Incidence of Phytophthora infestans (Mont.) de Bary on potato and tomato in Maine, 2006–2010

Abstract

Abstract Late blight, caused by Phytophthora infestans (Mont.) de Bary, is a devastating disease which is found worldwide. In Maine, United States (US), we recorded late blight on potato and tomato during the 2006-2009 cropping seasons. From 2006 to 2008, over 90% of the diseased samples were collected in potato fields from northern and central Aroostook County in Northern Maine, US. Then, in 2009, an unprecedented influx of inoculum on infected tomato transplants shipped to retail garden centers throughout the Northeast US significantly changed the late blight infection patterns. In 2009, 43% of diseased samples obtained were from tomato, and 57% from potato, and disease was found to occur all over the state. Moran’s index and spatial autocorrelation analysis of disease occurrence, geographical locations, host factors, and infection levels from previous years, were not statistically significant (p > 0.05). Therefore, random distributions of late blight incidences were recorded across locations and years. Nearest neighbor analysis revealed that mean spatial distances for late blight occurrence ranged from 1.51 to 71.4 km from 2006 to 2008, and 7.4 to 126.5 km in 2009. The frequency and locations of late blight outbreaks in 2009 were substantially greater than in 2006, 2007, and 2008, as affected by the influx of inoculum and movement of infected tomato seedlings as well as conducive environmental conditions. All were contributing factors for late blight occurrence in Maine. In 2010, few disease samples were collected, indicating that the influx of inoculum in 2009 did not persist to cause widespread disease in 2010. The reduction of inocula sources, fungicide protection of susceptible hosts, and the removal and destruction of infected tomato seedlings and potato cull piles or volunteer plants, can greatly reduce late blight occurrences and improve potato production. These actions should be considered as an integral part of late blight management programmes in regions where late blight commonly occurs