Plant Parasitic Nematodes
PLANT NEMATODES ATTACK ALL PLANTS grown in Florida. They cause farmers
and nurserymen millions of dollars in crop loss annually, but also can cause
problems in the urban world by damaging turfgrasses,
ornamentals and home gardens. We are often unaware of losses caused by
nematodes because much of the damage caused by them is so subtle that it goes
unnoticed or is attributed to other causes. Some scientists estimate that there
are over 1 million kinds of nematodes, making them
second only to the insects in numbers. However, few people are aware of
nematodes or have seen any, because: Most nematodes are very small, even
microscopic, and colorless; most live hidden in soil, under water, or in the
plants or animals they parasitize; and relatively few have obvious direct
effects on humans or their activities. Of all of the nematodes known,
about 50 percent are small animals living in marine environments, and 25
percent live in the soil or fresh water and feed on bacteria, fungi, other
decomposer organisms, small invertebrates or organic matter. About 15 percent
are parasites of animals, ranging from small insects and other invertebrates up
to domestic and wild animals and man. Some of the parasites of animals are the
largest nematodes known: some from grasshoppers can be several inches long, and
one from whales can reach lengths of more than 20 feet! Only about 10 percent
of known nematodes are parasites of plants. Morphology and
Anatomy. Plant nematodes
are tiny worms usually 0.25 mm to 3 mm long ( 1 / 100
" to 1 / 8 ") and cylindrical,
tapering toward the head and tail. Females of a few species lose their worm
shape as they mature, becoming pear-, lemon- or kidney- shaped. Plant parasitic
nematodes possess all of the major organ systems of higher animals except
respiratory and circulatory systems. The body is covered by a transparent
cuticle, which bears surface marks helpful for identifying nematode species. Life Cycle and
Reproduction. The life cycle of a plant-parasitic nematode
has six stages: egg, four juvenile stages and adult. Male and female nematodes
occur in most species, but reproduction without males is common, and some
species are hermaphroditic (Afemales@ produce both sperm and eggs). Egg
production by the individual completes the cycle. Most species produce between
50 and 500 eggs per female, depending on the nematode species and their
environment, but some can produce more than 1,000 eggs. The length of the life
cycle varies considerably, depending on nematode species, host plant, and the
temperature of the habitat. During summer months when soil temperatures are 80
to 90°F,
many plant nematodes complete their life cycle in about four weeks. Nematode
Feeding and Host-Parasite Relationships. Plant parasitic nematodes feed on living plant tissues,
using an oral stylet, a spearing device somewhat like
a hypodermic needle, to puncture host cells. Many, probably all, plant
nematodes inject enzymes into a host cell before feeding to partially digest
the cell contents before they are sucked into the gut. Most of the injury that
nematodes cause plants is related in some way to the feeding process.
Ectoparasitic
nematodes feed on plant tissues from outside the plant; endoparasitic
nematodes feed inside the tissues. If the adult female moves freely through the 61 soil or plant tissues, the species is said to
be
Amigratory.@
Species in which the adult females become swollen and permanently immobile in
one place in or on a root are termed
Asedentary.@ Migratory endoparasitic
and ectoparasitic nematodes generally deposit their
eggs singly as they are produced, wherever the female happens to be in the soil
or plant. Sedentary nematodes such as root-knot (Meliodogyne
spp.), cyst (Heterodera
spp.), reniform (Rotylenchulus spp.), and
citrus (Tylenchulus semipenetrans)
nematodes produce large numbers of eggs, which remain in their bodies or
accumulate in masses attached to their bodies. The feeding/living relationships
that nematodes have with their hosts affect sampling methods and the success of
management practices. Ectoparasitic nematodes, which
never enter roots, may be recovered only from soil samples. Endoparasitic
nematodes often are detected most easily in samples of the tissues in which
they feed and live (burrowing and lesion nematodes), but some occur more
commonly as migratory stages in the soil (root-knot and reinform
nematodes). Endoparasitic
nematodes inside root tissues may be protected from those kinds of pesticides
that do not penetrate into roots. Root tissues may also shield them from many
microorganisms that attack nematodes in the soil. Ectoparasites
are more exposed to pesticides and natural control agents in the soil. Foliar nematodes (Aphelenchoides spp.) are
migratory nematodes that feed on or inside the leaves and buds of ferns,
strawberries, chrysanthemums and many other ornamentals. They cause distortion
or death of buds, leaf distortion, or yellow to dark-brown lesions between
major veins of leaves. Other nematodes that attack plants above ground, but are
not common in Florida, cause leaf or seed galls. Still others cause
deterioration of the bulbs and necks of onions and their relatives. Diagnosing Nematode Problems
Determining if nematodes
are involved in a plant growth problem is difficult because few nematodes cause
distinctive diagnostic symptoms. A sound diagnosis should be based on as many
as possible of: symptoms above and below ground, field history, and laboratory
assay of soil and/or plant samples. Above-ground
symptoms. It is rare
that above-ground symptoms give sufficient evidence to diagnose a nematode
problem in the roots. However, they are important because they are almost
always the reason that nematode problems are first noticed. Since most plant
nematodes affect root functions, most symptoms associated with them are the
result of inadequate water supply or mineral nutrition to the tops: chlorosis (yellowing) or other abnormal coloration
of foliage, stunted top growth, failure to respond normally to fertilizers, small or sparse foliage, a tendency to wilt more readily than healthy plants, and slower recovery from
wilting. Woody plants in advanced stages of decline caused by nematodes may exhibit dieback
of progressively larger branches. AMelting
out,@ or gradual decline, is typical of nematode-
injured turf and pasture. Plantings stunted by nematodes often have worse
weed problems than areas without
them because the crop is less able than it should be to compete with weeds.
