Crown Gall in Apple Rootstocks: Inoculation Above and Below Soil and Relationship to Root Mass Proliferation

Trees of each of five apple ( Malus × domestica Borkh.) rootstocks were inoculated above and below ground with three strains of Agrobacterium tumefaciens (Smith & Townsend) Conn. (AT). These were compared with controls that were not wounded or wounded and treated with sterile deionized water. Percentage of inoculated sites forming galls above ground was significantly greater in Mark and Mailing 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 × strain interaction of AT both 4 and 18 months after inoculation, each measure of crown gall susceptibility above ground was significantly correlated with corresponding below-ground data at P ≤ 0.01. This high correlation and frequent occurrence of galls or protruding callus from wounds below ground in controls suggest that above-ground inoculations are suitable for evaluating crown gall susceptibility in apple rootstocks. Eighteen months after inoculation most of the Mark trees had developed characteristic root mass proliferation (RMP). Three years after inoculation few crown galls were apparent below ground in Mark rootstock, but all but one of trees examined had developed RMP. Among other rootstocks only three (38%) M.9 EMLA trees and one (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 scattered throughout the xylem were observed when Mark RMP were examined microscopically. RMP from other rootstocks and most crown galls contained similar pockets of callus. Although inoculation with AT did not enhance RMP development in Mark, experimental results are not inconsistent with some AT involvement in RMP. Mark RMP is more extensive but not qualitatively different from structures formed at a lower frequency in M.9 EMLA and M.7A inoculated with AT.