Vegetable Disease Update: August 29, 2018

The drier conditions the past few days have slowed the epidemic spread of cucurbit downy mildew across the region.
Vegetable Disease Update: August 29, 2018 - News


Irregular tan cracked lesions characteristic of anthracnose on a cucumber leaf. Photo: Beth Gugino, Penn State

There are continued reports of cucurbit downy mildew across the region although not on any new cucurbit host types. In Pennsylvania, it has been reported on cucumber, cantaloupe, butternut squash, jack-o-lantern pumpkin and Delicata squash. As cucurbit fields are done being harvested, consider plowing down the residue to reduce disease pressure both on your farm as well as for neighboring farms.

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.

Anthracnose on Cucurbit Crops

It is not uncommon to see anthracnose on cucurbit crops like cucumber or summer squash. It is caused by the fungal pathogen Colletotrichum orbiculare and only affects cucurbit crops and wild hosts. It initially causes water-soaked lesions that quickly become brown and necrotic with the lesion centers drying up and falling out. It can also cause a fruit rot that is covered in black spore-like structures with mats of salmon/pinkish spores. The disease is favored by warm wet conditions and is quickly spread by rain splash. Resistant cultivars are an effective tool, especially for cucumbers. If using fungicides, the rotation for resistance management is essential especially to prevent the development of resistance to strobilurin-type fungicides (FRAC code 11).

Late blight was confirmed on tomato in my research trial at Rock Springs in Centre County earlier this week. Disease incidence and severity were low, and all the symptomatic leaves were removed to determine the genotype. A late blight specific fungicide was also applied to the entire field. The cooler temperatures and wet weather last week created favorable conditions, but the exact source of inoculum is unknown since the plants were grown from seed at the research farm. Earlier this season, the previously unknown genotype isolated from tomato in two counties in New York has been determined to be a new genotype designated as US25.

A molecular DNA analysis indicates that it does not match any previously characterized genotype. Although not widespread, it is cause for concern because it is the opposite mating type of the most commonly occurring genotype US23 and interacting with US23 could enable it to survive to overwinter in the soil. All reports from PA have been determined to be US23, and I have no reason to suspect that the report from Centre Co. is any different. If you suspect late blight, or your local Penn State Extension Office. It is crucial that we collect a sample to characterize the genotype. The presence of a new genotype could alter our current and future management recommendations. For the latest reports visit

Black Mold on Onion

Seeing black soot between the bulb scales on your harvested onions? That black soot is likely the disease called black mold caused by a fungal pathogen, Aspergillus niger. The fungus is common in soil and crop residue and affects many vegetable crops. On onion, it exposed a black dusty fungal growth on and between the bulb scales and when severe, can lead to bulb rot by secondary bacterial organisms. It is primarily a post-harvest problem when the bulbs remain hot under high relative humidity (>80% RH), or there are fluctuations in temperature (e.g., coming out of cold storage) that result in the formation of condensation on the bulbs while in the bins and then exposed to high temperatures. Weather conditions this season have been favorable for this disease. Reducing exposure to high temperatures and storing at low humidity will help manage black mold.

Black soot (sporulation) characteristic of black mold on onion caused by the fungal pathogen Aspergillus niger. Photo: Beth Gugino, Penn State