Response of solanaceous cultivated plants and weed species to inoculation with A1 or A2 mating time strains of Phytophthora infestans

Hypersensitive response symptoms of late blight were induced when sporangia of Phytophthora infestans were inoculated on discs of excised leaf tissues of black nightshade, eggplant, green pepper, and tobacco. No symptoms due to disease or hypersensitive response were observed in stem piece tissues of these plant species, suggesting that these four are nonhosts for late blight. The A1 (US-1), the A2 (US-8), and the more recent A1 mating type (US-11) strains infected both leaf and stem tissues of climbing and hairy nightshade, potato, and tomato but only leaf tissues of petunia. This is the first report of the new strains of P. infestans causing late blight symptoms on climbing nightshade and petunia in Canada. Based on infection levels, infection rates, and spore production, the US-8 and US-11 genotypes were similar and both were more aggressive than the US-1 genotype. Disease severity and sporulation were greater on potato inoculated with US-1 than for the other host plants and were the least on petunia. All susceptible plants were infected at about the same level with US-8, except for petunia, which had a much lower level of infection. Spore production for US-8 was less on tomato and petunia than on the other plants. Infection levels and spore production by US-11 on tomato were as great as or greater than on potato. These results demonstrate that the new strains of P. infestans, US-8 and US-11, are generally more aggressive than the previous predominating strain, US-1, and that climbing and hairy nightshade and petunia may harbour late blight, while black nightshade, eggplant, green pepper, and tobacco plants do not.