2018 Faculty Update: James Kurle
By: James Kurle
The Soybean Pathology project focuses on managing diseases of soybean found in Minnesota. Minnesota agriculture is increasingly dependent on the productivity of two crops, corn and soybean. Together these two crops are planted on more than half of the cropping area of Minnesota. With essentially two crops grown on large areas, the disease management benefits of crop rotation are no longer available. In some instances diseases such as diseases caused by Pythium species, the pathogen infects both crops limiting plant stands and crop yields of both corn and soybean. Sudden Death Syndrome a pathogen of soybean proliferates on corn residues and infects corn plants without affecting corn yields. Fusarium root rots, particularly root rot caused by Fusarium graminearum causes both stalk rot of corn and seed and seedling rot of soybeans. Because of the limited disease management options available to farmers, I’ve emphasized identification of resistance, phenotyping for resistance, as the emphasis of research conducted on my Soybean Pathology project. Grace Anderson, scientist in my laboratory, has directed an energetic group of student laboratory technicians in screening 280 early maturity soybean varieties that are ancestral to public varieties in the University of Minnesota Soybean Breeding program. In collaboration with Dr. Aaron Lorenz and Dr. Jim Orf of Agronomy and Plant Genetics and Drs. Nevin Young and Yong Bao of the Plant Pathology Department this effort has resulted in identification of markers for resistance to root infection by the pathogen causing Sudden Death Syndrome, for tolerance to Phytophthora sojae, and for potential sources of resistance to Pythium irregulare, Pythium ultimum, and Fusarium graminearum.
To evaluate the varieties for resistance to infection by these pathogens or to induce symptom expression, we have investigated a variety of measures of disease resistance. These include traditional root lesions and foliar symptoms such as chlorosis, wilting, or defoliation and proxy measures of resistance including changes in leaf reflectance and leaf temperature. A novel but important aspect of our research has resulted from our collaboration with Dr. Cory Hirsch and doctoral candidate, Erin Gilbert, who are examining hyperspectral reflectance, changes in foliar reflectance occurring early in plant development when they are still invisible to the naked eye. This research is promising because it has the potential to accelerate phenotyping for resistance and reduce a process that can now take a month or more to relative short periods requiring only a week or two of plant growth. On the other hand, this research may also lead to identification of foliar signatures that enable recognition of diseased areas in soybean fields and facilitate application of precision pesticide applications or planting of resistant varieties.
Difficulties that we have experienced in producing disease symptoms indicate too that published procedures for establishing root and foliar diseases can be inadequate to meet the demands of molecular techniques for consistent, reliable, and repeatable disease symptoms. The process is complicated by environmental requirements; temperature, moisture, and light requirements, necessary for symptom expression. Variable conditions found in growth chambers can cause conflicting results. In the greenhouse, wide fluctuations caused by seasonal changes in light intensity that cannot be adjusted by supplemental lighting and accompanying temperature variations are an additional challenge.
To guide our breeding efforts we continue to conduct regular surveys of pathogen species and pathotypes present in Minnesota soybean producing areas. We recently completed a survey of Pythium species found in Minnesota soybean growing areas where we identified more than 30 species and three new, closely-related species from the genus Phytopythium. An additional result of this survey is the observation that many of the species found appear to be pathogens of both soybeans and corn. The large number of Pythium species infecting both soybeans and corn is an explanation for the difficulty we have experienced in detecting varietal resistance to Pythium spp. since previous surveys have acknowledged the presence of only three or four species and emphasized the role Pythium irregulare and Pythium ultimum in causing seed, seedling, and root rots of soybean and corn. Using molecular techniques to verify our species identification we have been able to phenotype soybeans for resistance using isolates confirmed to be P. irregulare and P. ultimum.
We have also begun a region-wide survey of Phytophthora sojae pathotypes. The survey is a collaboration among nine North Central soybean growing states with the objective of identifying the pathotypes or races that are present in soybean growing areas. Phytophthora sojae is an important yield-limiting pathogen of soybean that can be controlled by varietal resistance, the widely used Rps genes, partial resistance, and application of seed treatment fungicides. Reliance on single gene resistance selects for pathotypes capable of overcoming frequently used resistance genes. In the most recent pathotype survey, completed five years ago, we observed a pronounced increase in the number and complexity of P.sojae pathotypes throughout the region. In order to maintain the effectiveness of resistance available to farmers, soybean breeders will use the information gained in this survey in breeding new soybean varieties. In Minnesota we are working with farmers, extension personnel, and crop consultants to identify and sample fields exhibiting symptoms of Phytophthora. This cooperation is a win-win for both farmers and for us since the information obtained in the survey guides variety development generally and aids the individual farmers in selecting varieties resistant to the P. sojae pathotype found their fields.
This research brings together many different levels of cooperation, support, and effort. I represent cooperation among researchers in nine North Central states and by researchers here at the University of Minnesota. It is supported by grants from the Minnesota Soybean Research and Promotion Council, the North Central Soybean Research Program, and the Minnesota Agricultural Experiment Station.