Biological control of root rot of cauliflower (caused by Pythium ultimumvar. ultimum) using selected antagonistic rhizospheric strains of Bacillus subtilis

Abstract

Cauliflower (Brassica oleracea var. botrytis) with a root rot disease were found in a field in Kidwan village, 3 km south of El-Minia city, Egypt, during January 2001. Cauliflower plants showed wilting soon after transplanting. In addition, cauliflower seedlings collected from the same field showed browning lesions at the basal part of the stem and the root system and eventually led to wilting. Examination of root pieces and other infected parts yielded only one type of fungus characteried as Pythium ultimum var. ultimum by its typically saclike monoclinous and diclinous antheridia and aplerotic oospores. P. ultimum var. ultimum was shown to be pathogenic on cauliflower under glasshouse conditions. This is the first report of P. ultimum var. ultimum causing disease to cauliflower in Egypt. Bacteria producing antibiotics against the root-rot fungus Pythium ultimum var. ultimum were selected. Agar discs from rhizosphere soil of cauliflower containing colonies were transferred onto water agar plate culture of P. ultimum var. ultimum. After 2 days of incubation, colonies producing clear inhibition zones were readily detected. The two strongest antagonistic bacteria were identified as Bacillus subtilis. Subsequent plant growth experiments established that substantial disease control could be obtained by applying B. subtilis to the soil. Optimal control was obtained by mixing the bacteria with the soil rather than by dipping the cauliflower roots in the bacterial suspension immediately before planting. Disease was more severe in autoclaved soil than in non-sterile soil. © 2003 Taylor & Francis Group, LLC.