Nothing ruins a homegrown crop faster than these common plant problems. Here's how to identify and treat them right away so you can still enjoy your harvest.

By Deb Wiley
Updated May 06, 2020
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Juicy, sun-ripened, easy-to-grow tomatoes are among the most popular edible plants to grow at home. Whether you choose to grow them in containers or an expansive garden plot, tomatoes usually give you a good harvest and often taste much better than what you can find in the store. However, diseases like leaf spots and blights can pop up and ruin your garden party. Don't let these potential problems scare you away, though. Growing healthy tomato plants is relatively simple when you plant disease-resistant varieties, space plants properly, mulch, and water at least 1 inch per week.

Credit: Scott Little

Part of taking good care of your tomatoes means keeping an eye out for diseases caused by fungi, bacteria, and viruses. Many common plant diseases and pests lurk in the soil, so it's always a good idea to rotate where you place your tomatoes so they grow in the same ground only every four years or so. You may still see a few diseases turning up on your plants, so here's what to watch out for and what you can do about them.

Credit: Denny Schrock

1. Septoria Leaf Spot

A fungus causes septoria leaf spot, creating a small, circular patch with a grayish-white center and dark edges. Small black spots may show up in the center. Affected leaves turn yellow, wither, and fall off. Long periods of warm, wet weather help this disease flourish, and splashing water quickly spreads spores to other leaves.

Control leaf spot by not crowding your tomatoes. Leave enough space so air can circulate and dry off leaves faster. Avoid overhead watering. When watering tomatoes, water at the base of the plant. Also, water in the morning so wet leaves have time to dry before evening. A fungicide formulated for tomatoes can be used to treat affected plants.

Credit: Denny Schrock

2. Anthracnose

Follow the same procedures used for septoria leaf spot against anthracnose. This fungus shows up as a small, circular, indented area on tomato fruits. Eventually, rings surround the original spot. The flesh of the fruits 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.

3. Fusarium and Verticillium Wilt

These wilt diseases are caused by fungi in the soil that enter through young roots, then begin to plug the vessels that move water to the roots and stems of the plants. Without enough water, the plants begin 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, then die back from the tips. The process continues until the entire plant is affected.

Heirloom tomato varieties that have not been bred to withstand these diseases are commonly attacked by tomato wilt. To avoid these tomato plant diseases, plant tomatoes bred for disease resistance. They should be labeled V (for verticillium), F, FF, or FFF (for fusarium variations). Avoid overwatering tomato plants; just because a plant is wilted doesn't mean it needs more water. Check the soil; if it's dry, then water the plant.

If your tomatoes are affected by one of these tomato wilts, remove and destroy all affected plants. Don't place them in your compost pile. Avoid using this location for tomato, eggplant, potato, and pepper plants for four to six years, because the fungi that cause the tomato wilt can remain in the soil that long without a host to infect.

Credit: Peter Krumhardt

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.

Credit: Scot Nelson

5. Late Blight

The tomato plant disease late blight, caused by the fungus Phytophthora infestans, 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 tomato plant disease fungus also affects potatoes and can be transferred from them. Use the same controls as for septoria leaf spot.

6. Mosaic Virus

Mosaic virus attacks many kinds of plants and is common in tomatoes. While mosaic virus doesn't kill the plant, it diminishes the number and quality of fruits. The virus gets its name from the markings that resemble a mosaic of light green and yellow on the leaves and mottling on the fruits of affected plants. Leaves may also grow in misshapen forms, resembling 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 handled cigarettes or other tobacco products recently. So if you're a smoker, wash hands thoroughly with soap first and wear garden gloves when you're working with tomatoes. Planting resistant cultivars and not replanting in areas that previously hosted the problem will also help.

7. Blossom Drop

Brought on by temperature extremes, blossom drop occurs when temperatures rise above 85°F or drop below 58°F. The temperature extremes cause tomato plants to discard their developing 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 heatwave passes.

Credit: Cameron Sadeghpour

8. Blossom-End Rot

Caused by a lack of calcium, most often brought on by fluctuating water availability, blossom-end rot is a common tomato disease. 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.

Credit: Jay Wilde

9. Damping Off

A frustrating fungal disease, damping off causes sudden collapse of seedlings, or failure to germinate. There are many steps you can take to prevent damping off. First, plant seeds when soil is at optimum temperature. Presoak seeds to speed germination. If planting seeds in potting mix, use sterile potting soil and containers. Allow the soil to dry between waterings.

Credit: Scot Nelson

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.

Tomato Bacterial Diseases

Tomatoes can fall prey to a number of tomato plant bacterial diseases, including bacterial spot, bacterial speck, and bacterial canker. They're all slightly different but appear as spots on leaves and fruits. Use the same controls as for septoria leaf spot. Grow disease-resistant plants, and avoid rotating the same ground with peppers, which can host the same diseases. Avoid pruning and tying plants, because the bacteria can enter any openings made during these procedures. Fixed copper sprays may reduce the spread if applied as soon as symptoms begin.

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:

Verticillium wilt

F Fusarium wilt

F Fusarium wilt races 1 and 2

FFF Fusarium wilt races 1, 2, and 3

Nematodes

Alternaria alternata (stem canker or early blight)

T Tobacco mosaic virus

St Stemphylium (gray leaf spot)

TSWV Tomato spotted wilt virus

For example, the label on Big Beef VFFNTA Hybrid tells you that it is bred to resist verticillium wilt, fusarium wilt races 1 and 2, nematodes, tobacco mosaic virus, and Alternaria, and early blight.

Comments (1)

Anonymous
September 7, 2019
There's an easier way to avoid blossom-end rot. Get powdered milk (us oldsters used to have to drink it and called it "blue milk") and sprinkle about 1/4 cup around the base of every tomato plant as soon as you plant your seedlings in the garden. Water it in gently. The powdered milk offers immediately-available calcium and you won't lose any of the fruit to rot. It's a good idea to supplement that with crushed egg shells, but that first dose should be powdered milk. And don't forget the cardboard collars to thwart the dreaded cutworms!