The effect of fungicides and pod removal alone or in combination on green stem disorder of soybean and further investigation of genetic components
Green stem disorder (GSD) of soybean (Glycine max L. Merr.) is when stems remain green and fleshy after pods reach physiological maturity and are ready to harvest. Little is known about GSD and its causes. Although GSD has not been shown to directly impact yield, the immature green stems may lead to harvest issues. Harvesting at the right time is essential to preserve the quality of the seed. During 2014 and 2015, field experiments located in central Iowa were used to investigate the effect of fungicides, pod removal, and their interaction on GSD incidence. The objective of this research was to: (1) investigate how different cultivars get GSD after varying levels of pod removal, which was used to simulate the result of a pod reducing stress; (2) examine the effect of foliar fungicides on GSD at seven locations across Iowa; and (3) to determine how a combination of pod removal and foliar fungicide applications affected the incidence of GSD. Studies were conducted in separate field experiments planted in Iowa in 2014 and 2015. Results indicated that cultivars varied in their response to pod removal and that pod removal in combination with fungicides of certain strobilurin chemistries produced higher incidences of GSD (P=0.06). Results varied across years, as GSD was significantly higher in all treatments in 2015 (P<0.001).
The second portion of this thesis investigates the genetic components of GSD. The severity of GSD development has been seen to vary between cultivars and location. Selection of insensitive cultivars is a possible management option to avoid the development of GSD. In Iowa 2014, we evaluated a number of parent soybean lines and their likelihood to develop GSD under field conditions. Parents were seen to differ (P<0.001) both when pods were removed to simulate the result of a pod reducing stress and when pods were left intact. The parent Maverick exhibited a significantly (P<0.05) higher amount of GSD. In 2015, two populations of recombinant inbreed lines (RILs) from selected parents were investigated in Iowa and Illinois. Initial observations show differences (P<0.001) between lines within each population and location. Future studies will gain additional phenotypic data and clarify the genetic components for GSD. Soybean farmers should be aware of the added possibility that prophylactic fungicide applications may increase GSD incidence especially if plants experience a sink limiting stress.