Identification of crop and weed hosts of Ralstonia solanacearum biovar 2 in the hills of Nepal

Abstract

Common agricultural weeds and crops that grow in the high hills of Nepal were examined after artificial inoculation and under natural conditions in the UK and Nepal to determine whether such plant species could act as hosts to biovar 2 of Ralstonia solanacearum. Bacterial populations in the roots were determined 1 and 2 months after inoculation, and at various intervals after harvesting infected potato crops under natural conditions. Inoculated roots of the summer weeds Drymaria cordata and Polygonum capitata and the winter weeds Cerastium glomeratum and Stellaria media yielded 102−107 colony‐forming units per g root. High populations of the bacterium were recovered from these plants even after partial surface sterilization, indicating that systemic infection had occurred. Ralstonia solanacearum populations were recovered from root extracts of 75% of naturally growing D. cordata plants when sampled 3 months after harvest of a potato crop with bacterial wilt. Similarly, root extracts of 25% of P. capitata plants carried the bacterium. No potential winter weed hosts were infected under natural conditions when sampled 5 and 6 months after harvest of infected potato, indicating that winter conditions in the high hills of Nepal are not conducive to infection. Among crops, mustard (Brassica juncea cv. Fine White) developed typical wilt symptoms after artificial inoculation in warm glasshouse conditions (20–28°C). Mustard and barley are winter crops in Nepal. However, neither mustard (Brassica juncea var. Lumle Tori) nor barley (Hordeum vulgare cv. Bonus) was infected when planted into heavily infested plots under natural conditions. The results indicated that the role of nonsolanaceous summer weeds in the persistence of biovar 2 of R. solanacearum in the environment may have been previously underestimated.