Field application of selected bacterial strains and their combinations for controlling onion and garlic white rot disease caused by Stromatinia cepivora

Abstract

White rot caused by Stromatinia cepivora (Berk.) Whetzel, is one of the most important diseases of onion and garlic in Egypt. In in vitro assays, three antagonistic bacteria, i.e., Pseudomonas fluorescens, Bacillus subtilis, Bacillus pumilus and the triazole fungicide tebuconazole were compared for their ability to suppress S. cepivora. In greenhouse experiments, the chemical treatment was the most effective, with the lowest disease incidence and severity in onion and garlic compared with the control. The antagonistic bacteria tested either individually or in combination significantly reduced the incidence and severity of white rot on onion and garlic, occupying the second rank. In general, dual and triple combinations of bacterial isolates were more effective than the isolates used individually. The combination of all three bacterial strains was the most effective treatment, decreasing disease incidence by 65.7% in onion and 55.0% in garlic, respectively. Disease severity was also decreased by 61.1% in onion and 49.5% in garlic. In field experiments the bacteria were more effective in reducing white rot disease in trials with low inoculum density (40 sclerotia/kg of soil) than with high inoculum density (600 sclerotia/kg of soil). The chemical treatment was significantly the most effective, whereas the formulation containing three selected bacterial strains occupied the second rank. Reduction of white rot disease was accompanied by increased growth parameters of onion and garlic plants grown under field conditions.