By Tom Fowler, Field Specialist in Horticulture, University of Missouri Extension

This time of year we can start seeing fungal leaf spot diseases on tomato plants. You may see Septoria leaf spot and/or Early blight on the leaves.

Even though they call it Early bight we generally see it later in the season. Both leaf spot diseases are a fungus and they generally develop on the older lower leaves of the plant and can move up the plant and infect the tomatoes and reduce production. In severe cases it can kill the plant.

Septoria leaf spot is a small circular dark bordered spot that may have a tan or light gray center. They can eventually become so numerous the entire leaf surface becomes blighted. Early blight is a larger irregular shaped spot and will develop what is described as a target pattern of concentric rings or circles. If it is a heavy infection the leaves will turn yellow and drop off the plant.

To prevent diseases we can mulch the plants, stake or put a cage around the plant to hold it up off the ground. This keeps the leaves from touching the soil that may carry the fungus and to allow air circulation and sunlight to help dry the leaves. The fungus disease needs moist plant tissue to grow.

Using a mulch around the plant also helps prevent water from splashing the disease spores on the plant. Another strategy to try if you haven’t is to rotate your vegetable crops. Don’t grow tomatoes or related crops (peppers, potatoes, eggplant) in the same area of the garden for several years. In our vegetable plot we are on a three year rotation meaning it is three years before tomatoes grown there again. For small gardens this may not be practical but try your best. Even a few feet from where they were grown before would help.

Other strategies if you start seeing a few spotted lower leaves they can be picked off the plant and removed from the garden. Control weeds that may harbor disease in and near the garden especially horse nettle. When watering avoid getting the foliage wet. Using a soaker hose works well to prevent getting water on the leaves. Avoid working in the garden when the plants are wet. Clean your garden tools. Be sure and clean up the plant debris after the harvest to reduce the overwintering fungus spores.

Fungicide sprays can be used as a protectant. There are numerous brands of fungicides. A common fungicide for fruiting plants has the active ingredient Chlorothalonil. Fungicides protect the healthy leaves they don’t cure the spotted leaves. If you are going to use a fungicide it is important to start treatment as soon as you start seeing the disease and spray both the upper and lower leaf surfaces to protect the whole leaf. You may also find other fungicides labelled for tomato disease such as Mancozeb and copper based materials. Read and follow the label directions if you use one of these products.

For more information search on-line for the MU Extension guide titled Disease Prevention in Home Vegetable Gardens or contact your county MU Extension office for a copy.