Biological control of apple crown gall by nonpathogenic Rhizobium vitis strain VAR03-1

A nonpathogenic strain of Rhizobium vitis , VAR03-1, was tested as a biological control agent for crown gall of apple. When apple tree roots were soaked in a cell suspension of antagonists before planting in the field, treatment with VAR03-1 significantly reduced the number of plants with tumors and the disease severity. The inhibitory effects of treatment with VAR03-1 and the nonpathogenic R. rhizogenes strain K84, which is commercially used as a biological control agent throughout the world, on apple crown gall were almost identical. VAR03-1 was bacteriocinogenic, producing a halo of inhibition against two species of tumorigenic Rhizobium isolated from galled apple trees. Moreover, VAR03-1 established populations on roots of apple tree rootstock and persisted on roots for 437 days. This is the first report that the nonpathogenic R. vitis VAR03-1 effectively controls crown gall of apple.