Formation and survival of oospores of Phytophthora infestans under natural conditions

Abstract

Phytophthora infestans is able to produce oospores in leaves of potato and tomato plants after inoculation with a mixture of Al and A2 mating‐type isolates. Various conditions for oospore formation were analysed. Under controlled conditions, oospores were produced in potato leaves at temperatures ranging from 5 to 25° C. In leaves of potato cultivar Bintje incubated at 15°C, oogonia and antheridia were observed 6 days after inoculation and thick‐walled oospores appeared 3‐4 days later. In field experiments oospores were found in leaves and stems of potato cultivars Bintje, Irene and Pimpernel and in leaves, stems and fruits of tomato cultivar Moneymaker within 2 weeks after inoculation. A bioassay was developed to test the survival of oospores in soil under various conditions. To determine whether late‐blight infections derived from infectious soil were caused by oospwres, DNA fingerprinting was performed. DNA fingerprint probe RG‐57 was suitable for distinguishing asexual progeny from recombinant progeny arising from soil‐borne oospores. We demonstrated survival of viable, infectious oospores of P. infestans in soil during the winter of 1992–93. Oospores were not infectious from soil exposed to temperatures of 40°C or higher but in the range 35°C to as low as – 80°C for 48 h, oospores survived.