Septoria leaf spot and early blight are common foliar diseases of tomatoes in home gardens. Fungal diseases overwinter on plant debris in the soil. Fungal spores are splashed onto plant foliage by raindrops or splashing water and invade the plant tissue when leaf surfaces are wet. Rainy weather in spring and early summer favors development of foliar diseases on tomatoes.
Little can be done to control foliar diseases on tomatoes in the current growing season. However, good cultural practices can help prevent foliar diseases from reoccurring in future years. This fall, carefully remove as much of the tomato debris from the garden as possible and destroy it.
Plant tomatoes in a different location in the garden next year. Rotate crops so that tomatoes and other solanaceous crops (potatoes, peppers, and eggplants) are not grown in the same area for 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. There is no home garden treatment that effectively destroys the disease pathogens in the soil.
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 or train them to stakes. The foliage of tomatoes growing in wire cages and those trained to stakes dries more quickly than those sprawling on the ground.
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. Mulching tomato plants in early June allows the soil to warm up in spring.
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.
While cultural practices can help control foliar diseases of tomatoes, fungicides may also be needed. Apply fungicides, such as chlorothalonil, at regular intervals beginning 3 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.