The area bordering three 110-ha (270-acre) fields of blighted potatoes (Solanum tuberosum L.) in three northeastern Maine locations was surveyed during the summer of 2004 for the occurrence of late blight on cultivated and noncultivated host plants. Special attention was directed to solanaceous weed species. Hundreds of Solanum sarrachoides Sendt. ex. Mart. (hairy nightshade) plants with numerous leaf lesions and moderate defoliation were seen. The frequency of blighted hairy nightshade approximated the frequency of late blight in the adjoining potato fields. Lesions typically contained extensive, white, superficial mycelia colonizing the abaxial and adaxial leaf surfaces. Samples placed in a moist chamber produced lemon-shaped sporangia. On the basis of morphological characteristics, the pathogen was tentatively identified as Phytophthora infestans (Mont.) de Bary. Isolates were obtained by surface-disinfecting leaf sections collected from two locations for 2 to 3 min in 0.5% NaOCl and placing the sections on rye grain medium amended with antibiotics (100 ppm each of penicillin G, pimaricin, and polymyxin). P. infestans was confirmed after reisolating onto rye-lima bean medium. Pathogenicity was tested on detached potato, tomato, and hairy nightshade leaves; the undersides of all leaflets from replicate plants were inoculated with droplets of swimming zoospores (≥500 zoospores per droplet), the leaves were kept at 17°C and 100% humidity, and the extent of sporulation was evaluated after 4, 6, and 7 days. With eight isolates obtained from S. sarrachoides, Koch's postulates were completed on potato and hairy nightshade. Radial growth responses of these strains on rye grain agar amended with 1, 10, or 100 μg per ml of metalaxyl (Ridomil 2E) yielded 50% effective dose values greater than 100 μg per ml, since percentage growth at the highest fungicide concentration exceeded 50% of the no metalaxyl control. These resistance levels are typical of the metalaxyl-insensitive strains of P. infestans isolated from potatoes in this area in recent years, which were previously found to correlate with metalaxyl resistance in bioassays using potato tissues (1). Eight single-sporangial isolates were homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/111/122, Pep 100/100). All eight were A2-mating type and mitochondrial haplotype Ia, characteristics common to the US-8 clonal lineage of P. infestans from potato (2), which may infect a wider host range than the old US-1 clonal lineage. When evaluated on differential hosts, three isolates were tomato race PH-1 and complex potato race R 0,1,2,3,4,9,11. DNA fingerprint analysis with probe RG57 further established that the eight hairy nightshade isolates were identical to each other and to local P. infestans isolates from potato. To our knowledge, this is the first report of infection of S. sarrachoides by P. infestans in Maine. The pathogen was previously isolated from this host during field surveys in southern California in the 1980s in connection with late blight of tomato (4). Hairy nightshade has been shown to be a host for US-1, US-8, and US-11 isolates of P. infestans in a laboratory setting (3). The epidemiological significance of S. sarrachoides as an alternative or overwintering host of P. infestans is currently being assessed.
References: (1) K. L. Deahl et al. Am. Potato J. 70:779, 1993. (2) S. B. Goodwin et al. Phytopathology 88:939, 1998. (3) H. W. Platt. Can. J. Plant Pathol. 21:301, 1999. (4) V. G. Vartanian and R. M. Endo Plant Dis. 69:516, 1985.
