The Q&A Archives: Tomato Diseases

Question: I planted 6 varieties of tomato seeds in a seed starter, and produced excellent plants, which I transplanted into the garden. I ended up with beautiful plants that had lush green foliage and a large number of green tomatoes. Just as the first of these were starting to color up, with an orange pinkish tinge on the fruit, the trouble started. The foliage began to turn yellow and develop brown spots, then the green fruits developed hard brown patches. The plants were supported on trellises off the ground and the soil around plants was covered with black plastic. Any idea what the problem was, and how I can prevent it from happening again?

Answer: It's likely that one or more diseases attacked your tomatoes. It's difficult to diagnose exactly what disease is causing the leaf symptoms, and it's not all that critical to make a positive ID; the controls for most diseases are similar.

It sounds as though the tomatoes themselves suffered from sunscald. As the foliage died back the fruits were subjected to intense sunlight, which can cause discolored or hardened areas on the fruit. On healthy plants, the tomatoes are somewhat shaded by the foliage.

The first step is to choose at least some disease-resistant varieties. Then, be sure to provide a thick layer of mulch underneath plants, since many tomato diseases are soil-borne and spread when rain splashes soil up onto the leaves. Since your plants are trellised, this should help. Some people find they can slow down the spread of disease by diligently picking off affected leaves and removing them from the garden. There are some new organic, soap-based fungicides that are showing promise, and sprays made from neem oil also have shown some fungicidal properties.

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