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Downy Mildew Confirmed on Cucumbers in Pennsylvania

Posted: July 8, 2015

Yesterday downy mildew was confirmed on cucumber in Berks County. This is the first confirmed report in PA however there are likely more infected fields around so scout yours now if you haven’t already.
Highlighted counties are where CDM outbreaks have been reported as of 8 July 2015. Red indicated the reports were made within the past 7 days. Green is more than 7 days ago. Map source: Cucurbit Downy Mildew ipmPIPE website.

Highlighted counties are where CDM outbreaks have been reported as of 8 July 2015. Red indicated the reports were made within the past 7 days. Green is more than 7 days ago. Map source: Cucurbit Downy Mildew ipmPIPE website.

Infected cucumber fields are the primary sources of inoculum both north and northwest of PA in MI, OH, NY and Ontario while downy mildew has been confirmed on a more diverse range of cucurbit hosts to the south. These include watermelon in DE, MD, SC and GA as well as acorn, summer, and butternut squash and cantaloupe in SC. The vast majority of reports in the south however have been on cucumber.

Recent conditions have been very favorable for downy mildew infection in PA and the continued chance of rain and storms in the long range forecast will likely continue to put PA fields at risk.

If you are not already doing so, it is important to be regularly scouting your cucurbit fields especially cucumbers, cantaloupe and watermelon regularly for downy mildew. It takes several days for symptoms to develop after infection and conditions this past weekend were very favorable for disease development. Look at both the upper and lower leaf surface. Purplish-gray sporulation on the lower leaf surface is diagnostic for downy mildew. If you have been applying protectant fungicides then the sporulation may not be very obvious.

At this time it is recommended that you, at the very least, use a protectant fungicide program on your cucumbers and cantaloupe if you are not already using one and consider expanding it to include all your cucurbit crops due to the reports on watermelon in DE and MD. Also consider including a copper-based product if you are also concerned about angular leaf spot. For organic production, copper-based products are most effective and when alternated with Serenade have reduced disease severity. Oxidate is effective at killing the spores that it comes in contact with when applied but has not residual activity. Other possible rotational partners with copper are Regalia and Actinovate. Under favorable conditions, it can be very difficult to manage downy mildew either conventionally or organically. If you have succession planting, disking under or destroying the crop residue as soon as you are done with harvest will reduce disease pressure on the other plantings.

For the latest information on outbreaks and to receive email or text alerts please visit the Cucurbit Downy Mildew Forecasting website. Updates will also be made to the 1-800-PENN-IPM hotline weekly or more frequently if needed to provide growers with information that can be used to help make timely management decisions. The forecasted risk maps are also based on knowing where there are downy mildew infected fields (sources of the pathogen) so it is important if you suspect downy mildew on your farm to let me know either by email or by phone at 814-865-7328 or contact your local Penn State Cooperative Extension Office.

Contact Information

Beth K. Gugino
  • Associate Professor Vegetable Pathology
Phone: 814-865-7328