CROWN GALL IN APPLE ROOTSTOCKS - INOCULATION ABOVE AND BELOW SOIL ANDRELATIONSHIP TO ROOT MASS PROLIFERATION

Trees of each of five apple (Malus xdomestica Borkh.) rootstocks were inoculated above and below ground with three strains of Agrobacterium tumefaciens (Smith & Townsend) Conn. (AT). These were compared with co ntrols that were not wounded or mounded and treated with sterile deion ized water. Percentage of inoculated sites forming galls above ground was significantly greater in Mark and Malling 9 (M.9 EMLA) than in all other rootstocks except M.26 EMLA at 18 months after inoculation. At both 4 and 18 months after inoculation, gall formation below ground on M.7A was twice as high as above ground, but within other rootstocks, gall formation above and below ground was similar. For the rootstock x strain interaction of AT both 4 and 18 months after inoculation, each measure of crown gall susceptibility above ground was significantly c orrelated with corresponding below-ground data at P less than or equal to 0.01. This high correlation and frequent occurrence of galls or pr otruding callus from wounds below ground in controls suggest that abov e-ground inoculations are suitable for evaluating crown gall susceptib ility in apple rootstocks. Eighteen months after inoculation most of t he Mark trees had developed characteristic root mass proliferation (RM P). Three years after inoculation few crown galls mere apparent below ground in Mark rootstock, but all but one of trees examined had develo ped RMP. Among other rootstocks only three (38%) M.9 EMLA trees and on e (8%) M.7A tree produced swellings identified as RMP, and all but one (5%) of these trees had been inoculated with AT. Pockets of callus sc attered throughout the xylem were observed when Mark RMP mere examined microscopically. RMP from other rootstocks and most crown galls conta ined similar pockets of callus. Although inoculation with AT did not e nhance RMP development in Mark, experimental results are not inconsist ent with some AT involvement in RMP. Mark RMP is more extensive but no t qualitatively different from structures formed at a lower frequency in M.9 EMLA and M.7A inoculated with AT.