If you’ve ever grown tomatoes, then you’ve probably had to contend with tomato plant diseases. It’s easy to think of tomatoes as easy-to-grow garden vegetables, but that isn’t always the case. In fact, tomatoes can be pretty picky about their soil nutrients, water levels, and the way they’re spaced in garden beds. There are a lot of different diseases that can strike your plants, and sometimes I feel like I’ve seen them all in my own home garden. Learn how to identify the most common tomato plant diseases, and how to both avoid them and treat them effectively.
Early Blight
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Many tomato gardeners get frustrated when early blight (Alternaria solani) strikes their plants. As the name suggests, this disease afflicts on tomato leaves early in the growing season, and can cause trouble all season long if you don’t act quickly. This affliction first appears as little brown spots on the the plant’s leaves, which then spread outwards and become a lot more noticeable. Eventually, the infected leaves will fall off the plant.
Alternaria sp. is a tough fungus that can survive in soil even over the winter. The easiest way to prevent it is to mulch your tomato plants immediately after planting, which prevents the spores in the soil from creeping up and infecting your plants. When you see tomato blight, snip off infected leaves to prevent it from spreading. Next year, plant your tomatoes in an unaffected area of the garden.
Septoria Leaf Spot
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One of the most common tomato diseases—septoria leaf spot—appears just as its name suggests. This disease, shown here on wheat leaves, covers your tomato leaves with small, circular spots that have gray-white centers and darker edges. You’ll see this issue most often when the weather has been warm and wet.
Septoria (Septoria tritici) may look fairly harmless, but it will destroy your plants’ foliage. Leaves turn yellow, then wither away and fall off.
Prevent leaf spot by keeping a neat, tidy garden. Don’t place your tomato plants too close together, so air can circulate around the leaves. Since this filamentous fungus thrives in wet soil, you can avoid it by not overwatering. If leaf spot does appear, cut the infected leaves off your plants to slow the disease from spreading. Make a note of this issue in your gardening journal, and don’t grow tomatoes in the same soil next year. If the affliction persists, you’ll need to use fungicide to kill the spores, and/or dig out the beds and replace the soil entirely.
Fusarium Wilt
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Fusarium wilt (Fusarium oxysporum) is most commonly found in southern U.S. states, but it can occur anywhere. When it does, it’s capable of wiping out entire tomato fields. It starts with drooping stems, but from there it progresses to an all-over wilting, and eventual total collapse. If you suspect you’re dealing with this type of fungus, cut the main stem of your tomato plant open, and look for the hallmark dark streaks running lengthwise through the stem.
This disease can be a huge problem, because the pathogen’s spores can remain dormant in soil for years. Once active, they can (and will) spread easily to other garden areas, too. You can try to prevent it by disinfecting tomato cages and stakes at the end of every growing season. Use a mixture of 1 part bleach to 9 parts water to clean these items to disinfect them completely.
Once your soil is infected with Fusarium, it’s extremely difficult to get rid of. Rotate your crops to keep tomatoes away from infected soil, and look for wilt-resistant tomato varieties when you’re shopping for new garden plants.
Anthracnose
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When you see little sunken areas starting to appear in your tomato’s leaves, you’ll know they have a case of anthracnose. This fungal pathogen is one of the most common tomato plant diseases, and is caused by Colletotrichum phomoides fungus. It is extremely common, and it will eventually rot the entire plant, including the fruit. Anthracnose thrives in hot, wet weather, and can also afflict potatoes and onions.
Protect your plants from anthracnose by removing leaves from the plants’ lower 12 inches. This keeps foliage from coming into contact with wet soil, which encourages the growth of this fungal disease. One way to avoid this fungus is to refrain from over-watering your tomato plants.
You can also fend it off by rotating crops with those that aren’t members of the nightshade (Solanaceae) family, preferably every other year. So, if you plant tomatoes in your garden this year, avoid growing tomatoes, peppers, and eggplants in that spot next season. Try carrots, beets, and other root vegetables instead.
