Juicy, perfectly sun-ripened tomatoes are among the easiest vegetables you can grow in your garden. But taking good care of your crop means keeping an eye out for tomato plant diseases caused by fungi, bacteria, and viruses, which can cause leaf spots and blights. Temperature, nutrient levels, and moisture levels can also cause problems, potentially ruining your harvest and your dreams of enjoying slices of homegrown tomato on your next BLT. You can keep these potential problems at bay by familiarizing yourself with their symptoms and using a few simple strategies. With a little effort, your plants can stay healthy and productive all season long.
How to Control Tomato Plant Diseases
Selecting disease-resistant varieties, spacing tomato plants properly, using mulch, and watering at least 1 inch per week are some of the most important actions to keep your tomato plants disease-free. Many common plant diseases and pests lurk in the soil, which is why it's always a good idea to rotate where you plant your tomatoes so they're not growing in the same ground any time within four years. Diseases also can spread when leaves get wet, so avoid crowding your tomatoes too close together. This helps air circulate better and dries leaves faster. Aim at the base of your tomato plants when watering to minimize splashing onto the leaves and water in the morning so wet leaves have time to dry before cooler evening temperatures arrive.
Even when following all these best growing practices, some tomato plant diseases are likely to pop up. For those caused by fungi, bacteria, and viruses, it's almost impossible to cure your plants once they're infected. However, if you catch the problem early, you can try to slow down the spread by removing and destroying leaves or whole plants showing symptoms. You can also try to prevent diseases from taking hold in the first place by regularly treating them with a product labeled for controlling diseases on vegetables, which comes in spray bottles that are easy to use and rated safe for organic gardens.
The Most Common Tomato Plant Diseases and Problems
Tomatoes are usually vigorous growers that will reward you with a bumper crop when grown in full sun with plenty of water and nutrients (these plants tend to be heavy feeders, so they do best in rich soil with supplemental fertilizer). But it's almost inevitable that these plants will end up with a foliar disease or blemished fruit. So here's what to look for so you know what you are dealing with.
The 1. Septoria Leaf Spot
A fungus causes septoria leaf spot, creating small, circular patches with a grayish-white center and dark edges. Tiny black spots may show up in the center of each blemish. Affected leaves turn yellow, wither, and fall off. Long periods of warm, wet weather help this tomato plant disease flourish, and splashing water quickly spreads spores to other leaves.
The 2. Anthracnose
This fungus is a small, circular, indented area on tomato fruits. Eventually, rings surround the original spot. The fruit's flesh may rot completely through, especially on overripe tomatoes, so keep fruits picked as they ripen. Spores are spread by splashing water, and the fungus is most common during warm, wet weather.
The 3. Fusarium and Verticillium Wilt
These wilt tomato plant diseases are caused by fungi in the soil that enter through young roots and then begin to plug the vessels that move water to the plant's roots and stem. Without enough water, the plants start wilting on sunny days, although they'll appear to recover at night. Tomato wilt may first appear in the top or lower leaves of the plant, causing them to lose color and then die back from the tips. The process continues until the entire plant is affected.
To help control these tomato plant diseases, plant tomatoes bred for disease resistance. They should be labeled V (for verticillium), F, FF, or FFF (for fusarium variations). If one of these tomato wilts occurs, avoid using the location to grow tomato, eggplant, potato, and pepper plants for 4-6 years because the fungi that cause the tomato wilt can remain in the soil that long without a new host to infect.
The 4. Early Blight (Alternaria)
Another tomato plant disease fungus, Alternaria, causes early blight. Lower leaves show brown or black spots with dark edges, almost like a target. Stem ends of fruits may be attacked, showing large, sunken black areas with concentric rings. This fungus usually strikes after plants set fruit.
The 5. Late Blight
The fast-spreading tomato plant disease is caused by the fungus Phytophthora infestans and occurs during periods of cool, rainy weather that may come at the end of a growing season. It looks almost like frost damage on leaves, causing irregular green-black splotches. Fruits may have large, irregular-shaped brown blotches that quickly become rotten. This plant disease also affects potatoes and can be transferred from them.
The 6. Mosaic Virus
Mosaic virus attacks many kinds of plants and is a common tomato plant disease. While mosaic virus doesn't kill the plant, it diminishes the number and quality of fruits. The virus gets its name from the markings resembling a light green and yellow mosaic on the leaves and mottling on the fruits of affected plants. Leaves may also grow in misshapen forms that look like ferns.
Because the virus enters through cuts in leaves and stems, avoid handling the plant as much as possible. This virus also attacks tobacco plants and can be transmitted from them if you've recently handled cigarettes or other tobacco products. So if you're a smoker, wash your hands thoroughly with soap and wear garden gloves when working with tomatoes.
The 7. Blossom Drop
Brought on by temperature extremes, blossom drop occurs when temperatures are above 85°F or below 58°F. The temperature extremes destroy developing tomato blossoms. Often, you won't notice that there was any damage until you have fewer tomatoes to harvest at the end of the season. Prevent blossom drop by using row covers to raise night temperatures. Unfortunately, there's not much you can do about hot day temperatures; just maintain healthy plants so they'll set new buds after the heat wave passes.
The 8. Blossom-End Rot
Blossom-end rot is a common tomato plant disease that impacts the fruit. It is caused by a lack of calcium, usually brought on by fluctuating water availability. It appears as a sunken, dead area opposite the stem (the blossom end of the fruit). The area will expand as the fruit matures. Prevent blossom-end rot by promoting steady, stress-free plant growth. Water plants regularly to maintain moist but not waterlogged soil. Spread a 2-inch-thick layer of mulch around plants to conserve soil moisture.
The 9. Damping Off
Damping off is a frustrating fungal tomato plant disease that causes seedlings to collapse suddenly or fail to germinate. Presoak seeds to speed germination, and don't plant them in cold soil. If planting seeds in potting mix, always use a fresh bag and sterilize your containers first with a 10% bleach solution (add one part bleach to nine parts water and mix). Allow the top of the soil to dry between waterings.
The 10. Sunscald
Essentially a sunburn on a tomato, sunscald causes a section of the fruit to become soft, light in color, and dry. Prevent sunscald by maintaining enough foliage to shade fruits or shade fruits artificially with a shade cloth.
Understanding the Tomato Plant Disease Code
Disease resistance has been bred into many tomato varieties. The letters behind the names are codes showing what diseases and insects the tomato plants are bred to resist, including:
V Verticillium wilt
F Fusarium wilt
FF Fusarium wilt races 1 and 2
FFF Fusarium wilt races 1, 2, and 3
N Nematodes
A Alternaria alternata (stem canker or early blight)
T Tobacco mosaic virus
St Stemphylium (gray leaf spot)
TSWV Tomato spotted wilt virus
For example, the Big Beef VFFNTA Hybrid label tells you it's bred to resist verticillium wilt, fusarium wilt races 1 and 2, nematodes, tobacco mosaic virus, and Alternaria, and early blight.
Improve Your Tomato Harvest with These Tips
For the best and most abundant tomato crop, pruning takes priority. Once you've discovered whether your tomato plants are a determinate or indeterminate variety, you can learn how to proceed with removing suckers, leaves, thinning fruit, and keeping your plants as clean as possible. Knowing the best time to harvest your tomatoes will produce the tastiest addition to salads, sauces, and sandwiches. Keep pests from eating your carefully cultivated crop, and you'll have more tomatoes than you know what to do with.