VIABILITY, GERMINATION AND INFECTION POTENTIAL OF OOSPORES OF PHYTOPHTHORA-INFESTANS

Matings between five A1 and five A2 wild-type isolates of Phytophthora infestans from potato and tomato crops in the United Kingdom produced oospores in vitro in all cases examined. Oospores from the majority of crosses germinated, albeit at a low level (max 13.4%), after extraction from agar cultures by high-speed blending and treatment with novoZym 234. Viability of oospores from 20 crosses was tested by three methods. Two methods involved stains, either tetrazolium bromide (MTT) or phloxine B, and the third measured plasmolysis in 2 M NaCl. Both staining methods indicated a high percentage viability but gave false-positive results with heat-killed oospores. The plasmolysis method gave a lower percentage viability but no false positives. Oospores produced in vitro and stored either in sterile H(2)0 or in soil at temperatures between 0 and 20 degrees C survived for between 5 and 7 months, the length of the experiments. Oospores buried in non-sterile field soil survived for up to 8 months (January-August). Inoculation of potato with zoospores of A1 and A2 isolates produced oospores in stems but not in leaf tissue. In some, but not all cases, rapidly growing potato shoots (15-mm long) were successfully infected with oospores produced in vitro.