Phytophthora Root and Crown Rot of Apple Trees in Arizona. M. E. Matheron, Extension Plant Pathologist, University of Arizona, Yuma Agricultural Center, Yuma 85364. D. J. Young, and J. C. Matejka. Extension Agent, University of Arizona, Willcox 85643; and Research Assistant, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 72:481-484. Accepted for publication 18 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0481.

Five Phytophthora spp. were isolated from declining apple trees in commercial orchards in Arizona. Phytophthora cactorum, P. cambivora, P. drechsleri, P. parasitica, and an unidentified Phytophthora sp. (isolate A46) were recovered from 43, seven, three, one, and two trees, respectively, of 56 trees from 36 different orchard sites. Additionally, P. cactorum and P. cambivora were recovered from nursery-grown apple rootstocks and trees. All five Phytophthora spp. were pathogenic to apple seedlings grown in artificially infested potting mix in greenhouse tests. P. cactorum and P. cambivora were highly virulent, causing rapid plant decline and death. Phytophthora drechsleri, P. parasitica, and Phytophthora sp. (isolate A46) were less virulent, causing root necrosis and occasional plant death. These results implicate Phytophthora spp. in the decline and death of apple trees in Arizona. The association of P. cactorum and P. cambivora with roots of nursery-grown apple plants may partially explain the occurrence and spread of Phytophthora root and crown rot in this state.