BIOLOGICAL DIVERSITY OF RALSTONIA SOLANACEARUM STRAINS CAUSING BACTERIAL WILT OF SOLANACEOUS CROPS IN PAKISTAN

Abstract

Bacterial wilt caused by Ralstonia solanacearum is a major production constraint of solanaceous crops in tropical and sub tropical areas of the world including Pakistan. Therefore the biology of this disease was studied to elaborate current status of bacterial wilt in the country. The disease was mainly found distributed in Punjab and Islamabad Capital Territory. R. solanacearum was found to be associated with tomato, pepper, brinjal and potato. High prevalence of the disease was noticed in pepper, especially sweet pepper. During the surveys, other diseases of solanaceous crops were observed i.e Verticillium and Fusarium wilt, root knot nematodes (Meloidogyne spp), Phytophthora rot (Phytophthora capsici), stem rot/southern blight of tomato (Sclerotium rolfsii), pepper leaf curl virus, bacterial blights (Xanthomonas compestris and Pseudomonas syringae). According to farmers, bacterial wilt has been present in some fields of Punjab province for the past 20 years where out breaks of the disease were observed during monsoon season each year. Most of the farmers were not aware about bacterial wilt symptomology and its causal organism. Most of the commercial varieties used by farmers were imported whose disease resistance information was not known. Bacterial wilt was found in areas with diverse soil, pH, temperature, relative humidity, rainfall and topographical patterns. Mostly wilt affected soils were alkaline, calcareous in nature. Based on biochemical, serological and host tests specified for the identification of R. solanacearum, out of 326 isolates 45 (13.8%) were found to be R. solanacearum. The tests further revealed that biovar 2 and 3 were more prevalent in Pakistan with 80 % of strains belonging to biovar 3. The predominant biovar associated with potato was biovar 2. Studies further revealed that the rest of strains obtained from tomato, sweet pepper, hot pepper and brinjal were mainly biovar 3. Majority of R. solanacearum strains were isolated from plant and soil samples. In addition, some biovar 3 strains were also found to be associated with sweet pepper and hot pepper seeds. The strains obtained from sweet pepper formed a distinct cluster (based on serological and pathogenicity tests) as compared to the rest of strains. Studies further indicated that soil inhabiting bacteria (Pseudomonas aeroginosa, Klebsiella pneomoniae, Pseudomonas entomophila) or possible phenotypic conversion affect detection of R. solanacearum from soil. Enrichment ELISA combined with PCR proved to be a useful tool for detection of R. solanacearum from soil. Local pepper varieties i.e Sanam, Sangri and Ghotki showed good resistance to bacterial wilt. On the other hand local tomato varieties Roma and Rio Grande, and imported commercial sweet pepper variety California Wonder was found to be highly susceptible to bacterial wilt. UW 585 (Phylotype 1 American strain) was significantly distinct from UW 371 (Phylotype 1 Asian strain). Asian strain GMI 1000 and American strain K60 were equally aggressive on all tomato varieties. Results of these studies will provide basis for improved diagnosis, monitoring of bacterial wilt and formulation of management strategies.