Ohio State University FactSheet

Department of Horticulture and Crop Science

2021 Coffey Road, Columbus, Ohio 43210-1044

Weed Hosts of Soybean Cyst Nematode in Ohio

AGF-145-00

R. Venkatesh, S.K. Harrison
Department of Horticulture and Crop Science

R.M. Riedel
Department of Plant Pathology

Soybean cyst nematode (SCN) is a destructive pest of soybeans that was first detected in Ohio in 1987. A statewide survey conducted by OSU scientists indicated that by January 2000, SCN had been detected in 55 soybean-producing counties throughout Ohio, and is now regarded as the most economically important pathogen of soybeans in the state. Average soybean yield losses attributed to SCN in Ohio are estimated to be 15%, but yield damage can range from slight to near-total losses depending on various factors including severity of infestation, soil type, soybean variety, weather conditions, and other stress-inducing factors (e.g., presence of weeds or other pathogens).

Symptoms of SCN infection in soybeans may not be evident under low infestations or when good soybean growing conditions exist; however, yield reductions may still occur from relatively low levels of infestation. Soybean symptoms and yield losses due to SCN can occur in any soil type but are more apparent in light or sandy soils. Visible symptoms in moderate to severe cases include soybean stunting and foliar chlorosis, decreased nodulation, and slow canopy formation. Severity of symptoms increases under periods of low soil moisture or in combination with other stress-inducing factors that adversely affect normal soybean growth.

Above-ground symptoms in SCN-infected soybeans are most evident in July and August, which are also good months to carefully inspect soybean roots for the presence of SCN. When present, foliar symptoms of SCN-infected soybeans tend to occur in patches and may be confused with nutrient deficiencies or stress symptoms caused by other factors. As a result, field diagnosis of SCN infection is more accurately made by careful inspection of soybean roots by a trained individual. The egg-bearing cysts indicative of SCN’s reproductive stage are attached to the sides of roots and are whitish to brown in color. Though smaller than the head of a pin, SCN cysts are visible to the naked eye and are distinguishable from soybean nodules by their much smaller size, often lighter color, and lemon-like shape when viewed under magnification. Laboratory examination is required for positive identification of SCN and for determining actual SCN population levels in soil.

For detailed information on SCN’s life-cycle, detection, and recommended control measures, please refer to OSU Extension Fact Sheet AC-39-98: Soybean Cyst Nematode.

Weeds as Alternate Hosts of SCN

Soybean cyst nematode is a plant parasite that has a relatively broad host range, but the only major U.S. crop which serves as a host plant is soybean. It has been known for many years that certain species of weeds growing in and around soybean fields may serve as alternate hosts of SCN. Weeds that are highly compatible hosts of SCN interfere with nematode control efforts by providing secondary sites for SCN feeding and reproduction when soybeans are not present. Although use of SCN-resistant soybean varieties or rotation to non-host crops like corn, wheat, or alfalfa are effective ways to reduce SCN population levels in soil over time, the presence of SCN host weeds in any of those crops reduces crop rotation’s effectiveness in reducing SCN egg numbers in soil. One of the standard recommendations given to growers over the years for reducing SCN populations is to keep infested fields free of weeds. In fact, however, the majority of weeds common to soybean-growing areas of the U.S. do not serve as alternate hosts of SCN. To avoid the unnecessary expense of attempting to maintain SCN-infested fields completely weed-free while implementing best management practices for SCN control, it is essential to a) identify weeds that are present and determine if known SCN host weeds are present, then b) develop the most economical and effective way of controlling those weeds in and near infested fields.

Adaptation of SCN populations to new environments, cropping practices, and nematode control efforts has led to the development of several races of SCN that vary in their ability to reproduce on specific soybean varieties. At least 16 races of SCN have been identified thus far. Since some races of SCN may feed and reproduce on a given soybean variety at a greater rate than other races, it is probably safe to assume that races may also vary in their compatibility with weeds identified as alternative hosts of SCN. The most common race of SCN found in Ohio is race 3, but several other races including races 1 and 6 are also prevalent in SCN-infested Ohio soybean fields.

Identification of SCN Host Weeds in Ohio

In a recent study conducted at The Ohio State University, several weed species common to Ohio field crops were screened to determine if they were alternate hosts of race 3 SCN. Only broadleaf (dicot) weeds were selected for screening since there are no known grass hosts of SCN. Twenty-two broadleaf weed species representing 13 botanical families were selected for screening (Table 1). The screening process consisted of inoculating several replicates of container-grown weed seedlings with SCN eggs and then maintaining them under optimal growing conditions for a period of 30 to 50 days. The plants were then harvested and the number of SCN cysts formed on the roots as well as in the soil were counted. Weeds that tested positive as hosts of race 3 SCN were then tested as hosts of SCN races 1 and6.

The following weeds were identified as hosts of race 3 SCN: purple deadnettle, henbit, shepherd’s-purse, and field pennycress (see Figures A-D). The number of SCN cysts formed on purple deadnettle was almost equal to that produced on a highly-susceptible soybean variety. While number of cysts formed on the roots of other weed hosts were lower overall, significant levels of SCN reproduction were observed on henbit and field pennycress. Shepherd’s purse was the weakest host of the four species that tested positive, and the remaining 18 weeds screened tested negative and appear to be non-hosts of the nematode.

