of Soybean Cyst Nematode
[Races of SCN] [Management] [Back to Soybean Disease Notes] [Other Resources]
nematode (SCN) is the most serious soybean disease problem 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. Soybean cyst nematode should be suspected
as a possible production problem if any of the following apply: 1) irregular
patches of stunted and/or yellow soybeans - an up-and-down pattern of
soybean growth is common; 2) soybeans grown without rotation; 3) yields
declining over several years; 4) detection of cysts on roots or a positive
report on cyst nematode from the Plant Disease and Insect Clinic or the
North Carolina Department of Agriculture, Nematode Advisory Service; and
5) failures in weed control (weed problems are frequently more severe
in soybean cyst nematode infested fields).
of cyst-nematode problems on soybean can be accomplished by several means.
Generally, a soil sample must be processed in order to positively identify
the nematode problem. If you suspect a nematode problem, take systematic
(stratified) soil samples
in the fall when nematode numbers are high. Pull 20 to 30 soil cores 6
to 8 inches deep from 4 to 5 acres. Send soil in a plastic bag and appropriate
box (obtained from county Extension agent) along with the appropriate
form and payment to the North Carolina Department of Agriculture, Agronomic
Services, Nematode Advisory and Diagnostic Lab, 4300 Reedy Creek Road,
Raleigh, NC 27607. If you know you have soybean cyst nematode, you may
request an egg assay from the NCDA. Numbers of SCN eggs are more indicative
of the severity of the problem, but egg assays may overlook other nematode
problems. If you detect a problem during the growing season, plant and
soil samples should be taken. Infected plants (stunted, but not dead)
should be carefully dug from the soil. Small, white to yellow cysts (about
the size of the head of a pin) on the root system indicate that cyst nematode
is a problem. If you cannot identify cysts on roots, your county extension
agent can assist you in identification, or forward the samples (soil and
roots) to the Plant Disease & Insect Clinic or to the NCDA Nematode Advisory
Service for diagnosis. Analysis of the soil and/or root sample for nematodes
has the advantage that it may reveal other nematode or disease problems.
Include an accurate crop history (including soybean varieties planted
previously). Information about fertility, herbicides and cultural practices
can also aid in diagnosis.
are round worms which are generally microscopic. The soybean cyst nematode
is a parasite which can reproduce on only a few hosts, such as soybean
and snap bean. SCN cannot reproduce on crops such as corn, cotton, peanut,
tobacco or wheat. These crops are referred to as non-hosts. Eggs of soybean
cyst nematodes are contained inside a cyst which is the body of an adult
female. The eggs are dormant during the winter, but from April through
June, eggs hatch and juvenile nematodes migrate through the soil. If the
juveniles fail to locate roots of a host within a few weeks, they die.
When soybeans are planted and start to germinate, roots exude substances
that stimulate more hatch and attract juveniles to the root systems. Cyst
juveniles penetrate the roots, migrating in the root until they locate
a feeding site. This first event, penetration of the root system, may
cause considerable damage to the soybean plant. As many as 10,000 cyst
juveniles have been found in a single soybean root system as few as 10
days after planting. Nematode invasion of the root system frequently results
in seedlings that are more susceptible to Phytophthora, Pythium, and Rhizoctonia
The population density of soybean cyst nematode is relatively static during winter and early spring, but is changing constantly through the soybean growing season. Eggs are dormant until March or April when juveniles start to hatch. Hatching accelerates in May and June. Juveniles must penetrate soybean root systems within a few weeks in order to survive. If soybean planting is delayed to mid- to late-June the preplant nematode density may be cut in half. Once soybeans are planted, the population starts to build up again until soil temperatures become very high in July or August. Reproduction resumes in September through October with the onset of cooler weather.
maturity group largely determines the amount of reproduction in the fall.
Group IV or V soybean varieties mature in September and October, whereas
a group VII or VIII will not mature until late October or November. Late
maturing soybean varieties allow for an extra generation of cyst nematode
to develop, which generally doubles or triples the population density
of this nematode. The early infection of soybean root systems by SCN causes
the most serious damage to the root system. In the first 2-4 weeks after
planting, 30 to 50% of the soybean plant's yield potential is determined.
