Disease Resistance in Soybean
Soybean Disease Information Note 6
S. R. Koenning, Extension Plant Pathologist
E. J. Dunphy, Crop Science Extension Specialist

Soybean-variety selection is one of the most important decisions a grower makes when planning for the next crop. A farm manager should be aware of the diseases that threaten his particular crop and select varieties with resistance to match those diseases. No one variety is resistant to all diseases, but most varieties are resistant to many of the diseases that occur in North Carolina. Seed of resistant varieties generally costs about as much as susceptible varieties. There may however, be a hidden price if the resistant variety is planted in a situation where a susceptible variety is better adapted. The price a grower pays for an inappropriate choice of varieties may be less soybean yield at harvest. Resistant varieties often are the most cost effective means of managing plant diseases. Other tactics for controlling disease, such as rotation and other cultural practices, should also be used to supplement resistance. Reliance on a single tactic to control plant diseases is often ineffective.

This publication provides information about disease resistance of soybean to selected diseases that occur in North Carolina. In many cases, information on the resistance of a particular variety is not available. Often, it is best to assume that a variety is susceptible unless you have information to suggest otherwise. Two diseases caused by fungi (Phytopthora root rot and Frogeye leaf spot) and a number of diseases caused by nematodes (Soybean cyst, Root-knot, Columbia lance, and Reniform) are included in this information note. This list of diseases is not extensive, but includes the most important ones in North Carolina and those for which some information is available. The resistance of specific varieties can best be obtained through literature provided by seed companies.
Soybean Cyst Nematode

Soybean cyst nematode (SCN) is the most serious soybean pathogen in North Carolina. Since its discovery in North Carolina (and the United States) in 1954, it has spread to all counties in the Coastal Plain, Tidewater, and some Piedmont counties with large soybean acreages. Severe yield loss caused by this pest is especially common in sandy coastal plain soils. SCN, however, is not restricted to any soil type and often causes significant soybean yield losses which may go unnoticed.

Races of Soybean Cyst Nematode

Field populations of SCN are characterized as races (numbered 1 through 16). A race designation of cyst nematode is an indication of a field populations' ability to reproduce on each of several soybean varieties or lines. Knowing the race of cyst nematode in a given field can assist the grower in making decisions about which resistant varieties should be used. For example, if a field has race 1 or 3, then selection of a variety resistant to these races would be the appropriate choice. Centennial and Forrest are two examples of soybean varieties that are resistant to races 1 and 3. Centennial or Forrest, however, are susceptible to races 2 and 4 which are among the most common races in North Carolina.

Unfortunately, many growers have relied on resistant varieties as a sole means of controlling this pest. Continued use of one resistant variety generally results in a change in the nematode population's ability to attack "resistant varieties", referred to as a 'race shift'. If the grower starts with race 1 and grows the variety Forrest for 3 to 5 years, the population may shift from race 1 to race 2 or 4. Thus, the grower needs to know the race of cyst nematode present in addition to the population density. The Nematode Advisory Service (NCDA, North Carolina Department of Agriculture, Nematode Advisory and Diagnostic Lab, 4300 Reedy Creek Rd., Raleigh, NC, 27607-6465) will perform race determinations on a limited number of samples (at $2.00 per sample). Samples for race determinations should include 1-2 quarts of soil and roots taken from several locations in a field. Assays to ascertain the race of cyst nematode need to be taken during late summer before the nematodes enter a dormant state. The Nematode Advisory Service may not designate the race, but will advise you as to which resistant varieties are appropriate to your situation. A new soybean variety, Hartwig, apparently is resistant to all races of SCN. Hartwig, however, does not have the yield potential of the older resistant varieties. Thus, the use of the varieties resistant to the specific races present is still a desirable option for many growers.

Terminology for SCN Resistant Varieties

Some confusion exists over terminology used to describe SCN-resistant soybean varieties. Originally, four races of soybean cyst nematode were described (races 1-4). This was expanded to five and then to six. Around 1990, a system was developed to describe 16 races of soybean cyst nematode. However, all 16 races have not been discovered (are not known to occur) at this time. The most common races are 1-6, 9 and 14. Some companies and cooperative extension publications still use the old classification (this is and was a valid decision, since many researchers, extension personnel and industry representatives consider the new system to be confusing). Other companies and government agencies use the new system, believing it to be more accurate. Under the old system varieties fell into a few categories: i) resistant to races 1 & 3; ii) resistant to race 3; and iii) resistant to races 3 & 4. Centennial and Forrest are examples of race 3 resistant varieties that are not resistant to race 4 of cyst nematode. Many varieties resistant solely to race 3 may also be resistant to race 1, but were developed in the midsouth and never evaluated for race 1 resistance. These varieties are rarely resistant to cyst nematode populations found in North Carolina today. Varieties with resistance to races 3 & 4 generally possess a higher level of resistance to cyst nematode than race 3 resistant varieties. For the most part, however, varieties considered to be resistant to race 4 are actually resistant to races 3 & 6, 3 & 9, 3 & 14 or a combination of these. The point of this discussion is that varieties with resistance to races 3 & 4 are generally equivalent to these other varieties with resistance to races 3 and 9, or 3 and 14. An example would be TN5-95 which is listed as being resistant to races 3 & 4, but is actually resistant to races 3,9 & 14. In general these varieties (resistant to races 3 & 4 or 3, 9 & 14) are preferred when cyst nematode is present since they generally possess a higher level of resistance than earlier resistant varieties (resistant to races 1 and 3, or 3 only), such as Forrest or Centennial.

