Tomatoes are the most popular crop in the home vegetable garden. While tomatoes are relatively easy to grow, foliar diseases often occur in the home garden. Early blight and Septoria blight are the two most common foliar diseases of tomato. Early blight produces brown spots (up to 1/2 inch in diameter) on infected leaves. Concentric rings of darker brown often appear in the leaf spots. Septoria blight produces small brown spots (approximately 1/8 inch in diameter) with tan or gray centers and dark edges. Both diseases cause heavily infected leaves to eventually turn brown, die, and fall off. Lower leaves are infected first with the diseases progressing upward during the growing season. Wet spring and early summer weather favors development of early blight and Septoria blight. Defoliation may be severe when favorable weather conditions exist.
Early blight and Septoria blight overwinter on plant debris left in the garden. Fungal spores are splashed onto the foliage by raindrops or splashing water. A wet leaf surface is required for the spores to invade the plant tissue.
Home gardeners can help reduce blight problems on their tomatoes with good cultural practices. Fungicides may also be needed.
- Select stocky, healthy plants at a garden center or greenhouse. Unfortunately, there are no tomato varieties resistant to the tomato blights.
- Plant your tomatoes in a different location in the garden each year. Rotate crops so that tomatoes and other solanaceous crops (potatoes, peppers, and eggplants) are not grown in the same area for at least 3 or 4 years. Obviously, a 3 or 4 year rotation may not be feasible for gardeners with small vegetable gardens. However, small plot gardeners should rotate as much as possible. There is no home garden treatment that effectively kills the soil pathogens in the soil.
- When planting tomatoes, space plants approximately 3 feet apart. Adequate spacing allows good air movement and promotes rapid drying of plant foliage.
- Grow tomato plants in wire cages. The foliage of tomatoes growing in a cage will dry more rapidly than those sprawling on the ground. Gardeners can buy wire tomato cages at garden centers or make their own using concrete reinforcing wire or hog wire. A wire cage 2 feet in diameter and 4 to 5 feet tall should be adequate for most tomato varieties.
- In early June, apply a 2 to 3 inch layer of mulch around each tomato plant. Shredded leaves, dry grass clippings, and straw are excellent mulches. The mulch reduces the splashing of fungal spores onto plant foliage. Placing the mulch around plants in early June allows the soil to warm up in the spring.
- Avoid wetting tomato foliage when watering. Apply water directly to the ground around plants with a soaker hose or slow running hose. If a sprinkler must be used, water in the, morning so the foliage dries quickly.
- While cultural practices may help control tomato blights, fungicides are often needed. Apply fungicides (chlorothalonil, maneb, or copper-based fungicides) at 7 to 14 day intervals beginning 2 to 4 weeks after transplanting. Thorough coverage is essential. Be sure to spray both the upper and lower leaf surfaces as well as the centers of the plants. Spray to the point of runoff.
- If blight occurs, remove and destroy infected leaves as they appear. Prompt removal of infected leaves may slow the progress of the, blights. At the end of the gardening season, remove and destroy all infected tomato plants. Clean up and dispose of as much tomato plant debris as possible.
Tomato blights are common problems in the home garden. Good cultural practices and timely fungicide applications can help control these diseases and allow the gardener to harvest a bountiful tomato crop.
This article originally appeared in the April 26, 1996 issue, p. 60.