Interaction of temperature, soil moisture, seed treatment, cultivar, and soybean cyst nematode in root rot of soybean

The root rot complex comprised of Fusarium species, Rhizoctonia solani, Phytophthora sojae, and Pythium species is a major contributor to yield loss in soybean. Due to its wide distribution, Fusarium solani is possibly the most important contributor to losses caused by this complex in Minnesota. Heterodera glycines, commonly known as soybean cyst nematode (SCN), is the most destructive soilborne pathogen of soybean. Yield losses to Fusarium root rot and SCN for 2005 in Minnesota were estimated to be 85,000 tons and 250,000 tons, respectively. The etiology of seed, seedling, and root rots of soybean in Minnesota is poorly understood, and the interactions between the major root infecting pathogens of soybean are even less well understood. Root rot of soybean during germination and emergence is a major cause of stand establishment problems. Seed treatment fungicides have provided inconsistent control of pre-emergence seed, seedling, and root rot during field trials in Minnesota, and cultivars resistant to native isolates of F. solani are not available. Previous studies have investigated the interaction between SCN and Fusarium virguliforme, but not between SCN and F. solani. This is important, since SCN is prevalent in agricultural soils in Minnesota. My objectives were to (1) determine the relationship between soil temperature and moisture in root infection by Fusarium spp., Pythium spp., P. sojae, and Rhizoctonia spp. in natural soil samples taken from soybean fields in Minnesota, (2) determine the frequency of root infection as a function of seed treatment and sampling time after planting in natural soil, and the soil temperature and moisture conditions at which seed treatments are effective (or not effective), (3) determine the predominant native Fusarium species infecting soybean in Minnesota soil as a function of temperature and soil moisture, (4) screen soybean varieties in maturity group 2 and earlier for partial resistance to Fusarium spp. naturally present in soil, and for partial resistance to an isolate of F. solani artificially inoculated in sterile sand, and (5) determine the effect of soybean taproot infection by an isolate of F. solani on secondary root tip infection by SCN, and vice versa.