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145
Plant Nutrient Management in Hawaii’s Soils
Chapter 16
Plant-parasitic Nematodes and Their Management
D. P. Schmitt and B. S. Sipes
Poor plant growth can be caused by a wide range of
factors. A common reaction to poor growth is to
apply more fertilizer or irrigation water. This decision
may seem logical—wilting plants sometimes need
water, and leaf yellowing often indicates nutrient defi-
ciency. However, if the symptoms are caused by nema-
tode infection, adding more water or nutrients usually
intensifies the problem: additional water can result in
waterlogged soil, leading to death of the roots; exces-
sive nutrients can create a salt problem and contribute
to environmental pollution.
When plant growth problems occur, it is important
to determine whether plant-parasitic nematodes are in-
volved. If so, the nematodes must be managed to elimi-
nate or minimize the damage. Infection by these mi-
croscopic roundworms is a major cause of poor crop
yields in the tropics and subtropics. Crop damage from
nematodes is not readily apparent in most cases, and it
often remains hidden by the many other factors limit-
ing plant growth. This chapter is intended to provide
some insight about plant-parasitic nematodes, the dam-
age they cause, and their management.
Making the diagnosis
Diagnosis of nematode problems is difficult because
the symptoms vary with environmental conditions and
the plant growth stage. The assessment of the cause is
further complicated because the same symptoms may
be due to other causes. In tropical and subtropical en-
vironments, most observations of wilting, stunting, dis-
coloration, and general abnormal appearance should
trigger the thought of nematodes as a possible cause.
Because of the wide array of disorders that could be
caused by nematodes, an assay of soil and plant tissue
for nematodes is essential.
Assaying for nematodes
Soil samples are helpful in diagnosing a nematode prob-
lem or assessing other potential problems. Accurate
nematode diagnosis through a soil and root assay is the
key to developing a successful management program.
Proper sampling is necessary to obtain a reliable and
accurate estimate of the nematode population. Through
the assay, the kinds and numbers of nematodes are de-
termined—this is the necessary first step to making a
management decision. Sampling the soil before plant-
ing (“preplant” sampling) is a relatively reliable pro-
cedure for predicting the potential of nematode dam-
age. In the tropics and subtropics, however, precise or
even approximate damage and economic thresholds are
not established. Nevertheless, good judgments can be
made with the data generated from a soil assay.
For diagnostic purposes, samples from a crop field
or a group of plants can be collected any time after the
plants have been growing actively for at least 2–3
weeks.
Repeated sampling increases the reliability of sam-
pling. For preplant sampling, the repeat sampling can be
done immediately. During the cropping period, the sec-
ond sampling should be done a few weeks after the first.
From: Plant Nutrient Management in Hawaii’s Soils, Approaches for Tropical and Subtropical Agriculture
J. A. Silva and R. Uchida, eds. College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, ©2000
146 Nematode Management
College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa
Collecting samples for nematode assay
The proper sampling method is based on the species of
nematode and its location in the soil profile, which also
depends on the crop. It is important to sample in the
root zone if plants are present. With turf, for example,
the nematodes occur primarily in the top 4 inches (10
cm) of soil, whereas with many perennial crops such
as coffee and pineapple, the highest numbers of nema-
todes occur at a depth of 8–20 inches (20–50 cm) in
the soil.
Both large fields and small plantings such as home
gardens should be sampled in a systematic, zigzag pat-
tern. This sampling should consist of at least 15–20
uniform cores or shovel-fulls of soil composited into
one sample per garden or 2–3-acre (3⁄4 to 11⁄4 hectare)
section of a field. A systematic pattern of sampling (Fig-
ure 16-1a) provides a high probability of obtaining an
accurate sample.
Single plants (such as a tree) should be sampled
beneath the leaf drip line (Figure 16-1b). The number
of cores or shovels of soil taken around a tree will de-
pend on the size of the tree canopy. For a small tree
with a canopy <10 ft (3 m) in diameter, collect 6–8 cores
of soil and composite them into one sample. For larger
trees, sample 10–12 locations and composite them.
