Reports on Plant Diseases
RPD No. 501 - The Soybean
Cyst Nematode Problem
[ Symptoms ] [ Disease
Development ] [ Control ]
severely infected with the soybean cyst nematode, Heterodera glycines Ichinohe,
(SCN) become stunted and yellow or chlorotic and may be killed. The damage usually
is most severe on lighter, sandy soils, but drastic losses have been observed
on the heavy soils typical of much of the soybean acreage in Illinois. Symptoms
observed on heavier soils often are limited from none to stunting or uneven growth
not associated with yellowing. Yield losses can range from 5 to 80 percent, depending
on rainfall, soil fertility, the presence of other diseases, and the population
of the nematode.
The damage caused by the soybean cyst nematode can be greatly
accentuated if the infected soybean plants are exposed to droughty periods and
root-rotting fungi. Rotating to nonhost plants such as corn or small grains, eliminating
weed hosts, and using SCN-resistant soybean varieties will tend to delay or perhaps
prevent soybean cyst nematode populations from increasing to damaging levels.
soybean cyst nematode has been identified in most counties in Illinois (Figure
1). Undetected infestations probably are present in the remaining few counties.
An awareness of the problem will help in efforts to spot new and potentially devastating
on image for larger version
Unfortunately, the aboveground symptoms of damage on individual
plants and the appearance of infested fields are usually not specific enough to
allow positive identification. However, some symptoms are quite suggestive of
infection by this nematode. Heavily infected plants are stunted and may be yellow
or chlorotic, particularly in soils of low fertility or during drought conditions.
Severely infested portions of a field may be oval to somewhat eliptical in outline.
These areas may have a general yellowish cast and show the most severe damage
in the center, with less damage occurring toward the margin.
symptoms are not apparent. As slight to moderate stunting is difficult to detect,
the grower often is unaware of SCN presence in a field. Yellowing of plants will
not be apparent with low to moderate nematode populations unless plants are stressed
by other conditions. This explains reports of the sudden increase of soybean cyst
nematodes in new counties and fields in droughty years.
Many other conditions
may cause the same or similar symptoms. Therefore, identification cannot be made
entirely on the basis of aboveground symptoms alone. Growers should notify their
area Extension Educators at once if they see the conditions described above in
their soybean fields, or they should contact Extension Plant Pathologists at the
University of Illinois, Urbana, IL 61801. Special arrangements must be made for
collecting and shipping samples. Final identification cannot always be made with
the unaided eye. The nematodes must be recovered from infested soil or plant roots
where they can be identified under a microscope or with a hand lens. Older females,
which are brown, are not readily visible on roots.
Field diagnosis can be
done by digging up plants on the margins of damaged areas and gently washing or
tapping the soil from the roots. The presence of white to brown, lemon-shaped
cysts on the soybean roots provides positive identification. The absence of cysts,
however, does not mean that the soybean cyst nematode is not involved. The absence
of cysts is not uncommon where plants are nearing maturity or prematurely killed
by root-rotting organisms. Soil samples should be submitted in such cases.
NEMATODE AND ITS LIFE HISTORY
Soybeans are infected by the second-stage juvenile
a microscopic (1/60th-inch-long), colorless worm (Figure 2). Juveniles
penetrate the soybean by puncturing the roots with a spearlike feeding
structure, the stylet. They invade the root, then migrate toward food-conducting
tissues, where they feed and mature. Feeding alters the internal root
structure, thereby interfering with normal root functions and ultimately
causing plant damage. In approximately three weeks, under optimum conditions
(soil temperatures at 80 to 84 F or 27 to 29 C), egg-bearing females develop
from the juveniles. At 90 F (32 C) and above, nematode development is
slower and numbers reaching maturity are reduced.
The females enlarge greatly as they develop, becoming lemon shaped.
They break through the root surface while remaining attached to the root by the
head. The females lay eggs in a jellylike mass attached to their posterior end
and retain about two-thirds of the eggs within their swollen bodies. If an infected
plant is dug at this stage, the attached females can be seen with the unaided
eye as shiny, white, spherical bodies about the size of the period at the end
of this sentence. This is the so-called white female stage.
After death, the
white female stage changes from yellow to brown the brown cyst stage. By
the time brown cysts are formed, the cyst (the altered female body wall) has become
a protective structure containing up to 400 eggs. The cyst wall protects the eggs
from drying, chemical action, predators, and some parasites. For this reason,
the brown cyst stage is best suited for the spread of the nematode to new areas.
As many as four generations of the nematode can be completed in a single growing
season. Thus, if one cyst containing 400 eggs is introduced into a soybean field
in the spring, several thousand cysts could be produced in one growing season.
Thus, the introduction of even a single cyst into a field represents a potentially
high nematode population that could cause noticeable damage within a few years.
system for separating 4 races (designated as 1 through 4) of SCN, using 4 soybean
differentials and a susceptible check, has been in use since 1970. In 1988, a
new system of designating races increased the number of potential races to 16
(Table 1). Races are characterized by their ability to reproduce on certain soybeans
varieties. Race 3 is found most frequently in Illinois followed by race 1. Races
2, 4, 5, 6, 8, and 9 are known to occur in Illinois but do not appear to be as
common as races 1 and 3.
