Sudden death syndrome, SDS, of soybean [Glycine max (L.) Merr.] is caused by the soilborne pathogen Fusarium virguliforme (formerly called, F. solani f. sp. glycines; SDS; Aoki et al., 2003) and results in chlorosis, necrosis and eventual defoliation of plants if symptoms are severe enough. Sudden death syndrome was first observed in the United States in 1971 in Arkansas. Research concerning agronomic practices and management of SDS has been conducted in the south but to our knowledge this research does not exist for Iowa. Soybean cyst nematode (Heterodera glycines Ichinohe; SCN) and SDS are consistently the most yield damaging soybean pathogens in Iowa, therefore research regarding SDS and agronomic practices is needed in order to provide growers in the state with appropriate management recommendations. This thesis work was conducted to evaluate the effect of agronomic practices of maturity group, planting date and row spacing on SDS foliar symptom development and soybean yield. Chapter two is a literature review. The research comprising this thesis is presented as manuscripts in chapters three, four and five.

Chapter three evaluates the effect of planting date on SDS foliar symptom development and soybean yield at two locations in Iowa. Research has demonstrated that SDS disease expression is influenced by moisture and temperature. Greatest SDS disease expression results when temperatures are cool at planting and warm during reproductive growth while moisture is adequate throughout the season. Early planting of soybean is a critical practice for producers in Iowa to maximize yield potential. Because the environmental conditions favoring early onset of SDS foliar disease expression are similar to environmental conditions in Iowa during the time of early planting, we hypothesize that early-planted soybean in Iowa will experience higher levels of SDS disease expression than late-planted soybean.

Chapter four addresses the response of SDS foliar disease symptoms and soybean yield to row spacing and seeding rate at two locations in Iowa. Row spacing and seeding rate are agronomic practices employed by growers to achieve maximum yield. Both narrow rows and soybean seeded at higher populations exhibit greater leaf area than soybean planted in wide rows and lower populations. We hypothesize that this characteristic of narrow rows and high seeding rates will allow soybean to lessen yield loss to SDS as SDS can function to defoliate plants. No information exists on the relation of SDS foliar disease expression to row spacing and seeding rate.

Chapter five examines the effect of maturity classes and cultivar selection on SDS foliar symptom development and soybean yield. Research from Arkansas and Kentucky suggests that disease onset is a function of environment rather than maturity group. To escape yield loss to SDS in the southern US, SDS management recommendations include using early maturing cultivars so that disease onset will take place at a later growth stage compared to a later maturing cultivar. The objective of this study was to evaluate differences in SDS foliar symptom and severity among soybean cultivars with and without SDS-resistance in three classes of maturity in central Iowa. To our knowledge, this is the first study investigating the effect of classes of maturity on SDS foliar disease expression and soybean yield in Iowa.

The research presented in this thesis is the first attempt to understand the effect of common agronomic practices employed by growers in Iowa to SDS disease progression and soybean yield. This thesis provides a foundation for further investigation regarding the relation of SDS and soybean yield to planting date, row spacing and cultivar selection. From this information, recommendations to manage SDS and maximize soybean yield for soybean producers in Iowa can be made.