by Gabrielle LaTora, Agriculture and Natural Resources Agent, Fulton County Extension

Whether your tomatoes petered out early or held on past their prime, they probably didn’t look so good when you pulled them out. In our Georgia climate – hot, humid, and with irregular rainfall recently – tomatoes can struggle with diseases.

Now is the best time to look back on your tomatoes’ performance this season (while the pain and grief is still fresh) and strategize for next year. Prevention is the smartest and cheapest way to manage tomato diseases.

Common tomato diseases we see every year

Early blight (caused by Alternaria solani fungus) – typically starts as irregular brown lesions on leaves with concentric circles, often with yellowing around the spots. Starts on oldest leaves and moves its way up the plant. The fungus likes rainy, warm conditions. Be on guard when temps are 75-85F with rain and heavy dew.

Septoria leaf spot (caused by Septoria lycopersici fungus) – small, tan leaf spots with dark edges. Lesions are smaller than early blight, almost with a “shothole” look. Lesions start on the oldest leaves and move up. Look out for symptoms during wet and cool-ish conditions (68-77F).

Leaf mold (caused by Passalora fulva fungus) – irregular yellow spots on tops of leaves AND thick, fuzzy, brown fungal spores on leaf undersides. You are most likely to see this one in greenhouses or high tunnels and rarely in open field production. Likes humid conditions, temps 70-80F, and doesn’t need rain, which is why we see it so often in tunnels.

Buckeye rot (caused by several species of Phytophthora water molds) – you’ll see buckeye rot on the fruit, where it creates brown spots with concentric rings as they get bigger. As the spots decay, white, fuzzy mold may grow. The lesions don’t look rough and sunken, like late blight spots. No symptoms on leaves. Likes warm, wet weather.

Late blight (caused by Phytophthora water mold) – similar to buckeye rot but will infect leaves and fruit. The lesions are kind of irregular blotches rather than distinct spots, like buckeye rot. Fruit lesions are greasy-looking and become rough and sunken. Look out during cool, wet conditions.

Anthracnose (caused by Colletotrichum fungus) – small, sunken lesions on fruit that gradually expand. May develop concentric rings or salmon-colored spores during wet conditions. The fungus survives in the soil for a long time as microsclerotia and gets splashed up onto plants during rain or overhead watering.

Fusarium wilt (caused by Fusarium oxysporum fungus) – this is a vascular disease that happens when soil-borne fungi infect the xylem of the plant, preventing it from taking up water. Leaves first turn yellow and start to wilt, sometimes on only one side of the plant. Eventually, leaves turn brown and the plant dies. Only infects tomatoes and can persist in the soil for years.

Southern blight (caused by Sclerotium rolfsii fungus) – foliar symptoms look similar to Fusarium wilt – leaves turn yellow and wilty, then brown, and die. However, Southern blight causes the base of the stem to turn brown and rot, which is pretty distinctive. Produces distinctive sclerotia that look like mustard seeds that will survive in soil for many years and infect lots of other crops. Hot weather + poorly draining soils = perfect conditions for Southern blight.

Southern bacterial wilt (caused by Ralstonia solanacearum bacterium) – first you will notice the entire plant is wilty even with enough soil moisture. Leaves don’t turn brown and die as quickly as Fusarium wilt and Southern blight but will eventually. Split the stem lengthwise, and you will see dark lesions inside. Infections happen when weather is hot and soils are warm and moist.

8 ways to prevent diseases without chemicals

1. Avoid touching wet plants. Many of these pathogens move around on droplets of water. The less we disturb dew or rain on leaves, the less some diseases will spread.
2. Improve air flow. Pruning and trellising are essential to keep canopies open and get the leaves and stems off the ground. Don’t space plant too tight that air can’t move around them.
3. Keep leaves dry. Forget about sprinklers or sprayers and invest in drip irrigation. If you can grow under a tunnel, you will probably see fewer leaf spot diseases. Mulch or cover the ground to prevent water from hitting the soil and splashing up onto plants.
4. Buy resistant varieties. Plenty of tomato varieties have been developed through selective breeding to be resistant to common diseases. This is a major selling point for seed companies, so look closely at the variety descriptions. This is probably the easiest, cheapest, and most effective way to reduce tomato disease.
5. Rotate crops. A 3-year rotation away from nightshade crops (tomatoes, eggplants, peppers, or potatoes) is a good practice. Consider longer if you have identified Southern blight or Fusarium wilt.
6. Improve drainage. Check your soil drainage with a percolation test to see how well it drains. Add organic matter (please not sand) to improve drainage. Make sure you’re not overwatering.
7. Dial in soil fertility. Make sure soil pH is between 6.0-7.0 and that you fertilize with the right amounts of nutrients. Correct pH means the plant can access nutrients –> nutrients make healthy plants –> healthy plants don’t get sick as often! Make informed decisions by having the soil tested. Without tessin’, you’re just guessin’.
8. Clean up! Most of the pathogens that cause these diseases hang around in crop residues (dead plant parts that fall to the ground during the growing season or are left in the field) and infect new plants the next year. Get crop residues out of the field at the end of the season. Remove symptomatic plant parts when you see them to reduce inoculum. And make sure to sanitize your pruners, tools, and implements! 

Learn More

• Common Tomato Diseases in Georgia (UGA Extension)
• Keeping Your Tomato Plants Healthy (UGA Extension)
• Key to Common Problems of Tomatoes (UMD Extension)
• Commercial Tomato Production Handbook (UGA Extension)