Distribution.
The distribution of nematodes within
any site is very
irregular, so the shape, size and
distribution of areas with the most severe effects of nematodes will be erratic
within the field. Nematodes move very few feet per year on
their own. In the undisturbed soil of groves, turf and pastures, visible
symptoms of nematode injury normally appear as round, oval or irregular areas that gradually increase in size year by
year. In cultivated land, nematode-injured spots are often elongated in the direction of
cultivation because nematodes are
moved by machinery. Erosion, land leveling, and any other force that moves
masses of soil or plant parts can also spread a nematode infestation much more
rapidly than it will go by itself. Nematode damage is often seen first and most pronounced in areas under special stresses , such as
heavy traffic, excessive drainage because of slope or soil and dry areas
outside regular irrigation patterns. Below-ground symptoms
may
be more useful than top symptoms for diagnosing nematode problems. Galls caused on roots by root-knot nematodes, abbreviated 62 roots or stunted root growth, necrotic lesions in the root cortex, and root rotting may all be symptoms of nematode problems. An
experienced observer can often see cyst nematodes (Heterodera,
Globodera and Cactodera
spp.) on the roots of their hosts without
magnification. The young adult females are visible as tiny white beads, about
the size of a period on this page. After a female cyst nematode dies, her white
body wall is tanned to a tough brown capsule containing several hundred eggs.
Important cyst nematodes found in Florida include soybean cyst nematode (H. glycines) on soybeans and a few leguminous weeds, beet
cyst nematode (H. schachtii) on cabbage and
related plants, St. Augustine grass cyst nematode (H. leuceilyma)
on St. Augustine grass and cactus cyst nematode (C. cacti) on Christmas
cactus and related plants. H. cyperi is a cyst
nematode occasionally found infesting nutsedges (Cyperus spp). Field history.
Accurate field history can provide
valuable clues to the identity of nematode and other pest problems. A nematode
that has been present in the field in recent years is probably there yet, and
is likely to injure susceptible crops if environmental conditions are
favorable. Production records that show a gradual decline in yields over a
period of years despite no change in cultural practices may indicate
progressive development of a nematode problem. A nematode infestation in a new
field usually begins in a small area. It gradually intensifies in the original
spot and is spread through the field by cultivation, harvest, erosion and other
factors that spread infested soil or plant parts. Therefore, the total effect
of a recently introduced nematode is a gradual production decline for the
field, as the percentage of the field that is involved and the severity of
damage at any given area in the field increase over the years. Laboratory
assay. Laboratory
analysis of soil and/or plant tissue samples is often necessary to complete a
diagnosis. In the lab, nematodes are extracted from soil and plant tissues,
identified, and counted. Those results can be compared with research and field
observations to determine whether or not the crop is likely to be injured by
the population under those conditions. In some cases, specific steps to reduce
the numbers and/or effects of a particular nematode species are recommended
only if the population density exceeds some predetermined level felt to represent
the threshold for economic loss of that crop. Such thresholds are determined
through longterm experience of nematologists
with that pest and crop in growers=
operations and in controlled experiments. Principles of Nematode Management
For many reasons, nematode
management is not and should not be a matter of simply identifying a specific
pest and then applying a chemical nematicide that is
effective against it. There are many situations for which no safe, effective
chemical nematicide is available. Most chemical nematicides are relatively toxic, so they are hazardous to
people, pets, and other animals if handled carelessly. Most nematicides
are environmentally risky because of their toxicity. Unfavorable environmental
conditions and/or events can make all nematicides
less effective than expected. Nematicides are
expensive. Many cultural practices can affect how seriously nematodes affect a
planting and how effective nematicides are if they
must be used. Carefully combining many of the
following practices into an integrated nematode management program often will help keep nematodes below
damaging levels, and improve effectiveness of nematicides
if they are available and must be used. Preventing a nematode problem is far better than
trying to treat one after it is established. Many serious nematode pests are
widespread, but some are quite limited in distribution, either from one region
to another or from field to field. One can avoid carrying serious nematode
problems into uninfested land by knowing that
nematodes are spread in contaminated soil and plant parts. Good sense dictates
working in areas that are not infested with nematodes before moving to those
that are infested, to avoid carrying contaminated soil or plants to the uninfested field. Ornamental cuttings to be rooted should
be taken only from uninfested plants or portions of
plants from above ground that have never been rooted in potentially 63
contaminated soil. This prevents propagating populations of nematodes
that might seriously reduce growth and might cause the plants to be unfit for
shipment to many potential markets because of quarantines. Quarantine is
governmental action taken to prevent importing a pest into a previously uninfested area, usually by controlling movement of
contaminated soil and plant material. Crop rotation is
a very old practice for reducing soil-borne problems. Many nematodes,
soil-borne disease organisms and insects can reproduce and survive on only a
few plants. Repeatedly planting a field with the same crop without interruption
will enable any organisms that reproduce successfully on that crop to continue
to increase. Rotation to non-host crops may interrupt nematode reproduction and
allow natural mortality factors to reduce their numbers. By carefully planning
the sequence of crops to be planted in a particular field it may be possible to
avoid excessive build-up of pests of all of the major cash crops in the cycle. In a few instances, it is even
possible to include a crop in the rotation that will help control pests that
have built up in preceding crops in the cycle. For instance, hairy indigo can
be planted as a summer cover crop to reduce numbers of sting and rootknot nematodes, and pangola digitgrass is used to control burrowing and root-knot
nematodes in vegetable lands in Florida and in the West Indies. The many kinds
of plant nematodes in Florida complicate selection of rotation crops, because
crops that reduce some species of nematodes may favor the increase of others.
