, eradicate root knot, because it does not penetrate the jelly- like covering around the egg masses ; all infected roots should therefore be allowed to decay for several weeks in a warm, moist soil before any treatment with this material. Chlorpicrin is a tear gas and must be handled with caution. A gas mask should be worn during any work with this or other volatile fumigants, espe- cially indoors, because a person may be injured by the cumulative effects of small doses. Larger closes cause nausea, and skin burns may be serious. The fumes will kill plants in a greenhouse. Partial sterilization of almost any soil is followed by enormous improve- ment in plant growth. Chlorpicrin and other volatile fumigants produce this effect, but it is only incidentally due to the freedom of the plants from soil- borne diseases. Used in the hills before planting, a very small dose of chlor- picrin may greatly increase the yield of such crops as tomato and watermelon. There is a considerable interest at present in the use of other volatile organic chemicals as soil fumigants. Methyl bromide appears very effective, but it is difficult and dangerous to apply. Ethylene dichloride and propylene dichloride are used in various combinations, with chlorpicrin and other fumi- gants. A mixture of propylene dichloride with dichlorpropane, called "D-D," has given promising results in Hawaii and is being tested in several states. The treatment seems inexpensive mainly because the quantities used are sufficient for only a single season's control, and soil between rows is not treated at all. Additional materials in this group are being developed, and tractor- drawn equipment is being devised for their uniform application. 5 In California, carbon disulfide emulsion can be obtained from the Wheeler, Eeynolds, and Stauff er Chemical Co., San Francisco. 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. 1932. 7 Chlorpicrin is manufactured by Innis, Speiden and Co., 117 Liberty Street, New York, N. Y. A valveless type of applicator, for measuring doses and drilling the liquid into the soil, has been developed by this company. [CiR.330] The Root-Knot Nematode 21 Sodium nitrite has been claimed effective, from recent greenhouse tests. The nitrite changes to nitrate, but sodium may injure both plants and soil. Various other treatments have given promising results under optimum con- ditions, but have usually proved less successful in practice. Since all chemicals show this inconsistency of action in different soils and under different condi- tions, no standardization of their effectiveness is possible. Table 4 summarizes the more important chemicals 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 that could be fumigated in airtight compartments. The impossibility of adequate chemical treatment of soil, as explained above, is the reason for being skeptical of all panaceas unless they are well demon- strated. Points 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 condi- tions, though there are always larvae in the soil and egg masses in buried fragments of old roots. The large roots and as many galls as possible should be removed from the soil before fumigation, because egg masses contained in galls are extremely difficult to reach and kill with any treatment. In the field, where removal is not practical, decay of the old galls during 2 or 3 weeks of fallow may be fairly effective in releasing nematodes if the soil is warm and moist. For most treatments the soil should be reasonably dry, though some chemi- cals need to be dissolved and washed into the soil. Cyanide gas is soluble in water; but neither carbon disulfide nor chlorpicrin, unless emulsified, will penetrate wet soils. For effective fumigation the soil surface should be immediately packed, sprinkled, or covered with paper or wet canvas, to minimize the escape of gases and to prolong their action. Wetting the surface of the soil after applying chemicals will hold gases even better than a canvas or ordinary paper cover, but it helps protect nematodes in the wet surface soil from the chemical. This method holds in the gases so well that it must not be tried in orchard fumiga- tions, for an otherwise harmless treatment may thus become fatal to the trees. Residues from cyanide and some other chemical treatments are apt to be toxic to vegetation, and require that the soil be aired for several weeks before planting. With sodium cyanide, the time may be shortened by using ammo- nium sulfate to hasten the liberation of the cyanide gas. Heat treatments also require a fairly dry soil ; otherwise much heat energy is wasted in a limited area, in raising the temperature of the excess water. Steam does not spread through wet soil ; but, on the other hand, an exceedingly dry soil may be more injured by heat. 