Temperature and leaf wetness requirements for pathogen establishment, incubation period, and sporulation of Phytophthora infestans on Petunia x hybrida

The temperature and leaf wetness requirements for pathogen establishment (germination, infection, and colonization) and the temperature effects on incubation period and sporulation of Phytophthora infestans on petunia were compared with those on tomato. The responses to environmental parameters were found to be similar on petunia and tomato and agreed with those previously reported for late blight development on tomato and potato. In the current study, temperatures ranging from 13 to 23°C generally were conducive to establishment. Very little establishment occurred at 28°C. The minimum leaf wetness period that enabled pathogen establishment was 2 h, whereas most establishment occurred within 6 h of inoculation. The incubation period (time period from inoculation to lesion development) and the time required for development of sporangia after lesions were formed were shortest at 23 and 28°C, respectively. Production of sporangia was greatest (per square centimeter) at 18°C and was nearly absent at 28°C on both petunia and tomato. The sporulation density at 18°C was only slightly less on petunia compared with tomato (20,000 and 24,000 sporangia/cm2, respectively); however, the total lesion area on petunia was only 20% of that on tomato.