Tomato Leaf Spots

I’ve been surprised – and gratified – at how few calls I’ve received about tomatoes this year. Let’s face it, this has been as lousy a year for growing them as it has been for most other crops. In a year like this I often am not able to offer much help. But since they are the vegetable most commonly grown in Central Indiana home gardens I want to mention two diseases they are particularly susceptible to.

About this time of year two common fungal leaf-spot diseases often appear. Septoria leaf spot and early blight are both characterized by brown spots on the leaves. Septoria usually appears earlier in the season than early blight and produces small dark spots. Spots caused by early blight are larger and often have a distorted “target” pattern of concentric circles. For each disease, heavily infected leaves eventually turn yellow and drop. Older leaves are more susceptible than younger ones, so symptoms generally first appear at the bottom of the plant and progress higher.

These diseases can spread rapidly so it’s important to scout for symptoms. Plants usually become susceptible when fruit is about the size of a walnut. If you see symptoms, fungicides can help prevent their spread. Be sure to apply fungicide to both upper and lower leaf surfaces, and reapply if rainfall removes it. Chlorothalonil is a good choice for fruiting plants because most products have no harvest waiting period, meaning that fruit can be harvested once the spray is dry.

Chlorothalonil is an ingredient in several products. Be sure to start protecting plants when the disease is first seen. It is virtually impossible to stop it on heavily infected plants. As with all pesticides, always read and follow all label directions.

Several practices can help with disease prevention. Mulching, caging, or staking keeps plants off the ground. Better air circulation allows foliage to dry quicker than in plants allowed to sprawl. Mulching also helps prevent water and soil from splashing and carrying disease spores to the plant.

You can reduce many diseases by following proper sanitation at the end of each growing season. This includes removing and disposing of any plant material. Composting, if done properly, will also destroy disease organisms.

If you experience either disease and you are able to, consider rotating tomatoes out of that area of your garden. Other plants such as peppers, eggplant and potatoes are also susceptible so avoid replacing tomatoes with these crops.

Frequent rainfall and high humidity are conditions which favor the development of these diseases. We’ve had an abundance of both this year. For additional information on these and other tomato diseases, see Purdue Extension Publication “Five Steps for Healthy Garden Tomatoes,” by Purdue Extension Specialist Daniel S. Egel. It is available online at: https://www.extension.purdue.edu/extmedia/bp/bp-184-w.pdf.