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Recent increase in new locations reporting late blight in the Northeast

In the past week there have been eight new reports of late blight in NY and the New England States, several in counties where late blight had not yet been reported this season (map). A common theme in these reports is that plants looked fine one day and were heavily infected 2-3 days later. This could mean that the early stages of infection were missed, or that there were a large number of spores in the air that were deposited on leaves while they were wet from rain or dew.

The isolates of the late blight pathogen that have been identified in the Northeast to date this season continue to be exclusively US-23, which is susceptible to mefenoxam, the active ingredient in Ridomil, giving conventional farmers an excellent control option. See my pervious post for more information on fungicide options.

Stay vigilant! Scout tomatoes and potatoes regularly and thoroughly, focusing on parts of the field that are shaded or poorly drained, maintaining high relative humidity longer. Trials have demonstrated that copper fungicides are the most effective of those allowed in certified organic production. Be sure you are getting excellent coverage, and reapply often when weather is favorable for late blight. Keep in mind that leaf wetness from heavier dews at this time of year are enough to favor infection even if the weather is not rainy.

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