Long-term survival of Phytophthora infestans in liquid media prepared from autoclaved seeds

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.