Sample handling immediately after collection
The composited soil should be gently mixed before
withdrawing a subsample of about 1 pint (1⁄2 liter). Place
the subsample into a plastic bag and seal it to prevent
moisture loss. Label the sample with your name, ad-
dress, name of plant (including variety), and date of
sampling. This sample must be handled gently and kept
out of the sun, preferably in an insulated cooler or an
area that is about 70°F (21°C). Nematodes are sensi-
tive to heat and cold extremes, and to drying. Send the
sample in a cardboard box or styrofoam container to a
processing laboratory, such as the CTAHR Agricultural
Diagnostic Service Center.
General description of a nematode
Plant-parasitic nematodes are microscopic (usually less
than 1 mm long) and are armed with a spear-like de-
vice that they use for feeding (Figure 16-2). This appa-
ratus is inserted into the plant’s cell and is used to with-
draw the cell contents.
Of the hundreds of different kinds of nematodes
that infect plants, only a dozen or so species are known
to be economically serious root-feeding pathogens in
the tropics and subtropics; however, new species are
being found, so the number of species is likely to be
much greater than is currently recognized. There are
fewer kinds of plant-parasitic nematodes that cause sig-
nificant damage by feeding on foliage, but as with the
root feeding nematodes, new species are being discov-
ered that cause significant foliar damage. If the num-
bers of harmful nematodes are large, plant growth is
adversely affected.
The primary groups of nematodes recognized as
problems in the tropics and subtropics are the root-knot,
cyst, burrowing, lesion, foliar, and reniform nematodes.
Although these are associated with severe crop losses,
information on actual yield loss is generally lacking.
Other nematodes, such as the spiral, pin, and lance
nematodes, may occur in abundance but usually do not
cause sufficient damage to warrant concern.
The environmental conditions in the tropics and
subtropics are ideal for maximizing nematode damage.
Nematodes thrive at the temperatures under which most
a b
Figure 16-1. Collect soil samples in a systematic, zig-
zag pattern in fields (a) or around individual trees (b).
Illustration from the USDA Nematology Lab home page, <http://sun.ars-grin.
gov/ars/Beltsville/barc/psi/nem/>.
Figure 16-2. Schematic drawing of a female nematode
147
Plant Nutrient Management in Hawaii’s Soils
Some common plant-parasitic nematodes of the tropics and subtropics
crops and landscape plants are grown in these regions,
especially where frequent rainfall or irrigation keeps
the soil moist.
Rationale for making management decisions
A well informed management plan is necessary to en-
sure that the control practices selected can be effec-
tive, environmentally safe, and economical. Manage-
ment of nematodes must focus on reducing nematode
numbers to levels below the damage threshold. In an-
nual crops, the higher the nematode population num-
bers at the time of planting, the lower the yield. In pe-
rennial crops, the relationship between plant growth
and nematode populations is more complex. The ini-
Root-knot nematode
Root-knot nematodes belong to the genus
Meloidogyne. The two most common species in the
tropics are M. incognita (southern root-knot) and M.
javanica (Javanese root-knot); other species are
present but occur less frequently. The root-knot nema-
todes feed and mature inside the roots of plants. Their
feeding induces abnormal enlargements of the root,
called galls. The root-knot nematode does not sur-
vive very long without a host plant, except in very
low numbers and probably in the egg stage.
Diagnosis: Root galls are the primary symptom of
root-knot nematodes. Species identification requires
laboratory assay.
Reniform nematode
The reniform nematode, Rotylenchulus reniformis,
has a wide host range on cultivated and noncultivated
plants. The juvenile stages and males live in the soil
and do not feed. The adult female is swollen and is
the only parasitic stage of this nematode’s life cycle.
The female inserts her head and neck into the root,
leaving her body outside of the root. The reniform
nematode survives in the soil as eggs and coiled ju-
veniles. This nematode causes root rotting and re-
duced uptake of water and soil nutrients. The symp-
toms are general lack of vigor, discoloration of foli-
age, and (or) stunted plants.
