Bacterial biological control agents in the management of bacterial wilt(curtobacterium Flaccumfaciens PV.Flaccumfaciens) in the common bean

Abstract

Common beans (Phaseolus vulgaris) are widely grown throughout the world and provide valuable protein, vitamins and other nutrients. If the challenges related to its production are resolved, food security can be guaranteed for the growing population. Disease-causing pathogens are a major constraint in bean production. Curtobacterium flaccumfaciens pv. flaccumfaciens causes bacterial wilt of common bean disease. This pathogen causes high economic losses due to seedling death, yield loss and seed discoloration. Copper fungicides can reduce secondary spread but are expensive, harmful to the environment and lead to resistance of the pathogen. Bacterial control agents are effective and environmentally friendly alternatives to chemicals, as they provide protection throughout the crop period and are specific to plant diseases. However, less has been done in managing bacterial wilt using bacterial bio-controls. The objectives of this study were; to isolate wilt-causing pathogen in dry beans, to isolate bacterial antagonists against the pathogen from the rhizosphere of P. vulgaris and to control the pathogen in vitro and in vivo using the recovered bacterial bio-controls. The study was done in an experimental bean field located in Kimbimbi, Kirinyaga County. From infected leaves and seeds of bean plants, Curtobacterium species were isolated and characterized morphologically, biochemically, molecularly and by pathogenicity tests. Biological control agents were identified in the soil samples obtained from the rhizospheres of bean plants and screened against Curtobacterium flaccumfaciens using soft agar overlay assay. Primer pair 8F and 1492R were used for amplification of the 16S rRNA gene and sequencing. Analysis of the 16S rRNA gene was conducted to identify the Cff isolates and bacterial antagonists. Both a foliar spray and a seed treatment were used to apply bio-controls. A completely randomized design was employed for the experiment. Data on disease incidence and disease severity were analyzed using analysis of variance at a 95% confidence level. Phylogenetic analysis was done using the neighbor-joining method and phylogenetic tree constructed with MEGA software. Eleven Curtobacterium spp. were identified. Nine bacterial control agents showed antagonistic activities against Cff. A partial 16S rRNA gene sequence of these isolates identified species belonging to the genera Bacillus, Paenibacillus and Pseudomonas. Seed treatment coupled with foliar application of the antagonistic bacteria significantly reduced disease incidence and disease severity ranging from 16-29% and 4-10% respectively, compared to the control. Pseudomonas fluorescens was most successful in controlling the Cff pathogen with an efficacy of 81%, while B. cereus and P. polymyxa reduced the disease development by 79% and 68%, respectively. The use of rhizobacterial species provides a safe, effective and sustainable alternative to control bacterial wilt in the common bean. Herein, the rhizobacteria P. fluorescens, B. cereus and P. polymyxa species seed treatment coupled with foliar application significantly reduced the disease incidence and disease severity of bean bacterial wilt. The study recommends further research on distribution of bacterial wilt of common bean disease in bean-producing regions of Kenya and to determine if the isolated bacterial control agents are effective against other plant diseases