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Prevention of Crown Gall on Prunus Roots by Bacterial Antagonists. Larry W. Moore, Associate Professor of Plant Pathology, Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331; Phytopathology 67:139-144. Accepted for publication 4 August 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-139.

Biological control of crown gall on Prunus species was accomplished both in the greenhouse and field with strain 84 of Agrobacterium radiobacter. In the field, preplanting inoculation with strain 84 reduced the incidence of galling to 11% in seedlings that subsequently were inoculated with a mixture of six strains of A. tumefaciens, and completely protected seedlings against naturally-occurring crown gall bacteria in the soil. In contrast, more than 75% of the seedlings that had not received strain 84 before or after inoculation with A. tumefaciens developed galls. In the field, when the proportion of colony-forming units of strain 84 to pathogenic strains was 1:10, galling was reduced to 5% compared to 31% at 1:100. Nevertheless, 6% of the seedlings may have been infected by two of the six pathogenic strains that were insensitive to a bacteriocin produced in vitro by strain 84. When these two bacteriocin-insensitive strains were mixed individually with strain 84 and inoculated to tomato seedlings, 89% or more of the seedlings were galled, even with population ratios of 1:100 of pathogen to strain 84. Conversely, none of the seedlings became infected when inoculated simultaneously with 1:1 mixtures of strain 84 and each of the four bacteriocin-sensitive pathogens. A combined inoculum of all six pathogens mixed 1:1 or 1:10 with strain 84 resulted in >80% galled tomato seedlings contrasted to 5-6% galling of mazzard cherry seedlings inoculated in the field. Cells of strain 84 treated with chloroform did not prevent infection of tomato seedlings by strain Q51, whereas a similar number of viable strain 84 cells completely prevented galling. In the field, strain 84 was significantly more effective in reducing galling on inoculated cherry seedlings than were the bactericides commonly used in the nursery trade. Aboveground grafts of flowering cherry (Prunus pendula cv. ‘Double Subhirtella’) on mazzard cherry (Prunus avium) rootstocks also were protected from galling when scion wood was sprayed with strain 84 just prior to grafting. A mutant of strain 84 resistant to rifampicin and streptomycin readily colonized the roots of mazzard cherries following inoculation of the seed, roots, or crown. Three Oregon isolates of A. radiobacter of 32 tested prevented crown gall on mazzard cherry seedlings inoculated with A. tumefaciens, but two other isolates of this species significantly increased the number and size of galls on the seedlings.