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Etiology

Identity, Virulence, and Isolation Frequency of Seven Phytophthora spp. Causing Root Rot of Raspberry in New York. W. F. Wilcox, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; Phytopathology 79:93-101. Accepted for publication 1 August 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-93.

A syndrome of declining raspberry stands was typified by: premature chlorosis, wilting, and death of floricanes; poor primocane emergence and survival; and a red-brown necrosis of the root cortex of affected plants. The disease syndrome was observed throughout New York State, primarily on red raspberries but occasionally on purple or black raspberry cultivars, and frequently was correlated with physical or environmental conditions promoting excessive soil moisture in affected locations. Phytophthora spp. were isolated from symptomatic plants of seven red raspberry cultivars, two purple raspberry cultivars, and one black raspberry cultivar on 17 of 20 farms from which samples were taken. Isolates were identified as P. fragariae (13 farms), P. megasperma (seven farms), P. cryptogea (two farms), P. cactorum (two farms), P. citricola (one farm), an unidentified homothallic Phytophthora sp. (two farms), and an unidentified heterothallic Phytophthora sp. (two farms). When red raspberry plants (cultivars Heritage and Taylor) were grown in artificially infested soil mix and flooded for 48-hr intervals every 2 wk, P. fragariae and P. citricola were extremely virulent, causing complete root rot and plant death; P. megasperma, P. cryptogea, and the unidentified homothallic Phytophthora sp. were moderately to highly virulent, causing 46-96% root rot and a 20-80% incidence of plant death; and P. cactorum and the unidentified heterothallic Phytophthora sp. were only mildly virulent. In similar pathogenicity tests, P. citricola caused crown rot and death of Mahaleb and Mazzard cherry seedlings, whereas P. fragariae and the unidentified homothallic Phytophthora sp. caused only negligible root necrosis; the same isolates of P. fragariae also failed to produce typical red stele symptoms on inoculated Catskill and Blakemore strawberry plants.