Downy mildew confirmed on cucumber and butternut squash in Pennsylvania, USA

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 it was also confirmed on butternut squash in Lancaster County. 

 

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. Last week, 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.

 

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 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.