The occurrence of many fungal and bacterial diseases of vegetables is dependent on the weather. In fact, rainfall, overhead irrigation, or high relative humidity can activate pathogens and determine disease severity or whether the disease will occur at all during the growing season. May’s relatively mild and wet conditions have stimulated a number of diseases to develop in both backyard gardens and commercial fields. The following is a list of diseases to be on the lookout for in June.

Early Blight on Tomatoes

The disease is generally observed on mature plants. Dark-brown spots with dark concentric rings, often surrounded by a yellow halo, develop first on older leaves (figures 1 and 2). Infected leaves may die prematurely, resulting in defoliation. People in the South often refer to this as a firing up of plants.  This early defoliation can result in exposed fruit, which leaves the fruit more susceptible to sunscald and poor color.

Bacterial Spot on Tomatoes and Peppers

Symptoms appear as small, dark spots on leaves and stems, occasionally with a slight-yellow halo (figure 3). Spots may merge into larger, dark lesions that often drop out during windy weather, resulting in a ragged appearance to the plants. The pathogen can also cause a distinct fruit spot. Lesions can enlarge up to 1/4 inch and become brown and scabby (figure 4). On peppers, the fruit spots become raised and scab like.

Anthracnose on Cucumbers and Melons

Fungal leaf spots on cucumbers and muskmelons begin as water-soaked areas that enlarge rapidly, turn brown, and drop out to form a ragged hole within the lesion (figure 5). Leaf spots on watermelon turn black. Elongated lesions may form on petioles and stems. They can girdle the stem, causing death of tissue beyond the lesion. Depressed, dark, and bordered craters form on large fruit. These craters have creamy pink-colored spores oozing from the center under wet conditions (figure 6).

Downy Mildew on Cucumbers and Muskmelons

Angular, yellow-to-brown spots develop on the upper side of older leaves near the center of the plant. A fuzzy mold forms on the underside of the leaf spots on the lower leaf surface (figure 7). Leaves die as spots increase in size, spreading from the crown of the plant outwards.

Choanephora Wet Rot on Squash

This is a common problem on summer squash. The pathogen affects wilted blossoms and spreads to attached fruit. Infected fruit will rot rapidly, and grayish-white mold appears on the rotted area (figure 8). The growth resembles small, black-headed pins stuck in the fruit.

Leaf Spots on Sweet Corn

Many fungal diseases can occur on sweet corn, including Northern corn leaf blight (NCLB), Southern corn leaf blight (SCLB), and common rust.

• NCLB is characterized by long, elliptical lesions that run parallel to leaf edges and can reach 6 inches in length (figure 9). The lesions start grayish-green then turn tan and may have dark fungal spores in the center.
• SCLB causes tan, elliptical lesions on leaves (figure 10). The disease initially appears as small, yellow spots that enlarge into oblong tan-to-gray streaks that are 1 to 2 inches long.
• Common rust appears as small, reddish-brown, and oval-to-elongated pustules on both the upper and lower leaf surfaces (figure 11). When severe, the diseases can cause yellowing and death of infected leaves.

More Information

Disease management varies by crop and disease. Contact your county Extension office for information on management of these plant diseases.