Biological Control of Crown Gall on Peach and Cherry Rootstock Colt by Native Agrobacterium radiobacter Isolates

Crown gall caused by Agrobacterium tumefaciens is the major limiting factor in raising healthy stone fruit plants in nurseries. The incidence of crown gall varies from 4 to 97.5 per cent at different locations resulting in out right rejection of an average of 30 per cent stone fruit plants in nurseries. Agrobacterium radiobacter strain K-84 has been reported to provide nearly complete control of this disease, however this strain is not available in India. Therefore, the present work was aimed to evaluate the antagonistic activity of twelve native A. radiobacter isolates from peach cv. Redhaven, July Elberta and cherry cv. Stella rhizosphere soil against A. tumefaciens. Agrocin production was detected in three isolates of A. radiobacter UHFBA-8, UHFBA-11 and UHFBA-12, and in cross inoculation test against A. tumefaciens, all the three isolates completely inhibited gall formation into tomato stems, whereas inoculation of pAg variants of UHFBA-8 and UHFBA-11 resulted in 87.6 per cent less number of galls per wound and 96 per cent reduced gall size into inoculated tomato stems. Root dip treatment of peach plants with isolate UHFBA-11 resulted in 5.37 per cent incidence of crown gall in contrast to more than 54 per cent galled plants that had not received the treatment. In cherry rootstock Colt, isolate UHFBA-8 as root dip minimized crown gall incidence to 22.22 per cent as compared to 71.11 per cent incidence in untreated plants. Rifampicin resistant mutants of A. radiobacter isolate viz., UHFBA-8 and 11 efficiently colonized the root system of peach and Colt and their populations remained in the tissues of treated plants throughout the growing season. The data suggest that other mechanisms such as efficient colonization of root system, binding and physical blockage of infection sites are involved in biological control by A. radiobacter in addition to production of agrocin.