PROSPECTING FOR BIOLOGICAL CONTROL AGENTS AGAINST RALSTONIA SOLANACEARUM IN POTATO

Abstract

In Africa, cultivation of potato (Solanum tuberosum) is an enormous economic activity and represents an important food source. Potato contains essential amino acids and has the capacity to produce more energy and protein per unit land than any other single food crop. Despite the importance of the crop, its productivity is constrained by biotic and abiotic stresses. Key among them is the bacterial wilt disease caused by Ralstonia solanacearum. In Kenya, the disease affects 77% of potato farms causing yield losses of up to 100%. Existing management strategies have not been effective owing to the nature of the pathogen which possesses several strains that differ in host range, geographical distribution and pathogenicity. For effective control of the disease, there is need for an integrated management approach encompassing breeding for resistance against multiple strains and biocontrol strategies. This study therefore sought to: generate knowledge on specific Ralstonia solanacearum strains causing potato wilt disease in Nakuru county, Kenya and to bio-prospect for the potential of rhizopheral bacteria inoculants as biocontrol agents for potato bacterial wilt disease. Three major potato growing areas of Nakuru County were selected, diseased potato plants and soil samples around plants collected. Fifty-four (54) bacterial isolates of the pathogen were successfully isolated from diseased plants using the Triphenyl Tetrazolium chloride medium and 20 isolates selected for further characterization. The isolates were subjected to cultural and molecular techniques to further confirmed their status as Ralstonia solanacearum. Analysis of phylotypes showed that all four phylotypes - type I - IV were present across the isolated strains. Based xix on the ability of the pathogen to utilize sugars and alcohols, all the isolates were grouped as biovar III except two (Rs18 and Rs49 whose biovar was not identified). The aggressiveness of isolated pathogen was analyzed using a hypersensitive reaction test on tobacco and further confirmed their virulence using a susceptible potato variety Shangi under greenhouse conditions. All isolates elicited a hypersensitive reaction on tobacco. However, they showed varying levels of pathogenicity with isolate Rs6 being the most virulent. In the course of biocontrol assays, 62 rhizobacterial isolates were obtained from potato rhizosphere by serial dilution methods using nutrient agar medium and 20 were selected for antagonistic tests using the paper disc assay. Results showed that 5 isolates were highly antagonistic against four R. solanacearum strains. These antagonists were tested for plant growth promoting traits and disease suppression. The results revealed that Bacillus cereus, Bacillus subtilis, Paenibacillus sp, Providencia rettgeri and Providencia vermicola were dominantly active in potato rhizosphere causing resistance to bacterial wilt disease. The isolates Bacillus cereus and Bacillus subtilis were potential isolates possessing antagonistic activity along with several plant growth promoting (PGP) traits. Taken together, these findings provide baseline information for improvement programs targeting host-based resistance to multiple strains of the pathogen; and development of biocontrol strategies with the potential antagonists reported. Implementation of these outcomes to the existing control methods would lead to increased farmer incomes, reduced environmental risks and human exposure to chemicals.