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 o f these plant species, suggesting that these four are nonhosts for late bli ght. The A1 (US-1), the A2 (US-8), and the more recent A1 mating type (US-1 1) strains infected both leaf and stem tissues of climbing and hairy nights hade, potato, and tomato but only leaf tissues of petunia. This is the firs t report of the new strains of P. infestans causing late blight symptoms on climbing nightshade and petunia in Canada. Based on infection levels, infe ction rates, and spore production, the US-8 and US-11 genotypes were simila r and both were more aggressive than the US-1 genotype. Disease severity an d sporulation were greater on potato inoculated with US-1 than for the othe r host plants and were the least on petunia. All susceptible plants were in fected at about the same level with US-8, except for petunia, which had a m uch 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 resul ts demonstrate that the new strains of P. infestans, US-8 and US-11, are ge nerally more aggressive than the previous predominating strain, US-1, and t hat climbing and hairy nightshade and petunia may harbour late blight, whil e black nightshade, eggplant, green pepper, and tobacco plants do not.