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 "Partial Steril- ization of Soil by Heat") are lost with longer delay. The soil management for several months after partial sterilization, espe- cially by dry heat, should provide for frequent irrigations, with less than the usual amount of water at one time, because the water-holding capacity of the 22 University of California — Experiment Station TABLE 4 Chemicals Sometimes Recommended for Controlling the Root-Knot Nematode in Soil Material Amount Application Remarks Effectiveness Carbon disul- fide Carbon disul- fide emulsion Chlorpicrin Chlorpicrin, spot treat- ments (Chlorpicrin Ethylene dichloride Methyl bromide Dichlorpro- paneand propylene dichloride mixture Sodium cyanide Ammonium sulfate Cyanogas, or crude cal- cium cyan- ide 100 to 303 gals, per acre, or 2/3 to 2 fluid oz. per hole Dilute 1 part of the concentrated (68 per cent) emulsion with 49 parts of water; use 1 gal. per sq. ft. 150 to 400 lbs. per acre, or 1 to 4 cc (1/27 to 1/7 fl. oz.) per hole 2 cc (1/13 fl. oz.) per hill, or 5 to 25 lbs. per acre 3 parts ll0cc(l/3fl. }■ oz.) per 17partsJ hole 380 lbs. per acre, in field plots 1 fl. oz. to 100 cu. ft. of soil 200 lbs. per acre 600 to 1,200 lbs. per acre 900 to 1,800 lbs. per acre 600 to 1,200 lbs. per acre per year, in 1 or 2 applications Bury in holes 6 to 9 inch- es deep, 18 inches apart each way, in staggered rows; cover Pour evenly over sur- face; soil should be moist and loose; cover Insert 6 inches deep, in holes 10 to 14 inches apart each way, in stag- gered rows; cover* for 3 or 4 days One application per hill, 6 to S inches deep, one week before planting; cover* In holes 10? inches apart, rows 9 inches apart, staggered Release under an air- tight cover Inject into soil in tight container; fumigate 24 hrs. Inject in holes 12 inches apart Pour aqueous solution of cyanideon newly plowed land, irrigate heavily; pour on am- monium sulfate solu- tion the same day, irri- gate again lightly ; cover Spread dry over surface, disk in, irrigate Gas spreads only in light dry soil; plot must be aired a week or more before planting If a fungicide is needed, formaldehyde may be added, 2/3 gal. to 50 gals, of the diluted emulsion Tear gas— handle care- fully 1 Requires warm soil, at least 65° F, 6 inches deep ; plant after odor has disappeared, usually 2 or 3 days after removing cover Helps plants make a vigorous start Less expensive than effective doses of chlor- picrin Very volatile, but heav- ier than air The more practical use, for potting soil, etc. Used in crop rows only Ammonium sulfate is needed to liberate the cyanide gas; both com- pounds have fertilizer value; air soil at least 2 weeks The various commercial flake and dust forms contain 17 to 50 per cent of calcium cyanide; the gas is readily liberated by water Fair to good control, for one crop Most effective in greenhouses; this is a minimum dosage Temporary control in sand or loam Has considerably re- duced early plant infection Probably fails to reach deep soil in- festations Very promising Reduces infection until roots become well established Most effective form of cyanide; results vary with different soils and climatic conditions; little value in clay Variable * A special glue-treated gastight paper for covers is procurable from Western Waxed Paper Company, a divi- sion of the Crown Willamette Paper Company, North Portland, Oregon. Such a cover is applied immediately after the treatment, in unbroken sheets or rolls, and the edges buried at least 5 inches deep all the way around the treated area. This confines the gas in the soil and makes the treatment much more effective than when no cover is used. Keeping the soil moist at the edge of the paper adds still further to effectiveness. The paper maybe used repeatedly if handled with care. Wetting the surface of the soil with water is nearly as effective in confining the gas, especially if the wet soil is covered with wet burlap, kept wet for 2 or 3 days. See footnote 7, p. 20, for information on applicator. [Gir. 330] The Root-Knot Nematode 23 TABLE 4— Continued Material Amount Application Remarks Effectiveness 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 toxic to plants nides Potassium 300 lbs. per acre Mix dry salts, spread im- Xanthate contains car- Fair control, vary- xanthate mediately, irrigate the bon disulfide, which is ing with soil condi- same day ; this is a min- liberated by the acids tions; does not kill Super- 300 lbs. per acre imum dosage of the superphosphate eggs in galls phosphate and sulfur [ Sulfur 50 lbs. per acre Formalde- Dilute with water Sprinkle surface; cover Good fungicide; none Kills nematode eggs hyde 1 : 50; use 1 toSqts. lost in soil; toxic