Diagnosis: The reniform nematode can be accurately
diagnosed only through laboratory assay of soil and
root samples.
Burrowing and lesion nematodes
Adult burrowing nematodes (Radopholus similis) and
lesion nematodes (Pratylenchus spp.) cause root rot.
These worm-shaped nematodes are migratory, living
most of their life, including the hatching and feeding
stages, inside the roots and sometimes the lower stem
of their host plants. They can move through the soil
from one root to another during their life cycle.
Diagnosis: The root rot caused by this nematode is
not characteristic for diagnosis. It is necessary to have
the soil and roots assayed to determine the numbers
and kinds of nematode species present; root assays
are the most reliable.
Sugar-beet cyst nematode
The sugar-beet cyst nematode (Heterodera schachtii),
is primarily a problem on cabbage, broccoli, and cau-
liflower in Hawaii in the Kula, Maui, area. This nema-
tode penetrates the root, and the female enlarges as it
matures to become a white, lemon-shaped structure
that breaks through the root surface at maturity. When
the female dies, her body turns brown. Eggs survive
inside the dead female’s body (called a cyst) for many
years.
Diagnosis: Direct observation of the organism with
a magnifying glass is helpful, because the white cysts
are about the size of the period at the end of this sen-
tence. Root aphids are also white, so some experi-
ence may be needed to differentiate between these
two white structures clinging to the roots. Confirma-
tion by a diagnostic laboratory is recommended.
Foliar nematodes
Aphelenchoides besseyi, A. ritzema-bosi, and A.
fragariae feed inside leaf tissue. The entire nema-
tode life cycle is completed in the leaves. Plants can
be stunted with deformed, discolored, or dying leaf
tissue; “die-back” can also occur.
Diagnosis: Accurate identification requires labora-
tory assay of leaf-tissue samples.
148 Nematode Management
College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa
Important points about nematodes
•Nematodes are microscopic and transparent.
•Symptoms of nematode infection are often
indistinct and usually mimic nutrient deficiency.
•Soil assays for the presence of nematodes are
necessary for accurate diagnosis.
•Management strategies usually must be directed
at the particular nematode species present.
tial numbers of nematodes are still important because
they determine the early growth potential of the plant.
However, even if numbers are low at planting, nema-
tode populations will eventually increase and ultimately
damage perennial plants. Thus not only must the initial
populations at the time of planting be low, but also the
populations must be kept at a low level if plant growth
is to remain vigorous.
Manageming nematodes in tropical and subtropi-
cal environments is a challenge. There are a few con-
trol measures that are effective, and these must be used
under conditions in which they will work. For effec-
tive management of nematodes, the critical steps are
(1) accurate diagnosis, and
(2) proper selection of the most effective and environ-
mentally benign control method.
Control of nematodes
Nematode management should be multifaceted. Since
eliminating nematodes is not possible, the goal is to
manage their population, reducing their numbers be-
low damaging levels. Common management methods
used include planting resistant crop varieties, rotating
crops, incorporating soil amendments, and applying
pesticides. In some cases, soil solarization also may be
practical.
Control methods not involving pesticides
Use of resistant plant cultivars is limited because there
are only a few and their nematode resistance is very
specific. Because resistance is specific, accurate iden-
tification of the nematode species and race is neces-
sary before the proper cultivar can be selected. Crop
resistance is ideally combined with a long-term crop
rotation schedule and the best management practices
available to favor vigorous and healthy plant growth.
Crop rotation involves growing a crop that is not a
host for the nematode present before growing a crop
that is susceptible. The nonhost or immune crop will
cause nematode numbers in the soil to decline, giving
the subsequent host crop a chance to establish a good
root system. The success of this method depends on
growing the nonhost crop long enough to reduce the
nematode numbers. The rotation crop must be selected
carefully because some nematodes (such as root-knot,
reniform, and burrowing nematodes) have very wide
host ranges. Also, some undesirable species may emerge
on the rotational crop and become a pest. Variations on
the crop rotation concept include fallowing,
multicropping (intercropping), and green manuring.