Despite the difficulty, a good rotation program should be a basic component of
land/crop management plans because of the multiple benefits that can be derived
from it.
Crop root destruction
gets
far less credit than it deserves as a nematode management practice. Nematodes,
soil-borne diseases and many soil-borne insects will continue to feed and
multiply on crop root systems as long as they remain alive. When soil
temperatures are high, each month that a root system continues to live
represents an additional generation and potential increase of about 10-fold for
many nematodes. Even when soil temperatures are gradually declining, a
two-month period may support at least one additional generation. Therefore,
destroying root systems as soon as a crop is finished can stop nematode
reproduction and should encourage their decline through normal mortality. Flooding may sometimes be used to help reduce
numbers of nematode pests. It is practical only where the water level can be
controlled easily and maintained at a high level for several weeks. Where
flooding can be practiced, alternating periods of about two or three weeks of
flooding, drying and flooding again are apparently much more effective than a
continuous period of flooding. The soil should be worked during the periods of
drying to increase aeration and drying of soil and to prevent weed growth while
the soil is exposed. Flooding probably kills nematodes by providing a long
period without host plants rather than by some direct physical effect on the
nematodes. It is also important to consider the possibility that flooding with
contaminated water may actually spread some soil-borne pests such as nematodes. Fallowing is leaving a field with no plants on it
for a prolonged period to starve nematodes or other pests. Most nematodes will
decrease after a period of time without plants on which to feed. For fallowing
to be effective, the field should be cultivated regularly to prevent growth of
weeds and to expose new portions of the soil to the effects of drying and
heating. If weeds are allowed to grow in fallow land, many kinds of nematodes
may be able to survive and reproduce on the weeds, making the practice
ineffective. Resistance of plants to a specific pest is usually
the least expensive and most effective means of minimizing losses to that pest.
However, successful use of varietal resistance
requires knowing the extent and limitations of the resistance and which pests
are present in a particular situation. There are
Anematode
resistant@
varieties of tomatoes, soybeans, southern peas, sweet potatoes, cotton and
tobacco available for use in Florida, but each of these varieties has
resistance that is effective against only one, two or at most three species of
nematodes; none are
Anematode
proof.@ It is necessary
to know the pest species present in a field to select a variety with the
appropriate resistance. In addition, varieties with the appropriate resistance must 64 be adapted to cultural conditions and
requirements of your area. Another limitation to using nematode resistance as a
major management practice is that high temperatures often weaken or destroy the
resistant effect. Tomatoes
Aresistant@ to root-knot nematodes may not be able
to limit nematode reproduction or effects if soil temperature is hotter than 81°F. It
also is still necessary to use other methods to control any other nematodes
that are present, because the resistance against one or two species is not
going to affect the ability of any other nematodes to injure the crop. Biological Control
Many different bacteria
and fungi that are nematodes=
natural enemies have been isolated from nematode populations apparently being
kept at low levels by the bacteria and fungi. Nematologists
have been able to use some bacteria and fungi to reduce populations of some
kinds of nematodes under laboratory conditions, but successes at the full-scale
field level have been few. Most organisms recognized as
promising for biological control of one or more nematode pests are quite
specific in which nematodes they will attack, have been very difficult to
culture in sufficient quantities to be useful for field application, or both.
The conditions under which each is most effective are often quite specific and
limited. Commercially effective biological control as a means to reduce the
effects of nematodes on any cultivated crops may still be many years away. Nematicides sometimes can be very profitable when used
correctly in appropriate situations. However, their effects are almost universally
short-lived, so they should be used in conjunction with other practices that
minimize nematode re-infestation of a planting and reproduction.
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