The host range of the soybean cyst nematode includes leguminous
field crops plus some ornamental plants and certain weed species that are susceptible
and will increase nematode populations. Highly susceptible hosts are shown in
bold letters in Table 2. Other plants listed allow cyst production but
are not often associated with soybean production in Illinois.
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Identifying the problem is the first step in controlling SCN. Soybean
producers should be familiar with SCN symptoms and should suspect SCN
where yields are reduced without explanation. The best way to determine
if SCN is present and to determine population densities is to collect
a soil sample and submit it for analysis. A preferred time to sample is
in the fall, after harvest, so that analysis for SCN densities will provide
timely information for planning the next growing season. However, samples
may be collected anytime during the growing season for the reason of confirming
the presence of SCN.
Soil samples should be collected to a depth of 6 to 8 inches. Using a
zigzag pattern, collect 12 to 24 soil cores (using a soil probe). Place
cores from each 10 to 20 acre set into a bucket, mix thoroughly, and submit
about one pint in a plastic bag to a soil testing lab. The University
of Illinois Plant Clinic, 1401 W. St. Mary's Rd., Urbana, IL 61802 is
capable of assaying for SCN. Also, there are several private labs in Illinois
that are capable of processing soil samples. For a listing of these labs,
contact the Extension Nematologist at 217/244-2011. A charge will be made
by all labs for processing each sample (See Report on Plant Diseases #1107,
Predictive Soil Sampling and Analysis Procedure for the Soybean Cyst Nematode).
HOW NEMATODES SPREAD
Cysts occur throughout the root zone in the soil. Some accumulate on
the soil surface. From there, they can easily be transported alone or
with soil by man and by natural agents. Cysts may be found in the soil
adhering to farm implements, machines, vehicles, tools, shoes, or other
soil-carrying items. Nursery stock, transplants, bulbs, corms, and root
crops may carry cysts in adhering soil, even though the plants themselves
are not being attacked by the nematodes. Hay, straw, grain, or seed crops
that carry dust or soil peds may also serve as carriers. Basically, anything
that moves through an infested field in contact with the soil is capable
of picking up and transporting cysts. Equipment and contaminated soybean
seed that have not been thoroughly cleaned may be an important means of
Natural agents may also be important in the spread of the soybean cyst
nematode. Wind, runoff water, and wildlife can carry cysts into clean
areas. Even waterfowl and other birds feeding in infested fields may ingest
cysts and carry them considerable distances.
Control: An Integrated Approach
Ideal programs to manage SCN infestations
ahve successfully integrated the following: detection through scouting and sampling
procedures, and crop rotations utilizing nonhost crops and SCN-resistant soybean
varieties. Maintaining proper soil fertility and pH, managing other soybean diseases
and pests, and proper planting methods also help to keep plants vigorous and better
able to buffer the effects of SCN. The most effective management system has and
will continue to involve integrated approaches.
value of crop rotation should not be underestimated. A grower will achieve higher
yields on all crops involved in the rotation than on any crop that is planted
continuously. Crop rotation has proven to be a powerful tool for controlling SCN
as well as other diseases.
The goal of crop rotations in the presence of
SCN are to: improve soybean health and yield; reduce SCN numbers; and to preserve
yield potential of SCN-resistant varieties.
Rotations with nonhost crops
such as corn, small grains, red clover, alfalfa, and sunflower and with SCN-resistant
soybeans are effective because SCN juveniles hatch from eggs as long as soils
are between 60 to 90 F (16 to 32 C). Optimum hatch occurs at 72 F (23 C). The
second-stage juveniles have food reserves to last seven to fourteen days, depending
on soil temperature, after which time they must establish a feeding site in a
host plant. Thus, if no host plant is available, data indicate that 50 percent
or more of the SCN population is eliminated by starvation each year. Even greater
reductions may occur where eggs or juveniles are attacked by parasites and predators
such as fungi, predacious nematodes, and bacteria.
Two crop rotations are
suggested below that should minimize and may eliminate yield losses due to SCN.
These two suggested rotations are based on the preceding crop. They are as follows:
Crop Rotation #1 (following soybean)
Year 1--nonhost crop (e.g., corn, small
grains, sunflower, alfalfa, or red clover)
Year 2--SCN resistant varieties
Year 3--nonhost crop
Year 4--susceptible soybean variety if soil analysis
shows SCN populations are below threshold level. If not, year 2 can be repeated
using a different genetic source of resistance.
Year 5--repeat year 1
Crop Rotation #2 (following corn)
Year 1--SCN-resistant varieties
Year 3--susceptible soybeans if soil analysis shows SCN populations
are below threshold. If not,
repeat nonhost crop or SCN-resistant varieties,
preferably those with a different genetic source of resistance than used in year
Year 4--If SCN-resistant varieties were used in year 4, repeat year 2.