Blossom End Rot
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Do your tomatoes have dark spots on their bottoms? If they do, you’re probably dealing with blossom end rot. This common disease is usually caused by soil that is deficient in calcium. It can also appear if tomatoes have been planted in alkaline soil, or have been watered unevenly. You can easily prevent this disease by enriching your soil with calcium, and watering regularly and evenly. If your tomato plants are having problems with drying out, use mulch to help keep the soil moist.
Be sure to test your soil’s pH level as well! Tomatoes thrive in soil that has a pH level of 6.0-6.5. If your garden beds are too alkaline, you can add sphagnum peat or other amendments to make it more acidic.
If you see blossom end rot in your tomatoes, simply remove the fruits that have been affected. This can help new fruits grow disease-free.
Powdery Mildew
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Look for powdery mildew on your tomato plants in late summer and fall, or if you grow your plants inside a greenhouse. It appears on leaves as discolored, yellowish spots that have a fuzzy, powder-like substance on top of them. This will eventually progress to brown, dead areas all over the leaves.
Powdery mildew is also among the most common tomato plant diseases. It often shows up during periods of high humidity, or inside greenhouses (which are usually quite humid). In fact, there are three different types of powdery mildew that can afflict your plants: Leveillula taurica, Erysiphe orontii, and Oidium neolycopersici. Once these fungi show up, they can spread quickly.
Neem oil has proven to be effective at treating powdery mildew, as has sulfur. If the mildew has spread to several plants, a more widespread fungicide treatment might be needed. You can avoid this issue by spacing your plants well apart, and watering the soil at ground level, rather than pouring water over the leaves from above.
Bacterial Canker
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If the edges of your tomato leaves turn brown, and yellowish closer to the center of the foliage, they have bacterial canker (Clavibacter michiganensis pv michiganensis). It usually appears on only on side of a tomato plant. Bacterial canker is a huge problem in the garden, because it’s one of the most difficult tomato plant diseases to treat. It spreads quickly to other plants, and will eventually cause them to wilt and die.
Since this issue is often seed borne, make sure to buy heirloom, organic seeds from trusted companies. Should bacterial canker strikes, move healthy plants to a new area of the garden, and treat the afflicted plants with copper hydroxide or streptomycin spray. As a preventative measure, don’t plant your tomatoes in the same spot next year. In fact, keep tomato plants out of the area for at least three years so the disease won’t make a comeback.
Fruit Cracking
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If your tomato fruits are cracking, you’re dealing with a common problem that’s not quite a disease—but is definitely an illness. Fruit cracking is caused by insufficient light, water, temperature, or nutrients in tomato plants, which basically creates traumatized fruit.
You can prevent cracking fruit by growing tomato plants in raised garden beds that allow you to control the moisture levels of the soil. Use compost and mulch to maintain more constant soil moisture, and make sure your soil has plenty of calcium. Remove cracked fruits the minute you see them to avoid pest problems, since ants and other insects will be drawn to the exposed flesh within.
Verticillium Wilt
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Don’t let the name of this disease fool you. Verticillium wilt doesn’t always cause wilting, but tomato leaves will become yellowish and dry when it strikes. This fungal disease can affect other garden plants in addition to tomatoes, which makes it particularly virulent. Since it affects the plant’s xylem vascular tissues, wilt makes it difficult for tomato plants to get water and nutrients from the soil. Deprived of both food and water, the plant eventually dies.
You’ll notice this disease most often in cool weather conditions. If verticillium wilt has affected your tomatoes, move healthy plants away from this area of the garden and don’t plant new ones there for at least four years. Avoid planting other nightshades such as peppers, eggplant or potatoes in that spot in future, or they might succumb to wilt as well.
Late Blight
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Late blight is named not for the time of year it appears in the garden, but for the part of the tomato plant it strikes first: the oldest. The most mature leaves will be the first to develop this issue, which appears as misshapen, greenish-black patches on leaves. This blight grows on foliage quickly, and spreads throughout the garden if it’s not addressed.
Once late blight strikes tomato plants, remove and destroy all infected plants and move healthy ones to a new area of the garden. Don’t plant any other nightshades in this soil for at least four years.
Heal Your Garden
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Hopefully this article will help you to identify common tomato plant diseases, avoid them, and treat them effectively if they appear in your garden. Once you know how to manage these issues, you’ll end up with healthy, beautiful plants, and delicious fruit.