[Root-Knot Nematodes] [Reniform Nematode] [Phytophthora Rot] [Frogeye Leaf Spot]
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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.
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.
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.
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.
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.
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
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
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.
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.
to information: May 2000