Isolation, Distribution, Pathogenicity, and Identification of Phytophthora spp. on Asparagus in California. Peter G. Falloon, Crop Research Division, D.S.I.R., Private Bag, Christchurch, New Zealand. R. G. Grogan, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 72:495-497. Accepted for publication 7 December 1987. 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, 1988. DOI: 10.1094/PD-72-0495.

A survey was made in 1982 and 1983 of 57 asparagus fields throughout the main asparagus production areas of California. Phytophthora spp. were detected in all fields surveyed in the Sacramento and San Joaquin valleys, five out of six fields surveyed in the Salinas Valley, seven out of eight fields in Orange County, but no fields in the Coachella and Imperial valleys. Phytophthora spp. were isolated from spear, crown, or root tissue and baited from soil in fields planted with the asparagus cultivars Mary Washington, U.C. 72, U.C. 800, and U.C. 157. Out of a total of 84 isolates, 74 were identified as P. megasperma var. sojae and all were pathogenic on asparagus seedlings. Ten isolates of heterothallic Phytophthora spp. were isolated. Of these, four were pathogenic on asparagus seedlings and were identified as P. cryptogea (A₁ mating type); six that were not pathogenic could not be identified. Some fields in which Phytophthora was isolated from asparagus had previously been cropped with alfalfa, but no evidence of cross pathogenicity was found between three isolates of P. m. f. sp. medicaginis and asparagus (cv. U.C. 157), or P. m. var. sojae ex asparagus and alfalfa cultivars either resistant (Apollo 2, Washoe) or susceptible (Rere, Wairau) to P. m. f. sp. medicaginis. However, one isolate of P. cryptogea ex asparagus caused mild symptoms on Wairau and may be able to survive on alfalfa in the absence of asparagus. The results of this survey show that Phytophthora spp. were widespread in asparagus fields in 1982 and 1983 and could potentially have a major effect on asparagus production in California.

Keyword(s): Asparagus officinalis L., Phytophthora rot, resistant cultivars.