A quick and dirty explanation of 11 common tomato diseases and problems.

1. Blossom end rot. Symptoms and causes: The tomatoes get an ugly black rotted spot on their undersides. The main cause is lack of calcium in the tomatoes. Solutions: Avoid this three ways: mulch properly, water regularly, and amend the soil with bone meal fertilizer. Watering properly is the most important, so make sure your tomatoes plants don't dry out between rains or manual watering.
2. Catfacing. Symptoms and causes: You end up with deformed, scarred, and otherwise weird-looking tomatoes because of cool weather during pollination time. Solutions: Some varieties (like heirlooms) are more susceptible to catfacing. Avoid this problem by keeping the plants warm (50 degrees or more) during the early part of the season by mulching properly and covering with mini-greenhouses. The fruits still taste good, they're just ugly and hard to work with, so cook them up instead of slicing for salad.
3. Verticillim wilt, alternaria, and septoria leaf spot. Symptoms and causes: Older leaves yellow and wilt around midsummer, eventually wilting and killing the entire plant. Cause: fungus! Solutions: Avoid this problem by rotating your crops every year and by mulching properly to keep water (and fungal spores) from splashing onto your plants. Remove affected leaves immediately and throw them away or burn them well away from your garden.
4. Anthracnose and blight, otherwise simply known as fruit rot. Symptoms and causes: Big, ugly, stinky, rotten spots on the fruit at any time from fruit set to harvest because the foliage stays wet for too long. Solutions: Stake or cage indeterminate (vining) tomatoes to keep them up off the ground, and prune determinate varieties to increase air circulation. Never allow any part of the plant or the fruit to come into contact with the soil.
5. Sunscald. Symptoms and causes: Too much sun on the fruit, which may come about because of overpruning or if the plant has dropped leaves because of wilt. Fruits get sunburned with big white or yellowish patches. Solutions: Cover the fruits with loose straw and do better next year. Don't over-prune your tomato plants.
6. Timber rot. Symptoms and causes: White mold fungus which creates rotted points at branch nodes or near the soil line, usually during too much rain or too much overhead watering. Solutions: Increase air circulation so your plants can dry off, and keep tomatoes away from other susceptible crops like beans and melons, especially if these crops have caught timber rot.
7. Tobacco Mosaic and Cucumber Mosaic viruses. Symptoms and causes: mottled leaves, stunted plants, and rough, fernlike leaves; sometimes with mottled fruit. Solutions: Avoid tobacco products (yet another reason to quit smoking) and buy transplants from a reputable greenhouse. Always keep the weeds down and control insects, and wash your hands and tools to avoid spreading these viruses from one plant to another. These viruses also infect potatoes, cucumbers, melons, marigolds, and weeds.
8. Leaf roll. Symptoms and causes: Lower leaves roll up tight, usually during excessively rainy times and cool weather. Overfertilization and excessive pruning contribute. Solutions: This is not a serious disease, actually. Just handle your plants right and you will probably never see it. Otherwise, don't worry too much, but do check to make sure you aren't actually having spider mites or some other nasty thing instead.
9. Herbicide injury. Symptoms and causes: Leaves are thick and cupped, fruits may be catfaced and not ripen. Herbicides drift on the wind from your lawn and get on your tomato plants, or herbicide injury may be caused by mulching with affected grass clippings. Solutions: Don't use herbicides! If you still think you must apply a blanket of herbicide to your lawn, check the wind and don't apply close to the garden.
10. Cutworms. Symptoms and causes: Suddenly all your nice tomato plants are lying on the ground like dead soldiers on a Civil War battlefield. Either your garden gnomes came alive in the night or you've got cutworms. Solutions: Cut both ends off an empty condensed soup can. Wash throughly with detergent and a mild bleach solution (10 percent bleach). When you transplant your new babies, set them inside the soup can so it sticks up halfway out of the ground.
11. Tomato Hornworm. Symptoms and cause: These nasty moth caterpillars chew giant holes in your tomato plant's leaves and may strip and kill younger plants. Solutions: Till deeply in the fall and again in the spring to reduce infestation. Pick the little buggers off your plants as much as you can when you see them. If the infestation is incredibly severe, contact your local extension service to find out about chemical control methods.

I hope these quick and simple explanations help you if your tomato garden is attacked. Call your local extension service if you need additional help - that's why they are there!

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