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Etiology

Interactions Between Actinomycete-Like Organisms and Young Apple Roots Grown in Soil Conducive to Apple Replant Disease. S. W. Westcott III, Graduate research assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853, Present address: Department of Plant Pathology and Physiology, Clemson University, Clemson, SC 29634; S. V. Beer, and H. W. Israel. Associate professor, and Senior research associate, respectively, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Phytopathology 77:1071-1077. Accepted for publication 26 January 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1071.

Procaryotic mycelial organisms, similar to actinomycetes, were consistently found in lesions on young apple roots after seedlings had grown for 2 wk in soil conducive to apple replant disease. Although other organisms were found in the lesions, only the actinomycete-like hyphae were seen at the advancing lesion margins. These hyphae had penetrated walls of living cortical cells and were sometimes found associated with papillae at penetration sites in the cells. By 3 wk, invaded tissues had collapsed and actinomycete-like hyphae had penetrated to the endodermis. Further penetration was limited by the periderm, and infected tissues were sloughed as roots expanded in diameter. These observations indicate that the actinomycete-like organisms actively invaded and damaged living cells of the root epidermis and cortex and that they should therefore be considered pathogenic. Rhizosphere soil collected from roots of seedlings grown in a soil conducive to apple replant disease promoted high levels of infection by the actinomycete-like organisms. Severity of infection of roots was associated with inhibition of both apple seedling growth and lateral root development, which are symptoms that have been associated also with apple replant disease. These results support other evidence indicating a causal role for organisms similar to actinomycetes in apple replant disease.