The North Central Soybean Cyst Nematode Project

  • Gregory L. Tylka, Ph.D. and Peggy R. Thorson, Ph.D.
      • Department of Plant Pathology
      Iowa State University
  • Ward C. Stienstra, Ph.D.
      • Department of Plant Pathology
      University of Minnesota

Abstract

A regional effort was initiated in 1993 to determine the effects of susceptible and resistant soybean varieties and nonhost crops on population densities of soybean cyst nematode (SCN), to assess yield loss due to SCN, and to increase awareness of the nematode and its management among growers in the North Central Region of the USA. This project is being conducted by cooperating scientists from universities in Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, Ohio, and Wisconsin. The project is funded by a grant from the North Central Soybean Research Program. Research and education objectives were collectively developed by the cooperating scientists. Experiments are conducted in three to five locations per state each year with as much uniformity as possible to facilitate combined analyses of data from across the region. The project coordinator visits all participating states each year, works to promote uniformity among the experimental sites, and participates infield days to promote education about SCN. Results obtained to date indicate differences in effects of standard management practices among locations throughout the North Central Region.

Introduction

Scientists from throughout the midwestern United States met in St. Paul, Minnesota in July 1992 to exchange information and discuss needs and priorities for research and extension efforts related to SCN, Heterodera glycines. At the meeting, it was decided that a coordinated regional effort was needed to standardize research methods and management recommendations and to make more efficient use of research funds. A grant proposal for a regional SCN research and education project was submitted in October 1992. In January 1993, funding for the project was approved by the North Central Soybean Research Program, an organization of eight midwestern states which coordinates and supports regional soybean research.

Once funding for the project was secured, research and extension faculty from universities in the 10 midwestern states infested with SCN were invited to participate in the project, and an organization meeting of project participants was held in St. Paul, Minnesota in March 1993. Experimental and financial details of the project were discussed and agreed upon at the organizational meeting. Detailed experimental protocols were prepared and distributed to all cooperating scientists in May 1993. A full-time project coordinator, Dr. Peggy Thorson, was hired in June 1993 to coordinate the efforts of scientists in the 10 states, to participate in extension education efforts in all participating states, to determine SCN population densities from soil samples collected at all experimental sites, and to compile, analyze, and summarize all experimental data. Following is a description of the specific project objectives and selected research results from the 1993 and 1994 growing seasons.

Project Objectives

1. To determine the effects of susceptible and resistant soybean varieties and nonhost crops on SCN population densities in SCN-infested fields in the North Central Region, and to determine whether effects of these practices vary by geographic location.

2. To develop a method to accurately assess soybean yield losses due to SCN throughout the north central United States.

3. To identify regional testing sites and develop standard operating procedures for unbiased evaluation of SCN-resistant soybean varieties.

4. To increase awareness of SCN and its management among farmers throughout the north central United States.

5. To determine distribution of SCN races throughout the north central United States.

Cooperative Research

Research is being conducted by university faculty in 10 Midwestern states. Although a list of cooperating scientists is presented in Table 1 , the scope of this project involves many others including plant breeders, extension personnel, and growers. To maintain cohesion among cooperating scientists across this large geographical area, the project coordinator visits all states each growing season, frequently participating in field demonstration days.

Soybean Variety Experiments

To determine the effects of susceptible and resistant soybean varieties on yield and SCN population densities, experiments are conducted in two SCN infested fields and one noninfested field in most participating states (Fig. 1 ). Data were collected from 27 experimental sites in 1993, and 32 sites were established in 1994. Four replicate plots of several SCN-resistant and susceptible soybean varieties (maturity groups I, II, III, and IV) are planted in a randomized complete block design at each location. Individual plots usually consist of four rows, each 20 feet or 6.1 meters long. Soil samples are taken from the center 16 feet or 4.9 meters of the middle two rows per plot at planting and at harvest. All soil samples are sent to Iowa State University, where they are processed to determine SCN egg densities. At the end of the growing season, the center two rows of the plots are harvested. Data are statistically analyzed collectively and by soybean maturity group zones (Fig. 1 ). To adjust for differences in experimental sites and to facilitate comparison of results among sites, relative yields of varieties at each site are calculated by adjusting each plot yield to a percentage of the highest-yielding variety at that particular site. Relative yields subsequently are converted to standardized yields. expressed in bushels per acre, by multiplying the relative yields by the average soybean yield for the appropriate maturity group zone. Average standardized yields for varieties in each soybean maturity group zone in 1993 are presented in Table 2 , and unadjusted average yields for varieties in each zone in 1994 are presented in Table 3 .