to if sufficiently con- per sq.ft. plants centrated ; but field treatments are not promising Sulfur, 300 to 1,000 lbs. per Broadcast between Acidity must later be Reports contradic- ground acre plowing and harrowing neutralized with lime tory ; benefits prob- ably indirect, if any Lime or 1,500 lbs. to 2 tons Spread and plow Value depends on soil Reports contradic- quicklime per acre per year, conditions and need of tory; not a nemato- in 2 or 3 applica- lime cide tions Ammonia Not determined Drench soil with 1:400 Fertilizer, but may be Reported eff ecti v e dilution in water ; cover toxic to plants if im- pure on a small scale ; too expensive for field use; not tested in this country Crude naph- 850 lbs. per acre Spread and plow Air 2 weeks or longer thalene or creosote saltsf Cresy lie acid, 1 : 40 in water; 9 to 36 Irrigate Soil must be aired a full Not used in the called liquid gals, persq. yd. month United States carbolic acid Cyanogas 1,000 to 1,740 lbs. per acre Spread and irrigate Fair control under Naphthalene 1,000 to 1,740 lbs. greenhouse conditions flakes per acre Phytonomic J to 1 per cent con- Sprinkle; or apply with Worthy of experi- sodium centration of com- irrigation water ment hypochlorite mercial solution; J t gal. to sq.ft. Paradichloro- 300 to 600 lbs. per Inefficient; not benzene acre recommended Tobacco Spread b inches deep Rake, water heavily; Old recommendation Temporary reduc- stems, cut allow to soak in for 2 or tion claimed or ground 3 weeks t Vaporite is a mild form of these salts. t This is an especially prepared sodium hypochlorite with plant-injurious ingredients removed. It is being developed by the Research Department of Clorox Chemical Co., Oakland. Calif. 24 University of California — Experiment Station soil may have been lowered by the treatment. 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 manure or crop residues may benefit some soils, but nematode contamination must not be re- introduced with such material. No general rules can be given, because each soil presents a different problem, with many phases. APPLICATIONS OF THE PRECEDING RECOMMENDATIONS Getting Along with Nematodes. — Control measures are not usually intended to eradicate nematodes, but only to make the best of a bad bargain. Eradica- tion under field conditions is perhaps impossible. In most cases, however, a light infection will not seriously reduce yields in one season. Many of the treatments, both chemical and cultural, as ordinarily applied, reduce the soil population only enough to assure the success of a single susceptible crop. For economy of materials, volatile chemicals are sometimes used in the crop rows, just before planting, leaving infested soil between the rows. Or a small plot may be drenched with boiling water. When eradication is not possible, such procedure may be satisfactory from the standpoint of immediate returns. Whether or not a measure pays depends on the value of the crop grown as well as on the expense of the treatment. The grower feels entitled to a return for the effort and expense involved in any control program. When the population of a field has been reduced 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 with resistant crops might double or treble the value of the control already obtained. If the nema- todes are actually reduced to a minimum, the infection, and therefore the in- crease on crop roots, will be materially less the first season ; but of course the pest will eventually come back. With suitable measures, an infestation can be reduced to the point where it can be kept under control. Since, however, the menace of a serious new infestation always remains, only the cheapest and most practical measures should be attempted. The grower must be constantly alert, ready to apply some cultural control measure whenever it is practical. 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 favor- able to the nematodes than to the plants — for example, low temperatures or heavy soil. There is probably also a degree of dryness that would favor the nematodes somewhat less than the plants ; but this must be determined for each case by practical experience. If a plant can once get a good root system estab- lished under such conditions of reduced nematode activity, the later infections of new rootlets may be more successfully tolerated. Eradication of Nematodes. — As just stated, eradication of nematodes in the field is almost impossible. Control means fighting the infestation continually, even though the actual population may, with care, be kept below the danger point. Eradication means killing the very last nematode, so that the land is entirely free unless the pest is reintroduced. Wherever eradication is possible, [Cm. 330] The Root-Knot