A. Purple deadnettle, Lamium purpureum L. Purple deadnettle is a member of the mint family and has square stems and a minty aroma often produced by members of that family. Its oppositely arranged leaves have petioles throughout the life of the plant and are more triangular than round and are densely hairy. The top leaves tend to have a darker purple color than lower leaves and are more tightly clustered in an upright position at the ends of branches. It produces small, pinkish-purple flowers in the terminal clusters during early spring.		B. Henbit, Lamium amplexicaule L. Henbit is a member of the mint family and has square stems. Unlike purple deadnettle, the leaves of henbit are more deeply lobed and rounded or kidney-shaped, and the uppermost leaves are attached directly to the stem (no petiole), encircling the stem at the base of the leaf. Henbit produces pink or purplish flowers that are larger than those of purple deadnettle and begin to appear early in spring, and sporadically thereafter throughout the growing season.
C. Field pennycress, Thlaspi arvense L. Field pennycress is a member of the mustard family, a large family of plants that are often difficult to distinguish from one another prior to flowering. Field pennycress has round cotyledons and produces non-hairy leaves with rounded margins showing slight or no lobing. Flowers bloom in the spring and consist of 4 white petals. The fruits produced after flowering are very distinct, and appear on several branches at the top of the plant. The fruits are flattened, circular structures with winged margins and a 2-3 mm-deep notch at the apex.		D. Shepherd's-purse, Capsella bursa-pastoris L. Like field pennycress, shepherd's-purse is a member of the mustard family and produces round cotyledons. Unlike field pennycress, shepherd's-purse has hairy leaves with margins that are usually deeply lobed. The small, inconspicuous flowers have 4 white or greenish petals, and the fruits are a characteristic heart-shaped, flattened pod consisting of two halves. The flowering stalks are usually unbranched.

The four weeds identified as hosts of race 3 SCN were further screened as hosts of races 1 and 6 SCN. Purple deadnettle was a strong host of race 1 but a weak to moderate host of race 6. Henbit, field pennycress, and shepherd’s-purse proved to be non-hosts of Ohio SCN races 1 and 6.

Importance of SCN Host Weed Life Cycle

Each of the four weeds discussed previously have a winter annual life cycle in Ohio. Winter annual weeds may germinate throughout the fall and overwinter as small seedlings which remain close to the soil surface. The low-growing seedlings can continue to grow well into the fall, even after several frosts, until they go dormant in late fall or winter. The following spring, as temperatures increase, winter annual weeds resume growth and begin to flower early in the growing season. It is not unusual for winter annuals to complete their life cycle and produce mature seed at the same time or shortly after most of our summer annual crops like corn and soybeans have been planted. Three of the winter annuals identified as SCN hosts, henbit, field pennycress, and shepherd’s-purse, can also germinate early in the spring and will then usually complete their life cycle and produce mature seed by early to midsummer.

Winter annual weeds identified as hosts of SCN in our study are relatively easy to control; however, they are rarely targeted by specific control measures due to their life cycle in relation to soybean culture. That is, winter annual weeds complete most of their life cycle during the time when summer annual crops like soybean are not present. Perhaps of greatest concern in Ohio are the facts that (1) the populations of winter annual weeds like purple deadnettle, henbit, field pennycress, and shepherd’s-purse increases significantly when soybeans and corn are grown in no-tillage production systems, and (2) the same winter annual weeds are commonly abundant in declining forage stands or hay fields where no significant weed control measures are applied. The lack of fall or spring tillage in the case of no-tillage crop production makes it possible for many winter annual weeds to complete their life cycle before burndown herbicides are applied in the spring. In doing so, these weeds may provide sites where additional generations of SCN feed and reproduce when soybeans are not present.

Weed Control Recommendations

The winter annual weeds that we have identified as alternate hosts of SCN in Ohio can germinate in the fall (September - November) or early spring (March - April), with the exception of purple deadnettle, which germinates only in the fall in Ohio. Since SCN requires 3 to 5 weeks to complete its life cycle, soil temperatures in Ohio may remain high enough for SCN to reproduce on winter annual weed hosts that germinate in the fall. In no-tillage corn or soybean fields, winter annual SCN hosts that germinate in the spring will usually be controlled by preplant burndown herbicides before SCN can complete its life cycle on the weeds. As a result, we recommend in corn and soybean fields that special consideration be given to controlling weed hosts of SCN during the fall. Scouting fields for winter annual weed seedlings can begin in the fall after corn and soybean harvest, although positive identification of weed seedlings at this time can be difficult. All areas where soybeans may potentially be grown in the future, including hay fields, should also be scouted for the presence of winter annual host weeds during the spring and early summer when the weeds are mature and easy to identify. Although control efforts have little benefit when the weeds are mature, positive identification at this stage allows one to carefully plan weed control measures for the infested area when new weed seedlings emerge the following fall. Where SCN host weeds are identified, we recommend that weed control practices be delayed until mid- to late October to allow maximum emergence of weed seedlings to occur, yet while soil/air temperatures are still high enough for herbicides to be effective.

The SCN host weeds identified in Ohio thus far can be easily and inexpensively controlled with 2,4-D (1 pint/acre), Roundup Ultra (1 pint/acre) or Touchdown (2/3 pint/acre). Where perennial weeds are also present, apply higher rates of Roundup Ultra or Touchdown in the fall according to the herbicide’s label directions. In any case, herbicides should be applied to weeds that are actively growing and before average daytime temperatures fall below 50 degrees.

References

Hammond, R.B., R.M. Riedel, and R.E. Hite. 1981. Soybean Cyst Nematode Introduced into Ohio. Plant Dis. 65(10):846.

Riedel, R.M. and A.M. Gorden. 1988. First Report of "Heterodera glycines" on Soybean in Ohio. Plant Dis. 72:363.

Willson, H.E., R.M. Riedel, J.B. Eisley, C.E. Young, J.R. Jasinski, T.A. Wheeler, P.H. Kauffman, P.E. Pierson, and M.C. Stuart. 1996. Distribution of "Heterodera glycines" in Ohio. J. Nematol. 28(4S):599-603.

Acknowledgment

This research was funded by the Ohio Soybean Council in support of the Soybean Working Group at The Ohio State University.