Phytophthora Root and Stem Rot of Apple Rootstocks from Stool Beds. C. J. Tidball, Department of Botany and Plant Pathology, Oregon State University. R. G. Linderman, USDA-ARS, Horticultural Crops Research Laboratory, Corvallis, Oregon 97330. Plant Dis. 74:141-146. Accepted for publication 31 August 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/PD-74-0141.

Phytophthora stem and root rot occurred on EMLA.106 apple rootstocks in stool beds as early as May, and new infections occurred throughout the growing season. Phytophthora cactorum and P. cambivora were recovered from infected shoots or roots and were identified as the causal agents of the disease; P. cactorum was isolated more frequently. Isolates of both species caused necrosis on excised stem pieces at 15 or 24 C, but isolates of P. cactorum were more virulent than those of P. cambivora. Field applications of fosetyl-Al and metalaxyl to apple rootstocks reduced disease severity in the field, reduced lesions on excised stem pieces inoculated with P. cactorum, and reduced disease development on apparently healthy rootstocks while in cold storage for 3–9 mo. Short-duration root dipping of healthy or mildly infected rootstocks before cold storage in high concentrations of benalaxyl, etridiazol, fosetyl-Al, or metalaxyl reduced disease, but the degree of reduction varied. Fosetyl-Al and metalaxyl consistently produced the best results. None of the materials were phytotoxic.