2022 Pennsylvania Vegetable Disease Update for August 10

There continue to be no confirmed reports on pumpkin, winter squash, or watermelon north of eastern Tennessee (the Figure 2 map below on the right). Previously reported counties include Centre, Mifflin, Dauphin, Lehigh, Juniata, Snyder, Lancaster, Berks, and Bucks. There have also been additional reports in Michigan, New Hampshire, and Virginia, all on cucumber with a few on cantaloupe (see the Figure 2 map below on the left). The recent weather conditions have been ideal for downy mildew, so a regular spray program on cucumber and cantaloupe is recommended.

Figure 2.

If you are done with a crop for the season, burn it down or disk it under to reduce the source of inoculum across the region. 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.

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

Phytophthora Blight on Pumpkin

The Penn State Plant Disease Clinic received a pumpkin sample in August from a commercial field in Clinton County, PA. The pumpkin fruit submitted were rotted and had a grey, white sporulation covering most of the fruit. Microscopic examination of this sporulation revealed the fungal-like oomycete pathogen Phytophthora, which causes Phytophthora blight on cucurbits.

Figure 3. Powder sugar sporulation caused by Phytophthora blight on severely infected pumpkin fruit. Photo: Jennie Mazzone, Penn State

Phytophthora blight can affect all aboveground parts of cucurbits. Signs of the pathogen appear as a grey, white mold and may be present on any affected plant part. Leaf lesions are irregularly shaped and can range from light green to necrotic. Stem lesions are chlorotic to necrotic and can cause the plant to wilt. Fruit lesions may have a water-soaked appearance and are more commonly found close to ground level where moisture is high.

Moisture management is essential in managing Phytophthora blight. Plant in well-drained fields on raised beds. Avoid overhead irrigation. Sanitize equipment used in infested fields. Phytophthora can survive for years in the soil. If Phytophthora blight is confirmed in a field, avoid planting susceptible crops (cucurbits, tomatoes, peppers, snap beans, etc.) in that field for as long as possible. Also, avoid planting susceptible crops in adjacent, low-lying areas where water runoff may accumulate, as the pathogen could be carried to these fields in heavy rains. More information on Phytophthora blight management is available in the 2022-23 Mid-Atlantic Commercial Vegetable Recommendations.