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A Rapid Response System for Prediction and Management of Phytophthora Diseases in Massachusetts Farms Using Risk Mapping and Real Time PCR

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Phytophthora species consistently rank as some of the most devastating disease agents in Massachusetts farms. Two species, P. infestans and P. capsici, attack regionally important vegetable crops, including cucurbits, peppers, tomatoes, eggplant and potatoes. In 2007, over 8,000 acres of vegetable crops susceptible to infection by P. capsici and P. infestans were harvested in Massachusetts. Both species are exotic to New England, and have been responsible for disease outbreaks that have resulted in losses totaling hundreds of thousands of dollars in damages.

In 2009, an outbreak of P. infestans in the northern United States and eastern Canada devastated tomato and potato crops. This outbreak was caused by two unique genotypes of P. infestans, which differed in virulence, host preference, and fungicide resistance. This outbreak highlighted the need to determine the incidence of Phytophthora in Massachusetts farms, and for the development of a rapid disease response system that could accurately identify clonal lineages, new outbreaks, and also predict areas that are most likely to support disease development. The clonal lineage that was predominant in 2009 was US-22 which is highly sensitive to mefenoxam. Mefenoxam is highly effective against sensitive strains but it was not recommended in 2009 because most strains are known to be insensitive to the fungicide. The rapid detection of clonal lineages would allow more effective fungicide recommendations.

Phytophthora capsici has become firmly established in many fields in southern New England. Research has shown that P. capsici oospores were viable after exposure to freezing temperatures, allowing the pathogen to over-winter in northern regions. Soil fumigations can, at times, effectively manage P. capsici and P. infestans, while crop rotations are not always successful. Because Phytophthora species sporulate profusely under suitable environmental conditions (excess soil moisture and soil temperatures from 15 to 30 C), if outbreaks are not managed at early stages of development they become too expansive to control.

Current disease forecasting models for Phytophthora have been developed for use over large areas, and do not incorporate case-history information or site-level monitoring. While these forecasting models have enormous utility, they cannot provide the resolution required to adequately predict disease outbreaks at the farm-scale in Massachusetts (see The most fundamental component for an effective risk map, and often the most difficult to obtain, is accurate data on the current distribution of the targeted pathogen. Without this foundation, risk maps cannot accurately predict where and how a pathogen may spread and management plans often fail to meet their objectives.

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