Influence of clipping recycling on the incidence of Bipolaris cynodontis on common bermudagrass

Abstract

Field and laboratory investigations were used to determine the influence of turfgrass clipping recycling practices on the incidence of Bipolaris leafspot on common bermudagrass. Weekly counts of leafspot lesions on field plots subjected to three mowing treatments were used to assess disease incidence of B. cynodontis from May through September over a two-year period. Disease activity was highest in the months of April and May and August and September and was correlated with environmental conditions promoting excess moisture on the leaf canopy. Leafspot activity, where clippings were not collected, was 30 to 50 percent higher than on field plots where clippings were bagged and removed or plots where clippings were recycled with a mulching lawn mower. Laboratory studies determined the size of clipping debris influenced nutrient release and subsequently bacterial populations associated with decomposing debris. Carbohydrate analysis of leachates demonstrated an inverse relationship between clipping size and the amount of carbohydrate in leach water. Carbohydrate release from small clippings (0.3 cm) was three-fold greater than carbohydrates released from large clippings (4.8 cm). Cyclical patterns of wetting and drying of clipping debris (0.8 cm) demonstrated increased carbohydrate release during the initial drying cycle only. After 96 hours, bacterial numbers on bermudagrass leaf fragments were seven-fold higher on the 0.3 cm leaf fragments as compared to the 4.8 cm. Bacterial numbers on field plots were shown to fluctuate in response to nutrient availability by weekly mowing practices. Bacterial numbers were highest on mulching treatment plots and lowest on bagged plots. Dilution plating experiments demonstrated that bacterial growth on clipping debris was primarily attributed to rapidly growing Pseudomonas spp. A bioassay technique using bermudagrass clippings as substrate was developed to identify bacterial isolates that suppress saprophytic growth of B. cynodontis. Twenty-five of the 450 bacterial isolates recovered during the study inhibited the growth of B. cynodontis by seventy-five percent or greater when using the bioassay system. Bacillus spp. were determined to be more inhibitory than other bacterial species. Of the twenty-five isolates exhibiting activity against B. cynodontis twenty-two were Bacillus spp.