Keeping the soil free of plants (fallow) deprives
plant-parasitic nematodes of a host, which, over time,
reduces their populations. Maintaining good weed con-
trol is a critical component of fallowing for nematode
control because weeds are hosts of many species of
plant-parasitic nematodes.
Multicropping (intercropping) with plants that ei-
ther are not good nematode hosts or are antagonistic to
the nematodes also reduces nematode numbers.
Green manuring—tilling under a crop that grows
rapidly and produces a large quantity of biomass—adds
organic matter and, depending on the green manure crop
used, may add substances that repel or kill nematodes.
Sudangrass and corn are excellent green manure crops
that provide good nematode control.
Another nonchemical approach to controlling
nematodes is biological control—using other organisms
against the pest organism. A high level of natural bio-
logical control is ordinarily present in the soil. This
natural control probably keeps the nematode popula-
tions at 10–20 percent of what they would be in its
absence. Nevertheless, the level of natural control is
seldom adequate to prevent plant damage from nema-
todes.
The strategy of inoculating soils with biological
control organisms to increase or supplement the con-
trol organisms naturally present has proven to be unre-
alistic and is not recommended until more predictable
inoculants are developed.
A more realistic strategy for biological control of
nematodes is to incorporate soil amendments such as
manure (particularly poultry manure) and compost.
Such additions of organic matter contribute to biologi-
149
Plant Nutrient Management in Hawaii’s Soils
cal activity in the soil and enhance the natural activity
of organisms antagonistic to nematodes.
Solarization—heating soil under clear plastic tarps
that trap and increase the sun’s heat—can be an effec-
tive means of controlling nematodes in the soil. The
soil needs to be moist, well tilled, and heated to at least
140°F (60°C) for several days, preferably several
weeks. This method can be practical for home gardens,
but it should be done during the hot months and long
days of mid-summer.
Similarly, other heat and steam-based pasteuriza-
tion methods can be used to prepare potting soil. Healthy
plants grown in nematode-free media have a better
chance to survive after being transplanted to the field.
Pesticides (nematicides)
Nematicides are sometimes used in agriculture, but there
are few on the market. Most nematicides are highly toxic
synthetic pesticides commercially available only to com-
mercial growers. These products can be used only on
particular crops, and they usually must be purchased
and applied by a licensed pesticide applicator. How-
ever, several organically based nematicides are being
marketed that can be purchased without a license.
Two types of nematicides are fumigants and
nonfumigants. Fumigant nematicides are usually more
effective, but nonfumigant nematicides can also be used
effectively. Fumigant nematicides such as metam so-
dium and 1,3-dichloropropene are applied before plant-
ing. Some nonfumigant nematicides such as Nemacur®,
Mocap®, or Vydate® are moderately effective and can
be used both pre- and post-planting.
Some “natural” products claiming to provide con-
trol of nematodes have been developed from biologi-
cal sources. These products have not given adequate
control in research trials. Testimonials from growers
indicate some response, but the apparent recovery of
plants resulting from using these products can some-
times be due to growth enhancement by plant nutrients
contained in the formulations. In some of these cases
there is not actually a nematode problem, which re-
sults in a false indication of nematode control.
The label on the nematicide package provides ap-
plication and safety information. Follow the label in-
structions to maximize the material’s effectiveness,
minimize health risk, and to be in compliance with state
and federal regulations.
Suggestions for managing nematodes
in the home garden
•Susceptible plants can be grown in containers
with a nematode-free soil or growth media;
keep the containers off the ground.
•Sow seedlings for transplanting in clean media
in containers kept off the ground.
•
Check the roots of container-grown nursery stock
for nematode galls before transplanting them.
•Add large amounts of organic matter to the soil.
•Keep weeds controlled at all times.
•As soon as plants are not needed, pull them up
or till them in to stop their roots from hosting
nematodes.
•Infected plant root systems should be destroyed
and not composted.
•Rotate susceptible plants with resistant or
immune plants.