Nematode 25 no half-way control should be accepted. In some situations 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 eradication may be possible and is 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 avoid spreading the pest when land is cultivated or irrigated, the infested area must be separated from the clean land by a fence that will prevent all com- munication. Any possibility of drainage from the infested spot must be removed, if necessary by digging a ditch inside the fence. The extent of the infestation must first be determined, for it is probably wider than is suspected. Many indicator roots must be examined with the greatest care, and the fence must be 10 to 15 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 thoroughly dried or dis- infected. Roots should be turned under and allowed to decay. If dug, they should be burned inside the fence. Finally, control measures must be faithfully carried out. Frequent deep plowing, burning of brush over freshly plowed soil, fallow with sundrying, and chicken raising are recommended. Other measures discussed in this circu- lar may perhaps apply to the particular situation. If other measures are too difficult, a prolonged clean fallow should be enforced. Above all, every precaution against spread must be taken. Plants outside the fence, and from all parts of the field, must be frequently examined for galls. The fence must not be removed until careful examinations of indicator roots grown in the soil for three summer months have proved that the infestation is actually eradicated. Even then, watchfulness must not be relaxed. These points are not overemphasized. Unless all precautions are observed, the expense of a program for eradication may be wasted. No reasonable meas- ure for avoiding spread should be ignored, and no negligence permitted. Combinations of Practices for Field Control. — A well-planned combination of practices will go much farther toward controlling nematodes than any of the recommended treatments alone. Any chemical or cultural treatment will be more valuable if followed by fallow or by a resistant crop, with particular attention to the control of weeds. Field control by desiccation will be more effective if preceded by even a superficial 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 drying. Although fallow is an important control measure, its effectiveness in caus- ing exhaustion of nematodes is limited by its own conditions. The presence of growing plants is probably a main factor in stimulating eggs to hatch and larvae to be active. Since this factor is absent during fallow, a fallow may well 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. The danger of reproduction in new galls on the trap roots must be guarded against 26 University of California — Experiment Station with extra precautions in this case. For general field control, however, the weeds that grow for 2 or 3 weeks during fallow must perform this function. For an attempted eradication, the following program is suggested : 1. Eemoval and burning of old crop, with as much of the infected root systems as possible. 2. Dry or moist fallow for 2 or 3 months in warm weather, with cultivations frequent enough to prevent all weed growth. 3. A susceptible trap crop under good growing conditions, to be completely destroyed 2 or 3 weeks after sprouting. 4. Highly resistant rotation crops, without weeds. 5. Alternation of nos. 2, 3, and 4, as governed by practical considerations. 6. A test planting of a susceptible indicator crop at several points in the field, or extensive root examinations on the first susceptible crop planting. If any nematode galls are found, the entire planting should be destroyed, as a trap crop, and control treatments continued. Special Measures for Greenhouses and Nurseries. — Nurseries and seedbeds must be free of nematodes, both because of the loss from rejections and because of the danger to clean land in transplanting. A minor infection unobserved by the purchaser is more treacherous than a conspicuously heavy infection. In greenhouses, also, the warmth and cultural conditions favor a rapid increase of any nematodes present. All measures suggested in this circular should be thoroughly understood; the section on "Precautions against Introducing Nematodes" applies 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 walk through sheep dip before entering the plots. No suspected fertilizers should be used ; and all transplanted seedlings should be watched carefully. For complete eradication of nematodes after they are once introduced, more than one special measure may be needed. Although chemicals are thought of first, steaming or sundrying may be even more effective. New greenhouse beds should be equipped with a deep