Tomatoes are susceptible to many diseases and require some extra care. Some of these diseases will live in the soil from year to year, so rotating the location of your tomatoes to avoid reinfection of the next crop, working your soil every year, and replanting is a good idea.
Remember, the leaves closest to the soil are going to be the first infected when the soil around the plant is contaminated so it's best to make sure you are on top of your pruning. Along with staying on top of your pruning, be sure to add a layer of mulch under the plants to help block some of the diseases from transferring to your plants.
When pruning, you should trim your lower leaves up to 12 inches by the time the tomato plants are about 3 feet tall. Be sure to water with care to avoid getting soil onto the leaves. You can do this by using a water breaker nozzle and drip irrigation.
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Now let's talk about the different diseases you need to watch out for!
Early Blight: Keep an eye out for dark brown spots with dark circular rings. This will develop on older leaves. The spotted leaves will die early and the stems will have brown, elongated sunken lesions. This disease is common in warm, wet climates. Also, keep an eye on the fruit. If the disease has reached the fruit, you will find that they have developed leathery skin with sunken spots near the stem. This disease can be transferred from either the seeds, by transplants, or by the soil itself (fungus growing in the soil over the winter).
Late Blight: The leaves will show large brown lesions that look like sun-scald, the stems will have a white, moldy appearance, and the fruit will have irregular greenish-brown patches that will appear to be greasy and rough. You may also find greenish-black lesions on the stems. This disease is common in cool, wet climates.
Fusarium Wilt: The leaves will turn yellow, wilt, and then turn brown but still be attached to the stem. This disease progresses up from the soil, sometimes causing only half of a leaf or branch to become infected and die. The stem will show a brown discoloration. Since the disease enters the plant from the roots, it can spread easily from plant to plant. Unfortunately, this disease can remain in the soil for years.
Leaf Roll: This is not a disease, but something that needs to be watched as it is a sign of stress. In high temperatures, the tomato plants can abort their blooms and the leaves will begin to curl up. If caught in time, you can save your crop. To prevent any more stress, provide your plant with shade. Not enough watering or too much watering can also cause this.
Southern Blight: The entire plant will gradually start to wilt and die without changing color indicating that it has been infected. The fungus damages the stem at the soil line and brown lesions with a white fungal mat may appear. It's important to remove and discard the damaged plants quickly to help secure the rest of your crop. The disease can be spread from either infected tools or seeds and can survive within the soil from year to year.
Verticillium Wilt: Older leaves will begin to wilt around their edges and turn yellow, then brown. You cannot alleviate the wilting by watering the plant, you will need to cut the stems that show discoloration. If you catch this early enough, you save the plant and your crop.
If you are wanting some extra help to make sure your crop is secured, be sure to look for varieties that are more disease resistant than the rest. These varieties will have capital letters after their name. These letters are their disease resistance code and represent a specific disease or pest that the plant will not be troubled by.
A - Alternaria Alternate Fungus
EB - Early Blight
F - Fusarium Fungus
N - Parasitic Root Worms
St - Stemphylium or Gray Leaf Spot Fungus
T - Tobacco Mosaic Virus
TSWV - Tomato Spotted Wilt Virus
V - Verticillium Fungus