If a nonhost crop was used in year 4, repeat year 3.
The use of resistant
varieties is the key factor in managing SCN. However, resistant varieties should
not be planted consecutively for several years because of the possibility of developing
a population capable of reproducing on a given genetic source of resistance. Use
The success story of managing
SCN has been the use of resistant varieties. Three decades ago, only a small number
of SCN-resistant varieties were available. Today, through the efforts of public
and private soybean breeders, this list has expanded to approximately 700 lines
adaptable to Illinois growing conditions. Since 1977, the University of Illinois,
in cooperation with the USDA, has released 22 resistant varieties ranging from
Maturity Group I through IV. Newer releases have improved yield potential plus
the ability to resist up to 5 races of the nematode.
Marion Shier, University
of Illinois Extension, compiles yearly a list of public and private varieties
resistant to SCN. All varieties are listed by maturity group and relative maturity
within the groupings; a coding system that designates companies making the variety
available; and a listing of race resistance and source of resistance for each
variety. The Illinois Soybean Checkoff Board has published this listing in booklet
form and free copies are available. For more information on how to obtain copies
of this booklet, contact the Extension Nematologist in the Department of Crop
Sciences, at 217/244-2011 or by e-mail at firstname.lastname@example.org. A limited number
of copies may be available at local Extension offices.
are the foundation for IPM approaches in managing SCN as well as other plant diseases.
Some distinct advantages of using host resistance are that the pest control is
purchased with the seed, it is compatible with other management practices, has
few adverse effects on the environment, and in most cases the cost is minimal.
are a few nematicides that are registered for use against SCN. However, they are
not generally recommended for control of SCN because resistant varieties are more
cost effective than nematicides (nematicides increase the cost of production).
Nematicides may adversely affect the environment and frequently result in higher
cyst counts at the end of the growing season. The reason for this is that a nematicide
allows the soybean plant to develop a somewhat normal root system early in the
growing season because of nematode control. Later in the season, when the nematicide
has degraded to a non-toxic form, those nematodes remaining in the soil attack
the root system and are able to reproduce rapidly because there is more root mass
and consequently, more feeding sites. The end result is that population levels
at the end of the growing season may be as high or even higher in treated fields
than in untreated fields. Thus, the use of a nematicide does not permanently alleviate
the damage and subsequent yield loss caused by SCN. The use of SCN-resistant varieties
in rotations involving non-host crops is the most effective and economical approach
in controlling the nematode.
MAINTAINING PLANT HEALTH
Damage by SCN
is greater on plants that are under stress from other factors. Damage may be reduced
by providing plants with optimum growing conditions including adequate moisture
(irrigation, if available), maintaining adequate soil fertility, breaking soil
hardpans, improving soil aeration; and by controlling weeds, insects, and other
plant diseases. These practices help plants compensate for damage by SCN but do
not decrease nematode numbers. Research involving plant-parasitic nematodes has
shown that resistant varieties and crop rotations do not increase yield to their
full potential without the factors necessary for good plant growth and development.
SCN moves with infested soil, soybean producers with SCN-infested fields can reduce
but not eliminate the spread to other fields by washing soil from equipment used
in infested fields before moving the equipment to noninfested fields. Whereas
SCN is often carried in soil peds, seed produced on infested ground should be
thoroughly cleaned to remove soil peds before planting. Another common source
of SCN contamination is second-hand farm equipment. Such equipment should be washed
free of as much of the adhering soil as possible before it is brought on a farm
and used. Cleaning methods have involved the use of high-pressure water or steam.
Certain fields on a farm may be infested with SCN. Working, planting, and cultivation
of these fields should be done after uninfested fields have been worked. After
working infested fields, equipment should be cleaned.
SOYBEAN CYST NEMATODE
IN SNAP BEANS, KIDNEY BEANS, LIMA BEANS, AND PEAS
These crops are all good-to-excellent
hosts of SCN. SCN can reduce the yields of these crops, although populations higher
than those on soybeans may be needed to cause losses equal to those on soybeans.
At present, the best control approach should involve, through soil sampling, the
selection of fields for planting that do not have SCN or those with populations
below the economic threshold (see "Detection" section). Fields that
have previously been rotated to nonhost crops or SCN-resistant soybeans for 3
to 4 years may offer below threshold populations.
In Illinois, SCN has been
found causing damage on snap beans under field production systems. Snap beans
with usable resistance to races 3 and 4 include Wisconsin RRR (root rot resistance)
lines 36 and 46. Although little is known about the susceptibility of all commercial
varieties grown in Illinois, it is assumed that they are as good hosts as soybeans.
For further information concerning
diseases of crucifers and other vegetables, contact Mohammad Babadoost, Extension
Specialist in Fruit and Vegetable Diseases, Department of Crop Sciences, University
of Illinois at Urbana-Champaign.
of Illinois Extension provides equal opportunities in programs and employment.