Vegetable Disease Update: August 1, 2018

The wet weather is wreaking havoc in many vegetable fields.
Vegetable Disease Update: August 1, 2018 - News


Downy mildew lesions on pumpkin. Photo: Beth Gugino, Penn State

There have been several reports of suspected late blight that have turned out to be early blight and Septoria leaf spot causing significant defoliation. So still no new reports of late blight other than the reports from earlier in the season. If you suspect late blight, please contact your local Penn State Extension office or let Beth Gugino know via email at or by phone at 814-865-7328. For the latest reports check

Powdery mildew is a common sight in cucurbit plantings that are setting fruit. Remember that the effective fungicides for powdery mildew have a completely different mode of action than those for downy mildew. There is considerable variability in host resistance so scout cucurbit cultivars separately. The latest fungicide resistance management guidelines can be found in the 2018 Fungicide Resistance Management Guidelines for Cucurbit Downy and Powdery Mildew Control in the Mid-Atlantic and Northeast Regions.

This season several suspected potato black leg samples have been submitted for testing to determine if the symptoms were caused by the typical species of Pectobacterium or by a species of Dickeya, which causes similar symptoms but is more aggressive. Of the 15 samples so far, four tested positive for Dickeya using a molecular detection method. Of those, three of the four are suspected to have been sourced from the same seed supplier, and likely are from the same seed lot. Sourcing pathogen-free seed remains the primary management strategy. In some cases, Pectobacterium was also present along with Dickeya; not an either or situation.

Be on the lookout for squash bug in your cucurbit crops. The squash bug itself can cause significant foliar feeding damage leading to complete necrosis and collapse of the leaves. It can also transmit a bacterial pathogen (similar to bacterial wilt) which can cause squash and pumpkins (most susceptible) to turn yellow and collapse within two weeks of harvest due to a disease called cucurbit yellow vine decline .

Squash bug female laying characteristic bronze colored eggs in a diamond-shaped pattern on a pumpkin leaf. Photo: Beth K. Gugino, Penn State

High-Risk Alert for Downy Mildew on All Cucurbit Crops

The unrelenting wet weather has created ideal conditions for the spread and development of downy mildew on all cucurbit crops including cucumber, cantaloupe, summer squashes, winter squashes and pumpkin not only across most of PA but all along the east coast. Yesterday, downy mildew was confirmed on jack-o-lantern pumpkin in NJ and last week on butternut squash in NJ. It is advised that all cucurbits fungicide programs include products that are specific for downy mildew and be maintained on a tight spray schedule as long as the wet weather persists It is also recommended that for crops with successive plantings, once harvest is complete disk-under, rogue or spray the planting with an herbicide to kill the plant tissue. If infected with downy mildew, once the plant tissue is dead, the pathogen is dead. If not infected with downy mildew, then you are removing potential plant tissue that could potentially become infected and serve as an inoculum source for your later plantings.

If you suspect cucurbit downy mildew 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. Every confirmed report of downy mildew enables us to improve disease forecasting accuracy for the benefit of cucurbit growers not only in Pennsylvania but all along the east coast. Even reports that are made from previously reported counties. The latest information on reports of cucurbit downy mildew can be found at the CDM ipmPIPE website.

Downy mildew risk map for Thursday, August 2, 2018 (