2022 Pennsylvania Vegetable Disease Update for August 3

There have also been additional reports in Michigan, West Virginia, and Virginia, all on cucumber. There have been no confirmed reports on pumpkin, winter squash, or watermelon north of eastern Tennessee. Based on the USAblight map, there continue to be no reports of late blight on tomato or potato in the region.

If you suspect cucurbit downy mildew or late blight on your farm, please let me know by email at bkgugino@psu.edu or by phone at 814-865-7328 or contact your local Extension Office. Check out the Cucurbit Downy Mildew Forecast or USABlight website for the latest reports and forecasts.

Be on the lookout for basil downy mildew (Figure 1). Like other downy mildews, this one, caused by Peronospora belbahrii, is also host-specific. Symptoms include yellowing or chlorosis of the foliage, which looks similar to a nutrient deficiency. However, when conditions are cool (50 to 78°F) and wet (> 85% relative humidity), purplish gray to black sporulation can be visible on the underside of the leaf, similar in appearance to cucurbit downy mildew. Inoculum tends to build later into the season. The pathogen does not survive without a plant host and therefore does not survive to overwinter in field production (or home garden) situations.

Management primarily focuses on planting pathogen-free seed, selecting less susceptible cultivars, and applying fungicides. Conventional fungicides, including Ranman (FRAC 21), Revus (FRAC 40), Quadris (FRAC 11), Armicarb (FRAC NC), and phosphorous acid (FRAC 33) fungicides are registered for basil downy mildew. Several OMRI-approved products are also labeled for basil downy mildew, including but not limited to Procidic, Actinovate, Double Nickel, MilStop, Regalia, Cueva, Trilogy, and OxiDate. Practices that minimize leaf wetness and reduce humidity will also help manage disease. Once done with harvest, disk under or burn down the crop with an herbicide to eliminate potential inoculum sources for other plantings. If not already planting resistant varieties, consider them for future growing seasons, especially if growing the crop later in the season.

In the past few years, there have been outbreaks of Plectosporium blight on pumpkin and squash (also called Microdochium blight) across the region. The symptoms are relatively unique and include white diamond-shaped lesions that develop on the petioles and the veins underside of the leaves. The fruit can also develop small white spots that can coalesce, cover more significant portions of the fruit, and make the fruit more susceptible to soft rot. Warm and prolonged wet conditions favor the soilborne fungal pathogen, so symptoms may develop in lower parts of the field that retain water longer. Be sure to rotate cucurbits out of affected fields for 3 to 4 years to decompose crop residue thoroughly. Regular protectant fungicide applications with good coverage can effectively manage Plectosporium blight.

Figure 2. (Left) Plectosporium blight on a pumpkin handle post-harvest. Photo: Jeff Stoltzfus, Penn State Extension. (Right) Plectosporium blight on the fruit. Photo: Tom Maloney, Penn State Extension.

Clinic Corner

Septoria Leaf Spot on Tomato

In June, the Penn State Plant Disease Clinic received a tomato sample from a home garden in Bedford County, Pennsylvania. The sample had small, necrotic, circular leaf spots with dark margins. The spots were reported to be more severe on the lower leaves. Microscopic examination of the spots revealed the fungus Septoria, which causes Septoria leaf spot on tomato.

Figure 3. Necrotic leaf spots with dark margin on tomato caused by Septoria leaf spot. Photo: Beth K. Gugino, Penn State

Septoria leaf spot is a widespread disease on tomato. It can be confused with other foliar diseases, but some diagnostic features help distinguish this disease. Septoria leaf spots typically have nearly perfect circles with tan centers and dark margins. Close observation of the spots with a hand lens can reveal small, black circular fruiting bodies called pycnidia within the spot. Early blight (caused by the fungus Alternaria solani) can cause similar symptoms, but its more advanced leaf spots help distinguish it with the formation of a concentric circle (or bull's eye) pattern within the leaf spot. Bacterial spot and speck (caused by the bacteria Xanthomonas spp. and Pseudomonas spp., respectively) can also cause spots on tomato leaves, but these spots would lack fungal fruiting body signs. Remember that these diseases are not mutually exclusive, so more than one disease could affect the plants. If you suspect foliar disease on your tomato and are unsure of the cause, submit a sample to the Plant Disease Clinic.

Figure 4. Early blight causing necrotic leaf spots with concentric rink pattern surrounded by chlorosis on tomato (left). Photo: Jennie Mazzone, Penn State. Bacterial spot causing small necrotic spots on tomato (right). Photo: Beth K. Gugino, Penn State.

Septoria leaf spot is notorious for infecting the plant's lower leaves first and spreading upward, causing defoliation of severely affected leaves. The fungus overwinters on plant debris in the soil and can infect the plant during rain events where splash dispersal occurs. Control Septoria leaf spot by avoiding overhead irrigation, practicing crop rotation and proper plant spacing, trellising to increase air circulation, removing crop debris to reduce inoculum, and protectant fungicide applications. Please refer to the 2022-23 Mid-Atlantic Commercial Vegetable Recommendations for additional management recommendations.