Aggressiveness of Phytophthora Infestans Genotypes on Potato Stems and Leaves at Three Temperatures

Epidemics of late blight on potato in the semiarid Pacific Northwest since 1991 have been characterized by a prevalence of stem lesions in relation to leaf lesions. This study was conducted in 1997 to test the hypothesis that isolates representing immigrant strains of Phytophthora infestans were more adapted at causing stem lesions and more aggressive at higher temperatures than isolates representing the relatively older US-1 strain. A total of 23 Phytophthora infestans isolates representing US-1, US-8, and a new A1 compatibility type strain were tested for aggressiveness on leaflets and stems of whole potato plants (cv. Russet Burbank) in 11 trials. Plants in one set of trials were incubated at constant temperatures of 18, 23, and 28 °C for six days with a 16 h photoperiod. Plants in the second set of trials were incubated at the same three day temperatures but the night temperature in all treatments was reduced to 16 °C. Lesion establishment was recorded, and daily severity readings were used to calculate the area under the lesion expansion curve (AULEC). Lesion area, sporulation frequency, sporulation time, and sporulation capacity were also measured. Lesion establishment was higher on stems than on leaves for isolates of all strains. Isolates representing US-8 and new A1 strains often had higher AULEC values but had similar lesion establishment, sporulation frequency, sporulation time, and sporulation capacity values as US-1 isolates. A reduction in components of aggressiveness for all strains was noted at 28 °C, with leaflets being more affected than stems. Sporulation rarely occurred at 28 °C. Few differences in components of aggressiveness were observed between 18 and 23 °C. These findings indicate that isolates from the relatively newer strains (US-8 and new A1) were not better adapted in causing lesions on potato stems than isolates from the old US-1 strain, nor were they better adapted to higher temperatures. The relatively newer strains, however, were generally more aggressive as indicated by higher AULEC on stems and leaflets over the range of temperatures used in this experiment.