Infection of Apple Roots by Actinomycetes Associated with Soils Conducive to Apple Replant Disease. S. Wickes Westcott III, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Steven V. Beer, Department of Plant Pathology, and Warren C. Stiles, Department of Pomology, Cornell University, Ithaca, NY 14853. Plant Dis. 70:1125-1128. Accepted for publication 29 July 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-1125.

Apple seedling roots became infected by actinomycetes when grown in five soils collected from apple and pear nurseries in New York. Steam treatment of these soils (about 60 C, 30 min) eliminated actinomycete infection of roots and controlled symptoms of apple replant disease. Neither symptoms of apple replant disease nor root infections by actinomycetes were observed on seedlings planted in three additional soils collected from the root zones of apple trees. Soils collected from the same nurseries, but from sites not associated with previous planting of either apple or pear, contained low or undetectable levels of infectious actinomycetes. Seedling growth was not inhibited in four of these soils compared with growth in steamed samples of each soil. Inhibition of seedling growth in the other two soils appeared to be related to nutrient deficiencies. Concentrations of nitrogen, boron, and several other nutrients were significantly higher in shoots of seedlings grown in steamed vs. unsteamed samples from these two soils. Therefore, all six nonapple soils were judged not conducive to apple replant disease. Our evidence supports an association between infectious actinomycetes and the apple replant disease as proposed by several others. Actinomycetes may be important in the etiology of the apple replant disease.