Thus, the primary determinant of soybean yield is the initial population
density (or preplant population density) of this pest at planting. Later
infections may cause additional yield loss and provide a reservoir for
this pest to survive to infest the next soybean crop. These aspects of
life cycle determine the tactics we use to manage this pest. The key to
management of this plant parasite consists of limiting reproduction of
SCN such that the cyst nematode population density (the number of eggs
present at the beginning of the season) is minimized. This goal is accomplished
by a long term (2-4 years) strategy aimed at minimizing reproduction of
of SCN are characterized as races 1 through 16. A race of cyst nematode
is an indication of a field populations' ability to reproduce on selected
soybean host differentials (a set of varieties or breeding lines). Knowing
the race of cyst nematode in a given field can assist the grower in making
decisions about which resistant varieties should be utilized. 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.
Management of SCN requires the development of both long- and short-term strategies which promote soybean health and minimize reproduction by this pest. Elimination of other production limitations should take first priority. Good soil fertility and sub-soiling where a hard pan is present can be especially helpful since these problems can compound the damage caused by this nematode.
Rotation: Crop rotation is an effective means of managing
SCN. Planting a nonhost crop such as corn, tobacco, peanut, cotton or
grain sorghum for one year can significantly reduce the SCN population.
Two years of a nonhost may be necessary to reduce the SCN densities to
levels which will cause no damage, especially in sandy soils. Soybean
is more tolerant of SCN in heavier soils than in sandy soils, thus growing
soybeans every third year may be the best option on light land; whereas,
growing soybeans every other year on heavier land may be an economical
solution. Some damage from SCN will generally occur in rotations of less
than 3 years (soybean grown every third year), but it may be at tolerable
practices: Various cultural practices can impact cyst nematodes
and soybean yield loss. Damage caused by SCN tends to be greater in early
than in late planted soybean since the population density declines from
April to June. Yields of late-planted soybean, however, tend to be somewhat
lower than those planted early. Growers can take advantage of this fact
by planting the land most subject to damage by SCN last.
Chemical control: Several nematicides are labeled for use on soybean. The high rates required for control of soybean cyst nematode combined with the low price of soybean, however, has generally resulted in negative economic returns for growers. For these reasons, currently available nematicides are not generally recommended for control of SCN on soybean.
Resistant varieties: When available, resistant varieties are an economical means of managing SCN. Resistant varieties, however, are not a cure-all. The resistant variety must be matched to the race of SCN to which it is resistant and then used judiciously. Seed companies and the North Carolina Cooperative Extension Service publish information on soybean varieties and relative resistance or tolerance to various nematode species and races. Currently, SCN resistant varieties fall into several categories: resistant to races 1 and 3, resistant to race 3, resistant to races 3 and 4 (3, 9, and 14 by the new scheme), or resistant to all races. Many of the cultivars resistant to race 3 may also be resistant to race 1, and some cultivars resistant to races 3 and 4 may also be resistant to race 1 and partially resistant to race 2. When practical, resistant varieties should still be grown in a rotation with nonhosts and/or susceptible varieties.
Resistant varieties that limit reproduction of SCN are still attacked by SCN. A resistant variety may still be damaged by SCN if it follows a susceptible crop, but will still out-perform a susceptible variety. The more frequently a resistant variety is grown, the sooner a race shift will occur to end the usefulness of this resistant variety (this may or may not be the case with Delsoy 5710). Although many of the new resistant varieties have good yield potential, frequently a susceptible cultivar will outyield a resistant cultivar provided the nematode preplant population density has been reduced. A highly resistant soybean variety will suppress the nematode population density such that a susceptible cultivar could be grown in rotation with it. Varieties resistant or tolerant to SCN are available. There is a distinction between a resistant variety and a tolerant variety. Resistant varieties "resist reproduction by SCN" and thereby limit damage caused by this pathogen. Tolerant varieties "do not resist SCN reproduction". Nematode reproduction is unrestricted on most tolerant varieties, and yields of tolerant varieties are less affected by the nematodes than are susceptible varieties. Tolerant varieties may be an option if the race of SCN present cannot be controlled by a resistant variety. Tolerance of some soybean varieties has been noted, but has not been adequately evaluated in North Carolina.
1. Suggested rotations for managing soybean cyst nematodes with resistant
2. Suggested rotations for managing soybean cyste nematode without resistant
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.
update to information: May 2000