Viability of oospores of Phytophthora infestans under field conditions in northeastern North America

Abstract

Studies were conducted in potato fields on Prince Edward Island (PE) and New Brunswick (NB) to determine the overwintering viability, germination, and infectivity of oospores of Phytophthora infestans. Oospore viability was determined using plasmolysis (4 M NaCl) and tetrazolium bromide (MTT) tests. After seven months exposure to winter conditions in potato fields, the viability of oospores buried on PE and NB ranged from 2.5 to 13% and from 1 to 28%, respectively, with the plasmolysis test and from 13 to 44% and from 2.5 to 40%, respectively, with the MTT test. In other studies on PE, oospore viability ranged from 5 to 15% 12 months after burial. In laboratory studies, oospores exposed to temperatures of -20 and -50°C for up to 90 days had viabilities greater than 50%, whereas exposure to 0, 4, or 15°C resulted in average viabilities from 23 to 36%. The lowest viability, 22%, was recorded at the highest temperature tested, 36°C. Oospore germination was low, ranging from 6 to 19%. However, this level was sufficient to demonstrate that oospores could be a source of inoculum. A potato soil leachate medium was superior to others for stimulating oospore germination. Oospores of P. infestans present in potato stems and infested farm soil, maintained underground for seven months, were able to infect potato leaf discs in the laboratory. Although all studies were conducted in essentially gnotobiotic conditions, they demonstrate that oospores cannot only survive the winter conditions in northeastern North America but can also germinate and infect new potato tissue in the following growing season. While it would be useful to confirm these findings with new studies under native soil conditions, this is not practical at this time. The presence of oospores also poses a new disease risk that needs to be addressed in future late blight management strategies.