Many North Carolina populations of soybean cyst nematode are classified as race 2. The aforementioned varieties are not listed as resistant to this race of the nematode. Some research, however, has indicated that many of these varieties possess some resistance to race 2. New resistant varieties with yet higher levels of resistance to soybean cyst nematode are on the horizon or are available now. For example, the variety Fowler has high levels of resistance to races 2, 3, 5, & 14 of cyst nematode. The public variety Delsoy 5710 is resistant to all races of cyst nematode.
Columbia Lance Nematode

The Columbia lance nematode, Hoplolaimus columbus, has become established in much of the southern portion of the North Carolina coastal plain. The distribution of this nematode seems to be restricted to the sandier soils found in these areas. This pest can severely damage cotton and soybean. It may occur with other nematodes such as soybean cyst, reniform, lesion, root-knot and sting.

Management of the Columbia lance nematode is difficult because of the limited acreage of rotational crops available (peanut, tobacco and small grains). Several tactics to prevent soybean yield suppression caused by this pest can be used, however. Soils in the affected part of the state tend to have hard pans. Sub-soiling often is as effective as a nematicide treatment in increasing soybean yield. Hard-pan management should thus be a primary concern if this nematode is present in a field. Certain soybean varieties have some level of tolerance to this nematode. Tolerant varieties are not resistant, but these varieties will suffer only about a 10% yield loss if this nematode is at damaging levels. Growers should use tolerant soybean varieties with caution, since a cotton crop grown in rotation with a tolerant soybean variety may still be damaged.
Root-Knot Nematode

Management of root-knot nematodes can be problematical because four species of this nematode (Meloidogyne incognita, M. arenaria, M. javanica and M. hapla) may attack soybean. Rotations for managing root-knot nematodes also are difficult because of their wide host range, including virtually thousands of crops and weeds. The species of root-knot nematode must be known in order to properly formulate a control strategy.

Management of root-knot nematodes relies on the use of resistant varieties and crop rotation. Frequently, the rotational crop may be of greater value than the soybean crop. In these cases, selection of a soybean variety with resistance is the preferred choice and should benefit subsequent crops other than soybean. If numbers of root-knot nematodes are high following a cotton crop, a grower should consider using a root-knot resistant cotton variety in the rotation also. Until a few years ago, root-knot nematodes were rarely a problem in soybean in North Carolina. Currently the majority of soybean varieties grown in North Carolina range from moderately to highly susceptible to this nematode. The northern root-knot nematode, M. hapla, rarely causes problems in soybean in North Carolina. Some varieties have moderate to high levels of resistance to the Peanut root-knot nematode M. arenaria and the Javanese root-knot nematode M. javanica. Variety selection is the most important tactic to use against these nematodes and may aid in coping with nematode problems in rotational crop such as tobacco, vegetables and peanut that may be damaged by the high numbers of these nematodes that buildup on a susceptible soybean variety.
Reniform Nematode

The reniform nematode (Rotylenchulus reniformis) like lesion and Columbia lance nematode is largely restricted to the southeastern portion of the state. Unlike these other species, however, it may be found in any soil type. Cotton, many vegetables, tobacco and soybean are good hosts for this nematode. Corn and the certain soybean varieties are moderately to highly resistant to this parasite and resistant varieties should be considered as options, or in rotation, where reniform nematode is a problem.

Phytophthora Rot

This disease is caused by the soilborne fungus Phytophthora sojae. This fungus is widely distributed in North Carolina, but is most serious in heavy, poorly drained soils. It may cause a seedling disease or pre- and post-emergence damping-off, but is most common after soil temperature increases and high rainfall results in soils waterlogged for long periods. Soybean varieties differ in their tolerance and/or resistance to Phytophthora rot. Therefore, symptoms may vary in their severity, depending on the relative susceptibility of the variety planted. Symptoms on older plants are root rot, wilting, and death of plants. The stem of highly susceptible cultivars is usually dark brown, and this discoloration may extend from the base of the plant to more than 10 inches above the soil line. Leaves remain attached to wilted plants. The vascular and cortex tissue of the plant has a brown-to-black discoloration and "pith discing" may be visible in dead plants. The disease may appear to be restricted to low-lying areas of the field, portions of the field with heavier soils, or the entire field may be affected if poorly drained.

Soybean varieties resistant or tolerant to this disease are available. Resistant varieties should be used when disease pressure is very high. In most instances, however, varieties with field tolerance to this disease are adequate for conditions in North Carolina. Some stunting and loss of stand may occur with a tolerant variety, but this is usually not at levels that are economicaly damaging.

Frogeye Leaf Spot

Frogeye leaf spot is a disease of soybean leaves stems and pods. Lesions caused by this fungus (Cercospora sojina) are distinctive in that they are brown spots surrounded by a narrow red to reddish brown border. Seed from infected plants will often carry the disease. Many soybean varieties have a high level of resistance to this disease, while other varieties may show various levels of susceptiblity to this disease. If frogeye leafspot has been a problem in the past or disease is severe in a field this year, resistant varieties should be considered for use in subsequent years.

Other Resources

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For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.

Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data.

Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact your county North Carolina Cooperative Extension Service agent.

Published by North Carolina Cooperative Extension Service Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Last update to information: May 2000
Last checked by author: May 2000
Web page last updated Nov. 2000 by A.V. Lemay.