Occurrence of Metalaxyl-insensitive Phytophthora infestans on Solanum sarachoides in Northwestern Washington. K. L. Deahl, USDA, ARS, PSI, Vegetable Laboratory, Beltsville, Md. 20705 . D. A. Inglis, Washington State University-Mount Vernon REU, Mount Vernon 98273. Plant Dis. 79:540. Accepted for publication 12 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0540A.
An important potato production area of northwestern Washington was surveyed during 1994 for the occurrence of late blight on cultivated and noncultivated host plants. Special attention was directed to solanaceous weed species growing within and around fields of blighted potatoes (Solanum tuberosum L). Plants of Solanum sarachoides Sendtner ex. Mart, (hairy nightshade) with numerous leaf lesions and moderate defoliation were collected from one location within the border of an adjoining pea (Pisum sativum L.) field. Typical lesions contained extensive, white, superficial mycelia colonizing abaxial and adaxial leaf surfaces. When samples were placed in a moist chamber to induce sporulation, lemon-shaped sporangia developed. On the basis of morphological characteristics of the sporangial stage, the fungus was tentatively identified as Phylophthora infestans (Mont.) de Bary. Representative isolates were obtained by surface-disinfecting leaf sections for 2 to 3 min in 0.5% NaOCI solution and plating the sections on rye grain medium amended with antibiotics (100 ppm each of penicillin G, pimaricin, and polymyxin). Phytophthora infestans was confirmed after reisolation onto rye-lima bean medium. With randomly selected fungal isolates obtained from 5. sarachoides, Koch’s postulates were completed on S. sarachoides and cultivars of potato and tomato (Lycopersicon esculentum Mill). When evaluated for race and sexual composition on differential hosts, all isolates were tomato race 1 (T1), complex potato race R1, R2, R3, R4, R7, R10, and A’ compatibility type. Radial growth responses of these strains on rye grain agar amended with 1, 10, or 100 ?g metalaxyl (Ridomil 2E) per ml yielded ED50 values greater than 100 ?g per ml. since percent growth at the highest fungicide concentration exceeded 50%. These resistance levels are typical of the metalaxyl-insensitive strains of P. infestans isolated from potato hosts in this area in recent years, and were previously found to correlate with metalaxyl resistance in bioassays using potato tissues (1). This is the first report of infection of S. sarachoides by P. infestans in the Pacific Northwest, in association with late blight of potato. The epidemiological significance of S. sarachoides as an alternative or overwintering host of P. infestans is currently being assessed. The pathogen was previously isolated from this same host during field surveys in southern California in the early 1980s, in connection with late blight of tomato (2). The majority of these isolates were tomato race 1 (Tl) and A’ mating type, although neither metalaxyl response nor potato race designations were determined.References: (1) K. L. Deahl et al. Am. Potato J. 70:779, 199.1. (2) V. G. Vartanian and R. M. Endo. Plant Dis. 69:516, 1985.