Biocontrol agents from cucurbit plants infected with Xanthomonas cucurbitae for managing bacterial spot of pumpkin

Bacterial spot, caused by Xanthomonas cucurbitae, is an important disease of cucurbits worldwide. Considering the absence of resistant cultivars, along with the limited effects of chemical treatments, we evaluated the use of potential biocontrol agents for management of the disease. We collected 1,134 bacterial isolates associated with bacterial spot symptomatic leaves and fruits and screened in vitro for their antagonistic activity against X. cucurbitae. A total of 271 isolates inhibited colony formation of X. cucurbitae in a disc diffusion assay. Species of these antagonistic isolates were identified based on 16S rRNA primers and phylogenetic analysis using four housekeeping genes. Based on results from the in vitro assays, the strong inhibitors of X. cucurbitae were selected for greenhouse and field evaluations. We found that Bacillus amyloliquefaciens, Bacillus velezensis, Pantoea agglomerans, and Pseudomonas putida were able to prevent or limit development of bacterial spot on pumpkin plants in the greenhouse and field studies. Additionally, Bacillus amyloliquefaciens and Pseudomonas putida increased growth of 'Howden' pumpkin (Cucurbita pepo). Our results suggest that these biocontrol agents can be used for managing bacterial spot disease and enhancing the growth of pumpkin plants.