FORMATION AND SURVIVAL OF OOSPORES OF PHYTOPHTHORA-INFESTANS UNDER NATURAL CONDITIONS

Phytophthora infestans is able to produce oospores in leaves of potato and tomato plants after inoculation with a mixture of A1 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 degrees C. In leaves of potato cultivar Bintje incubated at 15 degrees 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 oospores, 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 degrees C or higher but in the range 35 degrees C to as low as -80 degrees C for 48 h, oospores survived.