Spatial Distributions of Rhizoctonia species in Soybean fields Undergoing Annual Rotations with Rice

Aerial blight is caused by Rhizoctonia solani AG1-IA. This pathogen also causes sheath blight of rice. In Arkansas, many soybean and rice fields undergo an annual rotation of these two crops which facilitates a continuous source of inoculum from one year to the next. Aerial blight is a two stage disease in that R. solani AG1-IA colonizes the plant during the early vegetative growth stages and then aerial blight develops after the soybean canopy closes in the later reproductive stages of development. As a result of the upper portion of the canopy often being asymptomatic, significant yield loss can occur in a field without the farmer being aware. The damage below the canopy is not visible unless an aggressive scouting regimen is practiced. In 2009, spatial sampling of fields undergoing an annual rotation of rice and soybeans was initiated. The overall objective of the work was to determine the spatial distribution of the pathogen as the fields were being rotated from rice to soybean. After, evaluation of several assay methodologies, it was determined that a modified toothpick baiting procedure was the most efficient along with plant sampling at growth stage V3. The toothpick method was determined to assay approximately 15.75 cm3 of soil at a bulk density of 1.3 g/cm3 and the isolation of Rhizoctonia spp. was made more efficient by the creation of a new selective medium (TSM) that was more effective in suppressing Trichoderma spp. and Zygomycetes. Spatial sampling determined that soil inoculum potential of the pathogen and the colonization of the plant at growth stage V3 were correlated in two fields and distributions in all fields were in agreement with the rice field levee system from the previous year\u27s rice crop. Further, aerial blight was modeled in 2009 and the greatest amount of disease was also in agreement with the levee system. The greatest recovery of inoculum and plants colonized by the pathogen occurred in the turns or areas of the levee system now referred to as logical areas of collection . Spatial analyses can be utilized to develop preferential scouting regimes and possibly precision fungicide applications to suppress pathogen activity at the stage one colonization of the soybean plant during the early vegetative growth stages of development. During the project, it was determined that Rhizoctonia solani AG11 was the predominant Rhizoctonia spp. in the fields. The distribution of inoculum of R. solani AG11 was mostly aggregated in larger agriculturally significant ranges. Spatial analyses indicated that the fungus preferentially colonized the soybean plants during the early vegetative stages of development in areas of the field that tended to be relatively dryer. Further, controlled stand experiments indicated the fungus was associated with greater plant stand across space