In 1993, differences in unadjusted and standardized yields of the resistant and susceptible soybean varieties were evident in many, but not all, experimental sites. Also, differences in yields of all varieties in SCN-infested and noninfested fields were detected. However, the magnitude of the aforementioned differences varied greatly between the western and the eastern experimental sites of the project. Initial SCN egg densities averaged 3,585 eggs/100cc soil in infested sites in maturity group zone I; egg densities averaged 4,913 and 3,034 eggs/100cc soil in zones II and III, respectively, in 1993. Final SCN egg densities were less than initial densities in most plots in several locations, even in plots planted with susceptible soybeans. Such results were typical of experiments located in the western areas of the experimental region, where excess moisture and cooler temperatures were common throughout the 1993 growing season. Generally, SCN egg densities increased on susceptible soybean varieties and decreased or stayed the same on resistant soybean varieties in the eastern experimental sites, which had average or below-average rainfall in 1993.

Results obtained in 1994 were more consistent across the experimental region than in 1993. Differences in average unadjusted yields between SCN- resistant and susceptible soybean varieties and between SCN-infested and noninfested fields were much greater in the second year of experimentation. Average initial SCN egg densities for infested sites in maturity group zones I, II, III, and IV were 1,973, 3,321, 2,217, and 1,482 eggs/100 cc soil, respectively, in 1994. SCN egg densities decreased in most plots where SCN-resistant varieties were grown, whereas egg densities increased in plots planted with SCN-susceptible varieties. Additional statistical analyses are being performed on the 1994 data.

Nonhost Crop Experiments

Experiments are conducted in several states each year to determine and compare the effects of growing corn, a SCN nonhost crop, on SCN population densities. Soil samples are collected at the beginning and end of each growing season, and SCN population densities are determined. Changes in SCN densities in soils cropped to corn are calculated and statistically compared. For example, average percent decreases in SCN egg densities in plots planted with corn in 1993 were 77%, 56%, and 39% in Iowa, Missouri, and Ohio, respectively.

Race Survey

To determine the distribution of SCN races throughout the North Central Region, SCN populations isolated from soil collected from infested soybean variety experiment locations are being evaluated. Race identification has been completed for many of the SCN isolates, but many more have yet to be tested. To date, race 3 is the most prevalent race among the experimental field sites, although races 4,6,9, and 14 also have been identified (Table 4 ).

Extension Activities

Cooperators in each state are encouraged to organize field day demonstrations to educate farmers about SCN biology and management practices. Field day Nebraska, Ohio, and Wisconsin, and the project coordinator has participated in numerous demonstrations throughout most of these states. Field days have increased growers’ awareness about SCN. Some growers have detected SCN in their fields after learning how to check for the presence of the nematode at a field demonstration, whereas others have collected soil samples and sent them to cooperating universities for testing. Since the project's inception, SCN has been discovered in counties in Iowa, Kansas, Michigan, Minnesota, Nebraska, and Ohio that had not previously been known to be infested with the nematode. Slides of the project logo, maps of the known distribution of SCN in the region (Figure 2 ), maps of experimental locations, and results are sent to cooperating scientists in participating states for use in educational efforts.

Summary

The North Central Soybean Cyst Nematode Project has been met with great interest among researchers, extension personnel, agribusiness, and growers in all participating states. The coordinated efforts of nematologists and plant pathologists from universities across the North Central Region have been instrumental to the success of the project. Grower awareness of SCN has increased dramatically as a direct result of the efforts of the project coordinator and the strategies may have very different effects on SCN population densities and soybean yields, depending on weather, soil type, and, perhaps, other factors. Results of 1995 research will be combined with existing data and analyzed in its entirety to attempt to better understand, and perhaps predict, the effects of standard SCN management strategies on nematode densities and crop yields across the region.