Soil Variables Associated with Sudden Death Syndrome in Soybean Fields in Iowa.

Sudden death syndrome, caused by Fusarium solani f. sp. glycines, has increased in prevalence in soybean production regions in the North-Central United States. Little is known about soil factors and environmental conditions that influence disease severity in this pathosystem. We studied associations between biological, chemical, and physical soil variables and severity of foliar symptoms of sudden death syndrome in nine commercial soybean fields in Iowa during 1995 and 1996. Disease was patchy in all fields, and soil samples were collected in each field along a transect that ran from a symptomless area through a diseased area. There were 25 sampling stops along each transect, separated by distances of 1.5 to 2.5 m. At each stop, soil samples were collected and soil strength, soil moisture, and foliar disease severity (at plant growth stage R6) were measured. Soil samples were assayed for population densities of F. solani f. sp. glycines, cysts of the soybean cyst nematode (Heterodera glycines), and for chemical variables (soluble salts, pH, organic matter, cation exchange capacity, and concentrations of P, K, Ca, Mg, Mn, and Fe). Cross-correlation analyses were carried out to test for associations between soil variables and disease severity in individual fields, while discriminant analysis was used to assess the effects of soil variables across all fields. Disease severity showed consistent associations with F. solani f. sp. glycines populations (strong effect) and H. glycines cyst counts (minor effect). Available K was identified as a possible disease-enhancing factor, but the magnitude of its effect was dependent on the overall K-concentrations in the fields. For example, as the median K-concentration increased, the correlation between K and disease decreased. None of the other soil variables showed consistent associations with disease. The results suggest that localized presence or absence of F. solani f. sp. glycines is the chief reason for the patchiness of sudden death syndrome in affected fields. Thus, manipulation of soil nutrient status or fertility level appears to have limited potential for reducing disease in the high-yield soybean production environment of Iowa. Instead, producers should focus on preventing the establishment or reducing populations of F. solani f. sp. glycines and H. glycines in their fields.