Vegetable Disease Updates for August 16, 2017

On August 15, 2017 late blight was confirmed on tomato in a commercial field in Cumberland County, PA. It has previously been reported in Chester and Indiana counties on tomato and/or potato.
Vegetable Disease Updates for August 16, 2017 - News


Severe foliar late blight symptoms on tomato. Photo: Beth K. Gugino, Penn State

Late Blight in Chester, Indiana and Cumberland Counties, PA

In addition this past week, there have been an increasing number of reports of late blight on tomato and potato in western New York as well as in Massachusetts. The unsettled weather has been favorable for late blight and even though the days might be sunny and warmer the cooler night temperatures and heavier dews will favor disease development. Continued scouting is recommended. A fungicide like chlorothalonil, applied preventatively and with good coverage can be used to successfully manage late blight. In counties and adjacent counties were late blight has been confirmed, consider including a late blight specific fungicide in your spray program.

If you suspect late blight on your farm, please contact your local Penn State Extension Office or let Beth Gugino know via email at or by phone at 814-865-7328. We are interested in collecting samples so we can better understand how the pathogen population is changing both within and across growing seasons. Also for the information regarding where the latest confirmed outbreaks have been reported and to receive email or text alerts about when late blight has been confirmed with a personally defined radius from your location visit

Increasing Concern About Northern Corn Leaf Blight in Pennsylvania

There are an increasing number of reports and general concern about Northern corn leaf blight given the wet weather and losses from previous seasons.

Northern corn leaf blight symptoms are usually first observed on the lower leaves and the spread up the plant. The lesions are initially small, elliptical and gray-green in color. As the disease progresses the lesions will expand to 1 to 6 inches long, become tan in color and are not restricted by the leave veins. Eventually, the lesions will coalesce and cover the entire leaf. Under humid conditions, the lesions will produce dark gray spores on the lower leaf surface giving them a dusty appearance. A new lesion can produce spores in as little as one week under favorable conditions. The spores are then disseminated by rain splash to the leaves of nearby plants or they can be carried in the wind longer distances during storms. The greatest losses from NCLB occur when severe necrosis develops on the upper 2/3 of crop canopy by silking. The reduction in photosynthesis due to the necrosis results in reduced ear fill and when symptoms develop on the husks they appear older and are less marketable.

Elongated tan lesions characteristic of Northern Corn Leaf Blight on corn. Photo credit: Allison Robertson, Iowa State University

The crop should be scouted regularly, focusing on the lower leaves where symptoms develop first. Protectant fungicides like chlorothalonil can be applied when there are reports of NCLB in the area but symptoms have not been observed in the field. Good coverage is critical. NCLB specific fungicides include those in FRAC group 11 (strobilurins; e.g. Quadris and Headline) and FRAC group 3 (triazoles; e.g. Tilt). There are also a number of products that contain both FRAC groups (11 + 3; e.g. Quilt and Stratego). Rotate between these FRAC codes and tank mix with a broadspectrum protectant for resistance management when symptoms are first observed in the field will help manage NCLB. PHIs vary between the products so read the labels carefully when the crop is near harvest. Also depending on the label, NCLB might be referred to as Helminthosporium leaf blight which is collectively refers to both Northern corn leaf blight and Southern corn leaf blight. Fungicides are most effective at mitigating yield loss when applied preventatively at early silking.

Cucurbit Downy Mildew Update

Downy mildew is continuing to spread with numerous reports on cucumber and an increasing number of reports on cantaloupe. Symptoms on cantaloupe differ from those on cucumber. The lesions are less angular and tend to be darker brown in color with a yellow halo (see picture). Purplish gray sporulation will develop on the underside of the leaves however a hand lens may be needed to see it since it is not as abundant as on cucumber. There have also been a few reports on butternut squash in the region in Maryland, Delaware and Massachusetts. This pathotype can go other Cucurbita spp. which includes jack-o-lantern pumpkin however the closest reports on jack-o-lantern pumpkin are in North Carolina and Indiana.

Characteristic downy mildew lesions on cantaloupe. Photo: Beth K. Gugino, Penn State

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 contact Beth Gugino by email at or by phone at 814-865-7328 or contact your local Penn State Extension Office.