Reports on Plant Diseases

RPD No. 501 - The Soybean Cyst Nematode Problem

November 1991

Soybeans 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.

The 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 infestations.

Symptoms

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.

Often these 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.

THE 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.

A 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.

HOST PLANTS

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.

Detection

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 spreading SCN.

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.