August 16, 2017 Update from Cheryl Smith, UNH Plant Diagnostic Lab:
Cucurbit downy mildew (CDM) has been identified in NH in both Hillsborough county (on cucumber) and Cheshire county (on muskmelon).
CDM symptoms begin as vein-delimited spots, but then expands and causes rapid collapse of vines. Excellent photos can be seen in the CDMpipe photo gallery. Cucumber and melon are susceptible to all pathotypes of CDM; for squash and pumpkin, reaction to pathotypes can vary, as can varietal resistance.
To control this disease with fungicides, growers should use fungicides specific for downy mildew. Commercial producers should reference the New England Vegetable Management Guide for fungicide options.
Options for organic producers are limited to copper fungicides (some of the Bacillus products may provide some suppression).
Home gardeners are also limited to copper, Bacillus products, and Agri-fos (Agri-Fos is not approved for organic production, and should not be mixed with copper products or applied immediately after a copper product).
Commercial producers and home gardeners in counties other than Cheshire and Hillsboro County should send samples of suspected CDM to the UNH Plant Diagnostic Lab for free confirmation, and to assist in the tracking of this disease. A sample submission form can be downloaded at http://unhpdl.org/ (be sure to write CDM confirmation at the top of the form).