Ames Farm Center: Tomato Diseases and Prevention Strategies

Gardening enthusiasts always look forward to growing tomatoes every year. Tomatoes are a peculiar produce, both fruit and vegetable depending on who you ask. The United States Supreme Court declared them a vegetable for import duties, even though botanically, they are classified as fruit. But did you know that tomatoes were once considered poisonous? It wasn’t until around 1850 that they were cultivated for consumption. In those days, tomatoes were known as “love apples” and were grown purely for decorative purposes due to their attractive but forbidden appearance. It was believed that tomatoes, being part of the nightshade family, were unsafe to consume.

Despite their interesting history, tomatoes remain a popular choice for gardeners, particularly in Michigan with its warm and humid summers. However, these favorable conditions also make tomatoes vulnerable to various leaf blights, such as Septoria leaf spot. This fungal disease primarily affects the leaves, not the fruit. The lower leaves are usually the first to show signs of infection, developing small, dark spots that rapidly enlarge and form a tan or gray center. In the center of the spots, you may notice small black dots, which are the fruiting bodies of the fungus. As the infection progresses, the leaves turn yellow, then brown, wilting and eventually falling off. The Septoria pathogen can spread through water splashing, leading to further defoliation. Michigan State University Extension has begun addressing questions about Septoria.

Septoria leaf spot
Signs of Septoria leaf spot on a tomato leaf. Photo credit: William M. Brown Jr.,

Septoria leaf spot
Leaf spotting and advanced decay of Septoria leaf spot. Photo credit: Paul Bachi, Univ. of Kentucky Research and Education Center,

Prevention is the key when it comes to Septoria, as there is no cure once it takes hold. If Septoria has been a recurring issue in your tomato plants, especially if they are always grown in the same spot, it is likely to persist in subsequent years. The pathogen thrives on tomato foliage but may also infect related Solanaceous weeds like Jimson weed, horse nettle, ground cherry, and nightshade. When conditions are wet, the fungus releases spores from the fruiting bodies on infected tomato leaves. If these spores land on a healthy leaf, spotting can appear within five days, given ideal weather conditions.

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There are several cultural techniques you can employ to limit the impact of Septoria. If the infection is relatively mild, picking off the spotted leaves can slow down the disease. Growing tomatoes in containers, using fresh potting mix, can be especially helpful. This ensures a clean and disease-free environment for your plants. Additionally, staking and properly spacing the tomato plants allow for adequate air circulation, reducing the time that the leaves are wet. Watering the plants via trickle irrigation keeps the foliage dry. If overhead watering is necessary, do it during the day to allow the leaves to dry quickly. Avoid watering in the evening, as prolonged moisture favors disease development. If you have a large enough garden, consider rotating your tomato plants to different spots each year. At the end of the season, remove any infected tomato debris and dispose of it properly.

In cases where Septoria persists despite your efforts, using a protectant fungicide may be necessary. Chlorothalonil is a common fungicide used to combat tomato spots and blight. Several brands offer products containing this active ingredient. Alternatively, there is an organic fungicide called “Serenade” that can be used. It may be a bit harder to find, but it is worth considering. Always read and follow the label instructions carefully. Typically, the spray needs to be repeated at seven- to 10-day intervals throughout the growing season for optimal protection. During rainy periods, the interval between sprays may need to be shortened, with applications made every seven days instead of 10. However, it is essential to adhere to the specified frequency on the label and avoid over-application.

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By implementing these strategies, you can effectively minimize the impact of Septoria leaf spot on your tomato plants. Your sandwiches, salads, and all tomato-based delicacies will thank you for the effort. For more information and resourceful gardening advice, visit the Ames Farm Center, your go-to destination for all things gardening.

Acknowledgment: Special thanks to plant pathologist Mary Hausbeck from MSU for her valuable expertise and input in the creation of this article.