Response of tubers of five potato cultivars to co-inoculation with US-1 and US-8 genotypes of Phytophthora infestans

The severity of late blight tuber rot in five potato cultivars (Green Mount ain, Sebago, Dorita, AC Brador and Island Sunshine) resulting from inoculat ion with US-1/US-8 combinations of Canadian isolates of Phytophthora infest ans was evaluated. In addition, the potential for mixed (Al:A2) inoculation s to result in oospore formation was examined. In studies conducted in 1996 and 1997, cv. Dorita (and Island Sunshine in 1997) developed the least amo unt of surface or internal necrosis relative to the other cultivars followi ng co-inoculation with US-1 and US-8 isolates of P. infestans. Differences also were revealed among isolate combinations used for inoculation. The com bination of a US-1 isolate from Prince Edward Island with a US-8 isolate fr om New Brunswick consistently produced the least amount of necrosis in tube rs of the various cultivars. There was no evidence of oospore production in any of the tubers examined.