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
M^ - '^
X
jjK^F^'^A
1 ^H
Jr
.-^
^:
Fig. 1. — Nematode galls on tobacco root.
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^ ^^
^-p— V
j
■;:-'^^^^
r^
V
.-'"i^^^^^^^SBiB
7 *
r
«c^^
,> ,- J "
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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