Irrigation and inoculation treatments that increase the severity of soybean sudden death syndrome in the field

The occurrence of sudden death syndrome (SDS), caused by the fungus Fusarium solani (Mart.) Sacc. f. sp. glycines (FSG) (syn. Fusarium virguliforme Akoi, O'Donnell, Homma and Lattanzi), is unpredictable in soybean [Glycine max (L.) Merr.] field trials making it difficult to evaluate soybean for resistance to the pathogen. Our objective was to evaluate the effect of field inoculation, soil compaction, and irrigation on the occurrence and severity of SDS symptoms. Six inoculation treatments were tested which included applications of FSG-infested grain planted in the furrow with the soybean seed, broadcasted and incorporated into the soil before planting, or placed below the soybean seed just before planting. Soil was compacted by driving a tractor across the field once in early spring. Irrigation treatments were applied at combinations of growth stages V3, V7, R3, R4, and/or R5. Significant increases in foliar SDS severity were observed from inoculation and irrigation treatments (P < 0.05), but not from compaction treatments. The inoculation treatments that placed inoculum close to the seed resulted in the greatest foliar severity. Irrigation treatments during mid to late reproductive growth stages resulted in significant increases in SDS foliar symptom development. These results increase our understanding of what environmental conditions increase SDS field symptoms and will be useful to researchers establishing SDS field nurseries.