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

Studies were conducted in potato fields on Prince Edward Island (PE) and Ne w Brunswick (NB) to determine the overwintering viability, germination, and infectivity of oospores of Phytophthora infestans. Oospore viability was d etermined using plasmolysis (4 M NaCl) and tetrazolium bromide (MTT) tests. After seven months exposure to winter conditions in potato fields, the via bility of oospores buried on PE and NE 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 studi es, oospores exposed to temperatures of -20 and -50 degrees C for up to 90 days had viabilities greater than 50%, whereas exposure to 0, 4, or 15 degr ees C resulted in average viabilities from 23 to 36%. The lowest viability, 22%, was recorded at the highest temperature tested, 36 degrees C. Oospore germination was low, ranging from 6 to 19%. However, this level was suffic ient to demonstrate that oospores could be a source of inoculum. A potato s oil leachate medium was superior to others for stimulating oospore germinat ion. Oospores of P. infestans present in potato stems and infested farm soi l, maintained underground for seven months, were able to infect potato leaf discs in the laboratory. Although all studies were conducted in essentiall y gnotobiotic conditions, they demonstrate that oospores cannot only surviv e the winter conditions in northeastern North America but can also germinat e and infect new potato tissue in the following growing season. While it wo uld be useful to confirm these findings with new studies under native soil conditions, this is not practical at this time. The presence of oospores al so poses a new disease risk that needs to be addressed in future late bligh t management strategies.