2022 Pennsylvania Vegetable Disease Update for August 24

A report on tomato in the adjacent county in eastern TN approximately a month ago was recently entered into the USAblight monitoring website. The cooler temperatures and more extended dew periods favor late blight development.

Late blight is characterized by irregular water-soaked lesions that are initially pale green before turning grey-brown. The lesions will tend to develop on the upper to the middle part of the plant canopy as opposed to early blight and Septoria leaf spot (tomato only) that start on the lower leaves and progress upward. Under humid conditions, the lesions on the underside of the leaf will sporulate, giving them a fuzzy white appearance (Figure 1).

Figure 1. Foliar symptoms of late blight. Photo: Beth Gugino, Penn State

Figure 2. Foliar symptoms of early blight. Photo: Beth Gugino, Penn State

In comparison, early blight lesions are initially small dark spots that develop characteristic concentric rings and are often surrounded by chlorotic leaf tissue (Figure 2). In contrast, Septoria leaf spot lesions are very circular, with a tan center surrounded by a dark margin (Figure 3). With the aid of a hand lens, small black pycnidia (spore-producing structures) can be seen in the center of the lesions. Septoria leaf spot does not cause fruit lesions.

Figure 3. Foliar symptoms of Septoria leaf spot. Photo: Beth Gugino, Penn State

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.

Bacterial soft rots are becoming more common in vegetable fields that have experienced some of the intense storms passing across PA this past week. Any injury, whether due to wind, hail, or insect damage, creates wounds that soft rotting bacteria can enter and cause tissue degradation in the form of soft rot. Once observed, there is no effective treatment other than to rogue out the symptomatic plants. Efforts to reduce damage and minimize soil splashing onto the plants and fruit will help the subsequent growing season.

Downy mildew continues to spread on cucumber across the region. Within the past week, there have now been reports on butternut squash in MA and on jack-o-lantern pumpkin in northwest OH against the MI border, as well as in western and southeastern VA. Continue to scout all cucurbit crops and burn down or disk under fields that are no longer being harvested to reduce inoculum. In PA, downy mildew was most recently confirmed in Adams County and previously on cucumber in Centre, Mifflin, Dauphin, and Lehigh, Juniata, Snyder, Lancaster, Berks, and Bucks Counties. If you suspect cucurbit downy mildew on your farm, please let me know either 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 4. Chlorotic lesions and purplish sporulation are caused by downy mildew on a pumpkin leaf's upper and lower side. Photo: Beth Gugino, Penn State

Clinic Corner

Alternaria Head Rot and Bacterial Soft Rot on Broccoli

In August, the Penn State Plant Disease Clinic received a broccoli sample from a commercial field in Columbia County, PA, with head rot symptoms. The heads showed varying degrees of brown, necrotic tissue. Severely necrotic areas were accompanied by water-soaking and soft rot. The soft rot symptoms made the sample quite odoriferous. Two diseases were identified on these broccoli heads, Alternaria head rot and bacterial soft rot.  

Figure 5. Broccoli sample showing head necrosis and bacterial soft rot Photo: Jennie Mazzone, Penn State

Figure 6. Broccoli sample close-up showing head necrosis and bacterial soft rot. Photo: Jennie Mazzone, Penn State

The Alternaria fungus can cause a leaf spot and head rot on broccoli. Leaf spots caused by this disease are brown, circular spots, usually with concentric rings, and surrounded by a yellow halo. The early stages of this disease on the head appear as small necrosis spots. As the disease progresses, head rot develops, and signs of the fungus may appear on affected tissue as dark brown sporulation. Follow the 2022-23 Mid-Atlantic Commercial Vegetable Recommendations for Alternaria leaf spot to manage this disease.

Bacterial soft rot typically occurs on already damaged tissue. In this case, the tissue already damaged by the Alternaria may have been the entry point for the soft rot bacteria. Several species of bacteria can cause this disease. Warm, wet weather favors soft rot development. Avoid conditions that keep heads wet for extended periods. Select resistant varieties. Varieties with tight, dome-shaped heads with small buds are less susceptible to this disease as water can more easily roll off the head instead of collecting on the head. Minimize injury to avoid this disease.