Potato tissue culture plantlets grown in culture tubes were successfully used to establish and monitor disease development in roots and other subterranean plant parts. Host tissue responses to infection and other disease processes were easily observed and received permanently (photographically with or without the aid of a microscope) in a nondestructive manner. Use of this technique for pathogenicity and other investigations is discussed
Isolates of Phytophthora infestans were obtained from individual fields of potato and tomato and from potato storages across Canada in 1994 (142 samples yielding 555 isolates), 1995 (185 samples yielding 914 isolates), and 1996 (300 samples yielding 1013 isolates). Characterization of these isolates according to mating type and sensitivity to metalaxyl revealed the changing nature of P. infestans populations in Canada. In 1994, isolates of the traditional Al, metalaxyl-sensitive (MS) phenotype were common, but by 1996, they were no longer recovered from any tissue samples. Isolates of the A2 mating type, which were predominantly insensitive to metalaxyl (MI), were prevalent in 1996, except in British Columbia (B.C.). In B.C., Al isolates highly insensitive to metalaxyl predominated. Isolates of the Al, MI phenotype were also found in a sample of tomatoes from Ontario in 1996. New populations composed of the A2 mating type were predominantly insensitive to metalaxyl, but they showed an intermediate response between the high sensitivity of the original Al population and the highly insensitive Al populations in B.C. The exception was a few A2, MS isolates obtained from B.C. in 1996. The correlation between recovery of MI isolates and metalaxyl use was poor. There was a tendency for the proportion of MI isolates from fields managed with or without metalaxyl to increase as the growing season progressed. Both mating types of P. infestans were found in the same tissue sample a maximum of three times in a given year; self-fertile isolates were not recovered. Of 627 tissue samples examined over 3 years, one was found to have oospores of P. infestans present, indicating that sexual reproduction had occurred. These results demonstrate that populations of P. infestans in Canada have changed dramatically from 1994 to 1996 and that the original Al, MS population was rapidly displaced.
Five substrates (rye, corn, pea, lima bean, and pea bean) were assessed for their ability to sustain the viability of two isolates of Phytophthora infestans in liquid culture. One isolate belonged to the US-1 genotype (A1 mating type) and the other to the US-8 genotype (A2 mating type). Media based on corn and rye were consistently the best for maintaining the viability of both isolates over time. The survival of the isolate of the US-1 genotype on all five media declined dramatically after 12 months in storage at room temperature (21 degrees C). Recovery of the isolate of the US-8 genotype remained possible after 30 months of storage at room temperature from corn, rye, and pea bean media but not from lima bean or pea media. Isolates of both genotypes grown on a rye kernel medium and kept at 4 degrees C were still recovered after 30 months in culture, although the percentage of successful transfers (mean survival) had declined. According to linear regression analysis, the US-8 isolate grown in sterile culture at 21 degrees C showed better survival over time than the US-1 isolate. To maintain isolates of P. infestans for 1 to 2.5 years without transfer, the use of liquid media based on corn or rye is recommended.
Survival of asexual (sporangia and mycelium) propagules of Phytophthora infestans in autoclaved peat moss exposed to various temperatures was assessed by ability to infect floating potato leaf discs. Infectivity of propagules in peat was measured after 0, 21, 35, 63, and 79 days of exposure to temperatures ranging from -33 C to 20 C. Propagules exposed to temperatures below freezing were no longer infective after 21 days. Exposure of propagules to temperatures of 15 or 20 C resulted in a rapid decline in infectivity over time; by the final sampling date, none were infective. In contrast, propagules exposed to a temperature of 5 C for 79 days remained infective.
