Question:
How can I prevent blight on my tomatoes?Answer:
Foliar diseases of tomatoes are a common problem in the home garden. Fungal diseases, such as early blight (Alternaria tomatophila and Alternaria solani), late blight (Phytophthora infestans), and Septoria leaf spot, overwinter on plant debris in the soil. Fungal spores are splashed onto plant foliage by raindrops or splashing water. A wet leaf surface is required for the spores to invade the plant tissue. Wet spring and early summer weather favors development of foliar diseases on tomatoes.
Home gardeners can reduce blight problems on their tomatoes with good cultural practices.
Rotating Crop Location
Plant 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. 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. No home garden treatment effectively destroys the disease pathogens in the soil.
Air Movement and Wire Support
When planting tomatoes, space plants approximately 3 feet apart. Wide plant spacing increases air movement and promotes rapid drying of plant foliage.
Grow tomato plants in wire cages. The foliage of tomatoes growing in wire cages dries more quickly than those sprawling on the ground. Gardeners can buy 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.
Applying Mulch
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 on plant foliage. Mulching tomato plants in early June allows the soil to warm up in spring.
Avoid Overhead Watering
Avoid wetting tomato foliage when watering. Apply water directly to the ground around plants with a soaker hose, slow-running hose, or watering can. If a sprinkler must be used, water in the morning so the foliage dries quickly.
Good Cultivar Selection
Some success can be found in planting varieties that are more resistant to blight diseases. There are not many varieities that show good resistance, but selections like 'Defiant F1', 'Seiger', 'Summer Sweetheart', and 'Chiapas' have shown some resistance. An extensive list of disease-resistant tomato varieties can be found in this article from Cornell University: Disease-Resistant Tomato Varieties.
Use of Fungicides
While cultural practices can help control foliar diseases of tomatoes, fungicides may also be needed. Apply fungicides, such as chlorothalonil, at regular intervals beginning 2 to 4 weeks after planting. 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.
Learn more in this article: Managing Tomato Diseases, Disorders, and Pests.