2022 Pennsylvania Vegetable Disease Update for August 17

There was one new report on jack-o-lantern pumpkin in northwestern Ohio in Fulton County, along the Michigan border. Downy mildew is currently confirmed on cucumber in Centre, Mifflin, Dauphin, Lehigh, Juniata, Snyder, Lancaster, Berks, and Bucks Counties. If you suspect cucurbit downy mildew 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 website for the latest reports and forecasts.

Figure 1. Select 2022 cucumber downy mildew fungicide trial treatments taken on August 10: A: Untreated control; B: Ranman, Previcur Flex, Zampro tank mixed with chlorothalonil rotation; C: Champ all applied weekly on a preventative schedule. Photos: Beth K. Gugino, Penn State

This 2022 season, a fungicide efficacy trial on cucumber was conducted at the Penn State Russell E. Larson Agricultural Research Center. Downy mildew moved rapidly through the trial, with disease incidence (percentage of leaves in a 16 ft plot showing symptoms) progressing from an average of 5 to 95% in the untreated control plots from 3 to August 10. Plots that were preventatively sprayed with a rotation of the targeted fungicides Ranman (FRAC 22), Previcur Flex (FRAC 28), and Zampro (FRAC 45 + 40) tank mixed with chlorothalonil had an average disease incidence of < 1%. OMRI-approved Champ WG (FRAC M1) applied as a weekly preventative had an average disease incidence of 11%. Plots sprayed with a single targeted product (not a registered use application) indicated that Ranman and Previcur Flex are still very effective for managing downy mildew on cucumber, while Presidio was ineffective. A full report will be available this fall.

Based on the USAblight map, there continue to be no reports of late blight on tomato or potato in the region. If you suspect 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.

Clinic Corner

Southern Blight on Tomato

The Penn State Plant Disease Clinic has received several tomato samples this summer with the disease southern blight caused by the fungus Athelia rolfsii (=Sclerotium rolfsii). One set of samples was associated with a research field in Centre County, PA, one from a commercial high tunnel in Allegheny County, PA, and another from a homeowner garden in Lehigh County, PA. The samples had lower stem/crown rot. Adventitious roots were forming above the rotted stem area. The stem lesions girdled the plant, causing wilt of the entire plant. Signs of white mycelium and reddish-brown circular sclerotia (overwintering structure) were present.

Figure 2. Tomato sample showing lower stem rot, wilt, and excessive adventitious roots caused by southern blight (A). Small red/brown sclerotia and white mycelium of southern blight (B) compared to the large black sclerotia caused by timber rot (C). Photos: A and C Jennie Mazzone, Penn State, and photo B, Beth Gugino, Penn State

Southern blight is a significant disease to be aware of but is not commonly found in PA. The disease is more commonly found in the southeastern U.S. and as far north as Maryland and Delaware. When we find it in PA, it is often associated with transplants sourced from the southeast. This disease may be confused with timber rot (also known as white mold) caused by the fungus Sclerotinia sclerotiorum. The color, shape, and size of the southern blight pathogen’s sclerotia help distinguish it from timber rot. The southern blight pathogen’s sclerotia are reddish-brown, spherical, and small (1-2 mm diameter), whereas the timber rot pathogen’s sclerotia are black, irregularly shaped, and larger in size (2-10 mm diameter). The southern blight fungus is favored by high temperature, moist soil conditions, and acidic soil.

Southern blight can be difficult to control. This fungus can infect many plant species, including tomato, pepper, bean, cantaloupe, carrot, potato, sweet potato, watermelon, tobacco, and soybean. Crop rotation with a nonhost like corn or small grains is recommended. More information on southern blight management can be found in the 2022-23 Mid-Atlantic Commercial Vegetable Recommendations.