Dieback of pome fruit rootstocks caused by Pseudomonas syringae

Abstract

Dieback (bark necrosis) of pome fruit rootstocks occurred in cuttings, potted plants in the greenhouse, and plants in the nursery when the freshly cut stems were inoculated with bacterial isolate ARS4, identified as Pseudomonas syringae pv syringae. Isolate ARS4, originally isolated from a diseased Malling 9 (M.9) rootstock from a commercial nursery, also caused papery bark (bark shredding) on apple and pear rootstocks indicative of severe dieback. Isolate 980 also identified as P. syringae pv. syringae, isolated from a sweet cherry branch canker, was associated with dieback of pome fruit rootstocks in greenhouse trials but was generally less pathogenic than isolate ARS4. Isolate ARS4 labelled with resistance to nalidixic acidic was recovered from lesions on five of nine M.9 rootstocks at the lesion boundary indicating that the isolate was present in diseased rootstock tissue. Of nine apple rootstock cultivars tested for susceptibility to isolate ARS4, M.4 and M.9 were more susceptible than the other five rootstock cultivars. The effect of inoculation during thawing of frozen M.9 rootstocks in pots led to significantly more dieback than if rootstocks were not frozen. Contamination of buds did not lead to dieback but isolate 980 reduced McIntosh bud growth to 1.7% of the total number of buds that were tested. Leaving a stub above the scion bud on M.9 rootstocks to control dieback did not affect bud development or rootstock stem diameter. Key words: Pseudomonas cichorii, Pseudomonas fluorescens, necrosis, papery bark, lesions, blister spot