tile system for steam treatments, which may be needed periodically for various diseases. Clean fallow, though seemingly wasteful, is recommended for infested nursery land. Its value depends on the thoroughness of weeding and on the duration of the treatment. Benches should be raised completely off the ground, or else insulated by concrete construction, with a 6-inch layer of cinders in the bottom. Drain tiles, if used, will also serve for steam treatment in case of a later need. Drying is a convenient method of destroying nematodes on implements, 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 unin- f ested soil. Boiling water in drenching quantities, or strong chemical solutions, such as lye, sheep dip, fresh hot whitewash, or a 10 per cent solution of elorox or phytonomic sodium hypochlorite (see table 4), may be used for various cleaning purposes. 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 certain other measures. If he has enough land, he should divide it into three plots, on only one of which he grows a susceptible crop. Another plot should grow a resistant crop; and the third should be treated for control, probably by fallow. The three practices should be rotated in each plot. All three plots should be weeded and well cultivated. [Cir.330] The Root-Knot Nematode 27 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 may be rather heavily attacked under unfavorable growth conditions. Some increase of the nematode population on any of these plants must be expected. Blackeye-bean hybrids (fig. 6) may have some place in the rotation. In truck gardens, particularly, the soil should be broken whenever there is no crop growing, for the numbers of nematodes are somewhat reduced by clean fallow in even a short time. If any weeds are allowed to grow, even along the edges of the field, they serve as breeding places for more nematodes. Fig. 6. — In the foreground, common blackeye beans, practically destroyed by root knot; in the background, flourishing blackeye-bean hybrids. (Photo- graph by W. W. Mackie.) Partial Control in Orchards. — When orchard ground is infested, the nema- todes cannot be eradicated; but it is highly important to limit their multiplica- tion. If nematodes have been introduced, the ground between the trees must receive particular attention. Any susceptible plants growing there will breed more nematodes. A covercrop should be chosen for its high resistance. The cowpeas listed in table 2 are suggested, but had better be rotated with other resistant plants. Indicator roots should be examined frequently, and the ex- tent of the infestation noted. Clean cultivation is sometimes necessary ; but if covercrop roots are removed and burned they will serve to some extent as a trap crop. Mulching may prove of considerable value. When trees once show serious nematode injury, it may not be profitable to save them. If worth keeping, however, they should be given as good a chance as possible to escape continued reinfection. Questions are often raised as to the advisability of setting out an orchard in 28 University of California — Experiment Station infested land. Because of the enormous increase each year in numbers of nema- todes and in injuries, it is unwise to use any susceptible rootstock in land even lightly infested. Chemical or cultural 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 trees have a fair chance of keeping ahead of their parasites. To keep them growing thriftily, the soil Fig. 7. — Galls of root-knot nematode on peach roots (natural size). The "beaded" arrangement distinguishes them from the large knots caused by the crown-gall disease. (From Cir. 359.) moisture should be kept above the wilting point at all times, and nitrogenous fertilizers applied if the trees respond to them. There are possibilities in the grafting of desired varieties to resistant roots. The California Agricultural Experiment Station is now working with resist- ant rootstocks for the deciduous fruit trees, and several of these show promise. Selected seedlings of the Shalil (P.I. No. 36485) are perhaps the most resistant of all the peach varieties under test, and appear to make satisfactory rootstocks for peaches, apricots, and plums. Several strains of myrobalan and other plums are resisistant and are readily propagated by stem cuttings. Marianna [Cir. 330] The Root-Knot Nematode 29 plum, also propagated by cuttings, is resistant. Test trees of these plums have been planted to determine their adaptability to the sandy soils in which nema- todes are most injurious to orchards (fig. 7). Mazzard cherry seedlings and Stockton morello cherry (propagated by root suckers) have been found resist- ant, though mahaleb seedlings are moderately affected. Certain varieties of quince and