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USA - Downy mildew now confirmed on cucumber and butternut squash in Pennsylvania                 


Pennsylvania, USA
July 17, 2015


Highlighted counties are where CDM outbreaks have been reported as of 16 July 2015. Red indicates that the reports were made within the past 7 days. Map source: Cucurbit Downy Mildew ipmPIPE website.

This past week reports of downy mildew are continuing to increase in the Great Lakes region with nine new counties reporting outbreaks on cucumber including Elk County, PA and as of today it was also confirmed on butternut squash in Lancaster County. 

Downy mildew on the upper leaf surface of cantaloupe.
Symptoms of downy mildew on the upper leaf surface of cantaloupe (Photo: Beth K. Gugino).

Infected cucumber fields continue to be 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 although the vast majority of reports to the south have been on cucumber. Yesterday, downy mildew was also confirmed in a commercial butternut squash field in Lancaster Co. To-date there have been no reports on jack-o-lantern type pumpkins.

Downy mildew on pumpkin
Symptoms of downy mildew on the upper and lower of a pumpkin leaf (Photo: Beth K. Gugino).

Symptoms of downy mildew vary depending on the cucurbit host. On cucumber as well as butternut squash the lesions are more angular and delineated by the leaf veins while on other cucurbits the lesions can be more irregular in shape. In all cases, the lesions are initially chlorotic or yellow before becoming more tan and necrotic. On cantaloupe, the necrotic lesions are often surrounded by a yellow halo (see picture below). On watermelon, the symptoms of downy mildew are very similar to anthracnose and the purplish-grey sporulation on the underside of the leaf can be sparse so the use of a microscope is often needed for confirmation. On pumpkin and winter squash, the lesions tend to be smaller than those of cucumber (see picture below). As the disease progresses, leaves will often turn brown and the edges turn up giving the leaves a scorched appearance. In all cases, observation of the purplish-grey sporulation on the underside of the leaf is necessary for confirmation. In the early morning, the lesions on the underside of the leaf may appear water-soaked.

If you are not already doing so, it is important to be regularly scouting your cucurbit fields. 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. If you have been applying protectant fungicides then the sporulation may not be as obvious.

At this time it is recommended that you, at the very least, use a protectant fungicide program on all your cucurbit crops and consider using downy mildew specific fungicides like Ranman, Previcur Flex and Zampro (see the 2015 Commercial Vegetable Production Recommendations for a more complete list of recommended products) especially on cucumber, cantaloupe and butternut squash. Also consider including a copper-based product if you are also concerned about angular leaf spot or other bacterial diseases. 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.



More news from: Penn State University


Website: http://www.psu.edu

Published: July 19, 2015

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