Effect of azoxystrobin on dollar spot disease deelopment in creeping bentgrass (Agrostis stolonifera) and kentucky bluegrass (Poa pratensis)

Dollar spot disease, caused by the fungus Sclerotinia homoeocarpa F.T. Bennett is common on intensively managed creeping bentgrass (Agrostis stolonifera L.) and annual bluegrass (Poa annua L.), and other turfgrasses. Frequent chemical applications during the growing season are made to control this disease. Previous observations state that application of fungicides such as azoxystrobin can increase the level of disease within the same growing season. The purpose of this study was to quantify the extent of disease increase caused by an application of azoxystrobin the previous year. In three field tests on creeping bentgrass between 2005 and 2007, a single application of azoxystrobin in the late fall was found to increase the incidence of dollar plot several fold on treated plots in the following summer. In addition, this study also found a slight enhancement of dollar spot incidence on Kentucky bluegrass (Poa pratensis L.), which has not been previously reported. Although the cause of this disease enhancement is not known, the physical and chemical characteristics of azoxystrobin, along with turfgrass cultural practices and the finding of the longer term effect from this study, implies that the phenomenon stems from non-target effects on microbial populations of leaves or soil rather than a direct long term effect on the plants. Studies on phyllosphere communities may shed light on this phenomenon and point out directions for future research.