This makes a total of six confirmed reports so far in 2021. Late blight is favored by temperatures between 65 and 70°F and high relative humidity or leaf wetness. If you suspect late blight on your farm, please let me know either by email at [email protected], by phone at 814-865-7328, or contact your local Extension Office. Additional information about late blight can be found on the USABlight website.
There have been no new reports of cucurbit downy mildew across the region this week. To date in Pennsylvania, cucurbit downy mildew has been confirmed on cucumber and cantaloupe in Centre, Huntingdon, Mifflin, Juniata, Luzerne, Lehigh, Bucks, Lancaster, Chester, and Lackawanna, Erie, and Butler Counties. There have also been no new reports of downy mildew on jack-o-lantern pumpkin in central Ohio or Kentucky.
The closest report on butternut squash is in eastern North Carolina. As crops mature, the threat of downy mildew impacting yield decreases since it is a foliar disease that does not directly impact the fruit. Scouting is still highly recommended. The inclusion of a downy mildew-specific fungicide in a fungicide program should be considered for crops that are not close to harvest.
Figure 2. Cucurbit downy mildew monitoring map as of 8:30 a.m. August 25, 2021. The left map represents confirmed reports on cucumber and cantaloupe. The right map is all other cucurbit hosts including pumpkin, watermelon, and squash (https://cdm.ipmpipe.org).
It is important to maintain a regular fungicide program on cucumbers and cantaloupes. As you finish with a planting, burning down the crop will reduce the spread to other succession plantings. Once the plant tissue is dead, the pathogen is dead. If you suspect downy mildew on your farm, please let me know either by email at [email protected] or by phone at 814-865-7328 or contact your local Extension Office. Knowing where the disease is an important component for area-wide management. See the Cucurbit Downy Mildew Forecast website for the latest reports and disease risk forecasts.
There have been several reports of postharvest fruit rots of cucurbit crops. The fruit visibly appear healthy at harvest however, after several days post-harvest in the bins – fruit rot symptoms develop. Fruit rots can be caused by several soilborne fungal/oomycete pathogens including phytophthora, fusarium, pythium, didymella (gummy stem blight), colletotrichum (anthracnose), and plectosporium as well as several bacterial pathogens. Even though the symptoms develop post-harvest, the fruit were infected in the field, typically on the side in contact with the soil.
Management of any one of these diseases requires an integrated approach. No single method is going to adequately manage the disease. In the future consider using cover crops, mulches, or strip tillage to reduce direct contact between the fruit and the soil. Integrate practices that improve soil health and drainage. Also, minimize drought stress to reduce potential cracking and minimize wounding at harvest. Reliance on fungicides for managing fruit rot is nearly impossible due to issues with coverage. Keep in mind that post-harvest washing of the fruit will not “cure” fruit that is already infected and the act of washing may actually spread the fruit rot pathogens in the wash water.
Bacterial leaf spot of cucurbits, beets and Swiss chard
Bacterial leaf spot (BLS) on cucurbits and chenopods (beet and chard) is caused by strains of the seedborne bacterial pathogen Pseudomonas syringae. In beet and chard, the most common initial symptom of BLS is pale brown, irregular to circular shaped, water-soaked necrotic lesions surrounded by a red halo. Yellow lesions, that subsequently turn necrotic as the disease progress, may be observed on the leaf margins. Although BLS symptoms can be confused with Cercospora leaf spot (CLS), these can be differentiated by the shape and size of the spots, as CLS tends to produce circular spots that are smaller in diameter.
Another way to distinguish CLS from BLS is by looking for signs of the pathogen; for example, stromata (fungus fruiting bodies that appear as black dots) can be easily seen in the center of the spots with a hand lens in the case of CLS. In cucurbits, circular necrotic lesions are observed during the early stages of disease development, but in later stages, lesions become irregularly shaped and are delineated by leaf veins (Newberry et. al., 2016). Lastly, wart-like eruptions may be present on the fruit.
Figure 3. Typical symptoms of bacterial leaf spots on beet, chard, and cucurbits. (A) Wart-like symptom on butternut squash fruit; Photo: C.T. Bull; (B) Leaf spots on watermelon, Photo: S. Da Silva, M.L. Paret; and (C) leaf spots on table beet, Photo: L. Coulter.
As part of a federally funded research project leading to more effective integrated management strategies by enhancing understanding of this pathogen’s genetic diversity and epidemiology, our research team will be searching for the disease on beet, chard, and cucurbits from vegetable producing regions in the state of Pennsylvania.
You can contribute to our search and management of this important plant pathogen by reaching out to members of our team (below) and extension educators if you suspect you have BLS on beet, chard, and cucurbits in your plantings or by sending your samples to the Penn State Disease Clinic.
For more information about this project, please visit the Seedborne cucurbits and chenopods diseases caused by Pseudomonas syringae website or read the article published in Penn State News, Plant Pathologist Awarded Grant to Aid Global Study of Seedborne Pathogens.
For more information about the project contact: Raymond O. García-Rodríguez; [email protected]; 814-865-7448; Carolee T. Bull; [email protected]; 814-865-7448; or Beth K. Gugino; [email protected]; 814-865-7328.
For detailed information on how to prepare and send your samples, please visit the Penn State Plant Disease Clinic website for more information.