Lebanon Reporter

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July 1, 2014

Two fungal diseases appearing on tomato plants

The most commonly grown crop in central Indiana gardens is the tomato. They do well in our climate, however, as any experienced grower knows, they are also susceptible to several diseases. About this time of year two common fungal leaf-spot diseases often appear on tomato plants.Septoria leaf spot and early blight are both characterized by brown spots on the leaves.Septoria leaf spot usually appears earlier in the season than early blight and produces small dark spots. Spots made by early blight are much larger and often have a distorted “target” pattern of concentric circles. Heavily infected leaves eventually turn yellow and drop. Older leaves are more susceptible than younger ones, so these diseases often start at the bottom of the plant and work up.This is the type of year where both of these diseases can become prevalent. The reason is that each of the pathogens responsible survive in the soil and old plant material. When we experience vigorous rainfall, such as we have had several times over the past few weeks, soil particles carrying the fungus are splashed onto the lower leaves of the plants. The warm, humid weather we’ve been having is also good for the development of these diseases.These diseases can spread rapidly so it’s important to scout for symptoms. If you find evidence of these, fungicides can be effective in preventing their spread. Be sure to apply fungicide to both upper and lower leaf surfaces, and reapply if rainfall removes it. Plants usually become susceptible when the tomato fruit is about the size of a walnut. Chlorothalonil is a good choice for fruiting plants because it has a zero-day waiting period, meaning that fruit can be harvested once the spray is dry.Chlorothalonil can be found in numerous products including Fertilome Broad-Spectrum Fungicide, Ortho Garden Disease Control, GardenTech Daconil and others. Be sure to start protecting plants when the disease is first seen. It is virtually impossible to stop it on heavily infected plants.There are several practices which can help to prevent these diseases. Mulch, cage or stake to keep plants off the ground, making them less vulnerable. 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 the disease organisms.If you experience either disease this year 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.Curt Emanuel is the Boone County Extension Educator for Agriculture and Natural Resources.

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