Isolation of the chitinolytic bacteria Xanthomonas maltophilia and Serratia marcescens as biological control agents for summer patch disease of turfgrass

A mycelial baiting method was used to isolate chitinolytic bacteria from soils known to harbor Magnaporthe poae, the causal agent of summer patch on Kentucky bluegrass. Two bacterial isolates, identified as Xanthomonas maltophilia 34S1 and Serratia marcescens 9M5, suppressed summer patch symptom development in Kentucky bluegrass cv. Baron by more than 70 and 50%, respectively, when compared to untreated control plants in growth chamber studies. Further studies indicated that time of application and concentration of bacteria influenced the degree of disease suppression. In general, bacteria were least effective in suppressing disease when they were applied to plants at dates closest to planting, and were most effective when applied up to 4 wk after planting. In bacterial concentration experiments, the highest degree of disease suppression was observed when plants were treated with the highest concentration of X. maltophilia 34S1. In contrast, the highest concentration of S. marcescens 9M5 did not provide the highest amount of disease control. Instead, less disease suppression was observed for plants treated with bacterial concentrations greater than the observed optimal concentration of 109 cfu ml−1. Survival of both bacteria was monitored in soil and the rhizosphere after application of three different bacterial concentrations. Populations of both bacteria in the rhizosphere stabilized 18 days after the final bacterial application, regardless of the concentration. For both bacteria, however, significant differences in non-rhizosphere soil populations, which steadily declined over time, were observed between the highest and lowest concentrations used throughout the sampling period.