REPRODUCTION OF SCN ON RESISTANT AND SUSCEPTIBLE SOYBEAN CULTIVARS UNDER FIELD CONDITIONS IN NORTH DAKOTA

FINAL RESEARCH PROGRESS REPORT
TO NORTH DAKOTA SOYBEAN COUNCIL
July 2006 to June 2009

Dr. Berlin D. Nelson, Principal Investigator, Dept. Plant Pathology, NDSU.
Cooperators: Dr. Ted Helms, Dept. Plant Sciences, and Dr. Carl Bradley, Dept. Plant Pathology, NDSU.

Project Duration: July 2006 to June 2009.

Research Goals: Develop management practices for the soybean cyst nematode (SCN)

Objectives: Quantify egg production by the soybean cyst nematode on susceptible and resistant soybean cultivars under field conditions in North Dakota and compare yields between those cultivars.

Progress Report:

In 2006, 2007and 2008 we established field experiments in Richland Co., ND, in fields naturally infested with SCN to evaluate reproduction of SCN on soybean cultivars adapted to this northern region. All field sites had sandy loam soils. In 2006 and 2007 there were two experimental sites each year while in 2008 there were three sites. In 2006 to 2007 there were 3 to 4 replications at each site. Soybean cultivars reported to have resistance to SCN and cultivars with no known resistance were evaluated. At the time this research was initiated there were few SCN resistant cultivars adapted to this northern growing area.  Soil samples were taken following planting and after harvest and processed to determine the number of eggs/100 cc soil. A reproduction factor (RF) for each cultivar was determined by dividing the number of eggs/100 cc soil at harvest by the eggs/100 cc soil at planting. Plots were harvested and yields were determined. The sites in 2007 and 2008 were joint research sites established by Dr. Helms and Dr. Nelson.  Dr. Helms evaluated cultivars for yield while Dr. Nelson evaluated reproduction of SCN. Dr. Helms published the yield data for 2007 and 2008 in a separate publication made available to growers. This report presents the data on SCN reproduction.

In one of our field sites (Ward) in 2006 many plants died prior to harvest or plant growth was poor and we were unable to obtain data on yield. The damage was apparently due to Spartan herbicide combined with dry soil conditions. At the other site (Freese) yields were obtained. Eighteen cultivars reported as resistant to SCN and 6 susceptible were evaluated. The Freese site was greatly stressed by drought and combined with the high level of SCN in the soil, yields were low. Soybean yields ranged from 2 to 37 bu/A, with significant differences occurring among the cultivars (Table 1).  Most of the SCN-resistant cultivars had greater yields than the susceptible cultivars.  The overall mean of the resistant cultivars was 24 bu/A compared to 7 bu/A for the susceptible cultivars.  Many of the SCN resistant cultivars were chosen for this research due to their use of different sources of resistance genes, and some had too late of a relative maturity (RM) to be commercially acceptable in North Dakota. The SCN-resistant cultivars ideal for growing conditions in Richland County, ND (RM 1.1 or less) generally had greater yields than the susceptible cultivars with similar RMs.  The low yields in the susceptible cultivars point out the significant damage that SCN can cause in ND, especially in a drought stress season.

Egg counts following harvest in 2006 indicated that most of the resistant cultivars increased egg numbers in the soil, although some increases were small.  The RF’s are shown in Table 2 for both sites.  RF’s greater than one indicate SCN reproduction on the cultivar. Averaged over resistant cultivars for the Freese site the mean increase in egg density was 2,995 eggs/100 cc soil. However, the susceptible cultivars increased the egg density by an average of 20,912 eggs/100 cc soil.  These results point out the how fast SCN can reproduce on susceptible cultivars, especially under dry, drought stress conditions, and how egg numbers can increase dramatically within one season.  They also indicate that environmental conditions affect reproduction even on resistant cultivars. Because of the damage to plants at the Ward site, poor plant growth resulted in poor SCN reproduction, but most resistant cultivars showed no SCN reproduction (RF 1 or less) while the mean RF for susceptible cultivars was 1.8.

The same cultivars were tested again in 2007 (Tables 3 and 4) in the same fields as in 2006, but in different areas of the field.  Most resistant cultivars did not show evidence of SCN reproduction in 2007.  The 2007 growing conditions at the two sites were more favorable for soybean, especially in soil moisture, than in 2006. The data in 2007 again points out the extremely high reproduction of SCN on susceptible cultivars. At the Freese site in 2007 some plots were destroyed by rabbit damage and no data was obtained on nine cultivars.

In 2008 we established three test sites and evaluated 35 cultivars provided by seed companies that wanted cultivars tested on SCN infested soils. To measure reproduction, data was obtained from one plot of each cultivar at each site. The data was then averaged over sites to obtain an RF for each cultivar. As in 2007, there was no evidence of reproduction or only slight reproduction on most resistant cultivars (Table 5). Some cultivars reported to have field resistance to SCN, but not single gene resistance, did allow reproduction of SCN. The three susceptible checks did not show a high level of SCN reproduction in 2008 which was unexpected. The plots of the checks in the spring had mean egg densities between 7,000 to 10,000 eggs/100 cc of soil, which is sufficiently high to result in extremely high egg densities at the end of harvest under good growing conditions.  However, these three sites had high soil moisture for much of the season and plant growth was retarded during the first several months of the season. Poor plant growth, coupled with cooler soils, may have slowed SCN development on the roots and resulted in lower reproduction by the end of the season compared to previous years.  Furthermore, the high soil moisture likely contributed to a higher mortality of eggs/J2 larvae from parasites and predators during the growing season compared to a drier year. Substantial reductions in egg numbers were found between the spring sampling and fall sampling in some of the plots of the check cultivars.

The results of this research indicated the following for SCN in North Dakota: 1) SCN can reproduce to high levels on susceptible cultivars under conditions in this northern growing area, 2) resistant cultivars can dramatically reduce SCN reproduction and contribute to lowering SCN egg levels in the soil, 3) not all SCN resistant soybean cultivars have the same level of resistance to SCN reproduction, 4) under stress conditions such as drought and higher soil temperatures, SCN can reproduce on resistant soybean cultivars and 5) the amount of SCN reproduction on any given cultivar will be impacted by environmental conditions during the growing season. Even a susceptible cultivar may not show high SCN reproduction under certain environmental conditions. Although reproduction of SCN is an important component of how well a cultivar may perform on SCN infested soil, growers should always examine the yield data from a cultivar growing under SCN infestations as an equally important component of performance.  

Managing SCN egg levels using host resistance is one part of an integrated SCN management plan. Other parts of the plan include soil sampling to determine egg levels, resistance to increase yields, the use of crop rotation to help lower egg levels, and rotation of resistance sources to avoid the buildup of forms of SCN with virulence to the prevalent SCN resistance genes. In May 2009 three more SCN test sites were established between Dr. Nelson and Dr. Helms to evaluate 28 soybean cultivars for yield and SCN reproduction. Those tests are currently in progress and the results will be made available to growers.

Table 1. Yields of soybean cyst nematode (SCN)-resistant and susceptible soybean cultivars at a location infested with SCN in Richland County, ND in 2006.

a R = resistant to SCN; S = susceptible to SCN, according to company.
b RM = relative maturity.
c Fisher’s protected least significant difference (LSD) test.