From Meg McGrath, Long Island Horticultural Research & Extension Center, mtm3@cornell.edu:
Downy mildew has again started developing early (mid-July) on Long Island, which was anticipated following early detection in New Jersey (6/11/22). Since 2007 when monitoring was started on LI, symptoms have first been seen here from 7/14 (2021) to 9/7 (2010). Often it has been about 1 month after finding in NJ. The pathogen survives over winter in south Florida then moves through the eastern USA with cucurbit production.
Symptoms were found on cucumber on 7/20 at a farm on the North Fork through the Cornell Cooperative Extension IPM Scouting Program. Anyone signed up to receive alerts from the Cucurbit Downy Mildew Forecast Program for a radius from their farm that includes this location would have received an e-mail or text notification on 7/20 as soon as this report was posted. No symptoms were found on 7/19 when scouting was done on the South Fork, but it is possible this disease is present in cucumber crops not examined there.
Ideal conditions for long distance spread of this pathogen are cloudy days, which protect its spores from the sun’s uv rays, plus rain, which move spores from air currents down to plants plus spores germinate and infect when leaves are wet. At the Cucurbit Downy Mildew Forecast website there have been two high risk forecasts for successful spread from NJ to LI (6/27 and 7/2) and two moderate risk forecasts (7/5 and 7/12). Based on symptom appearance, 7/12 is likely when spread occurred to the affected North Fork cucumber crop.
All cucumber and cantaloupe crops should be examined weekly. View images of symptoms. Look throughout the crop because there can be hot spots with symptoms, especially early in disease development, as was the case on the farm where found. The pathogen clade affecting these crops does not affect squash and pumpkin; that clade arrives in the northeast during late summer.
Please report when you see this disease.
While rain plays an important role in pathogen spread into a crop, once downy mildew is developing in a crop, additional infections occur when humidity is high (> 85% RH) during nighttime, especially combined with dew. Conditions are expected to continue to be favorable for downy mildew to continue developing in affected crops.
For more information about this disease and its management see the Cornell Vegetables website.
Research is underway at LIHREC this summer to evaluate a new conventional fungicide, biopesticides, programs with biopesticides and conventional fungicides, and IPM fungicide programs applied to resistant cantaloupe varieties.