Effect of azoxystrobin on turfgrass phyllosphere microbial populations and disease enhancement

This study documents dollar spot enhancement months after application of azoxystrobin to Agrostis stolonifera and Poa pratensis . One possible cause of disease enhancement is the perturbation of foliar microbial populations. Dilution plating showed that foliar populations on the grasses treated with azoxystrobin were reduced during summer. This correlated with a two-fold increase of dollar spot incidence. Cultural morphology after 9 days distinguished 11 morphotypes, but longer culturing revealed 26 submorphotypes among isolates of yeasts, bacteria and filamentous fungi. DNA-RFLP revealed 111 ribotypes and rDNA-sequencing revealed 94 species from 450 culturable isolates. While identification by morphological characteristics was useful for filamentous fungi, molecular analysis was required for yeasts and bacteria. Thirty-eight microbial species were present only on untreated plots, while 24 species were only associated with azoxystrobin-treated plots. Reduction in microbial populations and absence or gain of some species due to azoxystrobin may help explain increases in dollar spot incidence.