Updated November 30, 2020:
Resistance to Important Soybean Diseases
Midyear progress report, December 1, 2020
Berlin D. Nelson Jr.
The primary objectives of this research are to identify sources of resistance to major soybean pathogens such as Fusarium virguliforme and Phytophthora sojae and screen NDSU breeding material for resistance to major pathogens.
In August of 2020 a survey for sudden death syndrome (SDS) was conducted in soybean fields in Richland County to determine if the disease has spread since it was first identified in 2018. This was conducted in areas around where the first field with SDS was found in August 2018. In the 2019 survey, we did not find any new fields with symptoms. The 2020 survey covered an area of approximately 400 square miles which was about 28% of the county. Some fields within this area were brought to the attention of the survey crew by crop scouts working in the area. Plants with typical SDS symptoms were collected from fields and the DNA was extracted from root tissue. The DNA was then tested for evidence of Fusarium virguliforme by PCR analysis using F6-3 and R9 primers specifically for this pathogen. Positive and negative controls were used in the analysis. The PCR test verified 12 fields in Richland Co. were positive for SDS. In addition, a field in Cavalier County, ND, was also observed with SDS symptoms by extension agents in the area and plants were sent to be analyzed using PCR. That field was also verified to have SDS. Although most of the fields observed in this survey had small patches of plants showing foliar symptoms of SDS, several of the fields had large areas with symptoms. The results from this research point out that SDS is a potential future problem for soybean growers in North Dakota. The disease is now well established in the area and because this is a soil borne pathogen, it will readily spread from field to field by agricultural equipment and the natural movement of soil by environmental conditions. It will be important that growers, crop scouts, and others managing soybean fields can identify this disease when it first appears. An important management tool that will likely be needed in the future will be SDS resistant soybean varieties.
In May of 2020, we initiated a field experiment to test the resistance of soybean varieties/plant introductions for resistance to sudden death syndrome. This was part of our effort to identify sources of SDS resistance that the NDSU breeding program could use in development of SDS resistant lines. We identified seven SDS resistant and three susceptible varieties from published research with seven of them in maturity groups I and 0. Five of the resistant varieties (MN302, Evans, Merit, P15A and P19A and three susceptible varieties (Spencer, Barnes and McCall) were planted into soil infested with F. virguliforme in large plastic pots. The inoculum (isolate P10-2 from North Dakota) was wheat seed colonized by the pathogen over three weeks, then mixed into La Prairie silt loam. The pots were buried in Fargo clay with part of the bottom removed so roots could grow into the soil under the pots. There were ten seeds per pot with four replications. A known SDS positive control (isolate 13FV mol) was included in the test on Barnes. There were ten seeds per pot and four replications per soybean variety. A non-inoculated control for each variety was included in the test. Emergence was recorded at 9 days following planting, the number of plants with the first true leaves were counted at 16 days and the number of plants with the second trifoliate leaves were counted at 21 days. Then plants were thinned to 6 plants per pot. Plants were watered each week for 6 weeks to maintain high soil moisture and promote SDS infection and disease development.
The results of the field tests showed that soil infested with F. virguliforme significantly reduced germination on some varieties and retarded plant growth. The resistant varieties P19A and MN0302 had the highest germination in the infested soil compared to the other varieties. P19A averaged 9 plants per pot while Spencer, the susceptible, averaged 3.5 plants per pot. P19A also had the greatest number of plants with true leaves after 16 days. The SDS resistant varieties, however, did not have the highest number of plants with the second trifoliate leaves after 21 days. At 7 weeks following planting, infected plants that were still growing averaged six inches shorter than those growing in non-infested soil. All plants were monitored for foliar symptoms throughout their growth, but typical foliar SDS symptoms did not develop in this experiment. The reasons for that are unknown. Plants were harvested on October 7 and the number of pods and the seed weights per plant were determined. P19A and Evans had the highest number of pods per plant and P19A had the highest seed weight per plant in the inoculated plants. In the uninoculated plants, P19A also had the greatest number of pods per plant and the highest seed weight. The results, however, were mixed in the inoculated plants as the SDS resistant varieties did not always trend toward higher yield compared to the susceptible varieties. The resistant varieties tended to have a lower percentage of root rot on the tap root compared to the susceptible varieties Barnes and McCall.
During the summer of 2020 soil was collected from 147 soybean fields in nine counties of SE North Dakota to initiate a study on the virulence of Phytophthora sojae, the cause of Phytophthora root rot of soybean. This soil is stored in the greenhouse and will be used to bait P. sojae from soil and then test virulence of the isolates on a set of eight differential cultivars. In the summer of 2021, soil will be sampled from other soybean producing counties. This information has a direct impact on the types of resistance that will be needed in future soybean varieties for North Dakota.