ROOT COLONIZATION OF NON-SUSCEPTIBLE HOSTS BY RALSTONIA SOLANACEARUM
Melo M.S., Takatsu A.
Departamento de Fitopatologia, Universidade de Brasilia, C. Postal 4457, 70.919-970, Brasilia, DF, Brazil.
Root colonization of eleven species of vegetable crops considered to be non-susceptible hosts to Ralstonia solanacearum was studied in pots with sterilized soil, under glasshouse conditions (18-35°C). Seedlings of fifteen days after germination were inoculated with six isolates of R. solanacearum, biovars I, II and III, resistant to the antibiotics streptomycin, rhifampicin and chloramphenicol. Plants were inoculated by pouring 100 ml of bacterial suspention of 1x108 CFU/ml at the center of each pot. The colonization was evaluated in CFU/g of roots 45 days after inoculation by the plating method in culture media with the antibiotics and the fungicides benomyl and chlorothalonyl. Zucchini (Cucurbita pepo var. melopepo), table beet (Beta vulgaris), cowpea (Vigna sinensis), carrot (Daucus carota), chicory (Cichorium endivia) spinach of New Zealand (Tetragonia expansa), bean (Phaseolus vulgaris), cabbage (Brassica oleracea var. capitata) and parsley (Petroselium crispum) hosted biovars I and III. High populations of biovar II were found only in zucchini and cowpea. Coriander (Coriandrum sativum) hosted only the isolates of biovar I while onion (Allium cepa) was a non-host for any of the isolates of R. solanacearum tested. Chicory and spinach showed also symptoms of the disease by internal infection with one isolate of biovar III, being the first report of theese species as susceptible hosts of R. solanacearum. These data showed that many species considered to be non susceptible plants usually cultivated in rotation with susceptible crops, may be symptomless hosts of many strains of R. solanacearum, allowing them to survive for a long time in infested areas. Biovars I and III are known to be soil borne pathogens as they survive for a long time in soil due to the large range of susceptible host plants. It showed however that they have a larger range of non susceptible hosts which are better for long term survival once they are not killed by the pathogen. Present information is important for better understanding the epidemiology of bacterial wilt and also for methodology improvement for fast identification of non host plants to be used in regional programs of crop rotation for control of the disease.