apple show promise. Seedlings of ten selected varieties of pear are not resistant, though not severely affected. Several individual almond seed- lings have been found resistant ; but whether seeds of these will yield resistant seedlings remains to be determined. SUMMARY The root-knot nematode causes galls to form on the roots of most wild and cultivated plants. These galls interrupt the flow of sap so that the plants grow pale, wilt, and sometimes die. The nematode spreads most rapidly in sandy soils, but may also become a pest in clay. It has been found 8 feet deep in sandy soil. It tolerates a wide range of moisture conditions. It is carried by irrigation or drainage water from infested land, and may be introduced to clean fields in seed potatoes, transplanted roots, or soil adhering to implements. Precautions against introducing nematodes into clean land are urged. Many individuals survive winter freezing, but all are killed at -4° F. The larvae may live a year or longer in the soil, but they feed and grow only in living plants. All stages develop at temperatures between 50° and 90°. At the optimum temperature, about 81°, a larva may become an egg-laying female in 16 days. One female may lay 500 to 3,000 eggs, which can hatch without having been fertilized by a male. Susceptible perennials, especially orchard trees, should be started only in nematode-free land. Susceptible annuals should not be planted where an in- festation is at all serious. Resistant varieties of important crops and rootstocks are being developed. It is possible to get along with nematodes by raising resistant crops m rota- tion, with careful control of weeds. One susceptible crop may be grown every two or three years, or host plants may be grown in winter. The infestation will remain in the soil indefinitely, though starvation, enemies, and other natural causes of death destroy many 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 the length of the growth period. Complete air-drying kills all stages of the root-knot nematode, but incomplete drying may favor survival. A temperature of 140° F kills all stages instantly, 120° kills in 10 minutes, and 110° kills in 2 hours. A hot-water treatment for killing nematodes in roots and tubers is still in the experimental stage, and not all treatments may be successful. Either steaming or baking may succeed in eradicating nematodes from greenhouse soil. Boiling water has a limited application for ground beds. Chemicals are not recommended for general field treatments, since they give only a temporary control ; they destroy enemies of nematodes but never eradi- cate the nematodes themselves. In the field, a combination of measures gives much better control than any one method alone. 30 University of 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 Indus. Bui. 217:1-89. 4 tables. 3 pis. 3 text figs. (Out of print.) 8 Brown, H. D., 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. Hort. Monthly Bui. 2(12) : 737-56. 8 figs. (Out of print.) Day, L. H., and W. P. Tufts. 1939. Further notes on nematode-resistant rootstocks for deciduous fruit trees. Amer. Soc. Hort. Sci. Proc. 37:327-29. 1 fig. Godfrey, G. H. 1923. Root-knot: its cause and control. U. S. Dept. Agr. Farmers' Bui. 1345:1-26. 26 figs. (Out of print.) Goodey, T. 1933. Plant parasitic nematodes and the diseases they cause, xx -I- 306 pp. 136 figs. E. P. Dutton and Company Inc., New York, N. Y. Hutchins, L. M. 1937. Nematode-resistant peach rootstocks of superior vigor. Amer. Soc. Hort. Sci. Proc. 34:330-38. 8 figs. Johnson, J. 1930. Steam sterilization of soil for tobacco and other crops. U. S. Dept. Agr. Farmers' Bui. 1629:1-13. 7 figs. King, C. J., and C. Hope. 1934. Field practices affecting the control of cotton root knot in Arizona. U. S. Dept. Agr. Cir. 337:1-13. 5 tables. 8 figs. MlLBRATH, 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 others. 1940. Soil treatments for the control of diseases in the greenhouse and the seedbed. New York Agr. Col. [Cornell] Ext. Bui. 217:1-60. (Revised.) Sackett, W. G. 1927. Soil sterilization for seedbeds and greenhouses. Colorado Agr. Exp. Sta. Bui. 321 : 1-24. 8 tables. 16 figs. Snyder, E. 1936. Susceptibility of grape rootstocks to root knot nematode. U. S. Dept. Agr. Cir. 405:1-16. Illus. Tufts, W. P., and L. H. Day. 1934. Nematode resistance of certain deciduous fruit tree seedlings. Amer. Soc. Hort. Sci. Proc. (Sup.) 31: 75-82. Illus. Watson, J. R. 1921. Control of root-knot, II. Florida Agr. Exp. Sta. Bui. 159 : 29-44. 1 fig. Zimmerley, H. H., and H. Spencer. 1923. Hot water treatment for nematode control. Virginia Truck Exp. Sta. Bui. 43:267- 78. 1 table. 6 figs. 8 Publications that are out of print may often be found on file in libraries. 10w-7,'44(8694)