Tomato Diseases and Disorders

Blossom-End Rot

In my area, we have had enough rainfall this summer so no problems with blossom-end rot this year. This usually begins as a sunken spot on the blossom end of the fruit which turns black and leathery as it grows larger. Often you don’t notice it until you go to pick the tomato and find that the bottom half has rotted away—not a nice surprise!

It is a physiological disorder rather than a disease caused when the plant has trouble extracting enough calcium from the soil because moisture levels are too high or too low. There can be plenty of calcium in the soil but the plants are incapable of utilizing it properly. Stressed plants divert the little calcium they have away from the fruit and send it to the shoots to keep them growing. Along with uneven moisture, excessive nitrogen and high soil acidity can contribute to blossom end rot.

To prevent blossom-end rot from happening, ensure your soil is rich in calcium. Many “tomato toners” fertilizers include extra calcium. Also, eggshells are full of calcium, making them very handy for your tomato plants.

• First sterilize the eggshells by popping them into a warm oven for 20 minutes, or microwave on full power for two minutes. Crush them up then add them in and around your planting holes. Shells take a while to break down, but you can speed this along by grinding them up to increase the surface area, or even dissolving the grounds in water to water on at planting time. Aim for about two eggshells per plant. Often, though, the simple reason behind blossom end rot is irregular watering, which makes it harder for the plants to absorb all the nutrients they need.

Also, mulch your plants to keep the area around the roots consistently moist. See more about blossom-end rot solutions.

Early Blight

Early blight is the most common tomato disease.

It is a fungus that begins on the lower leaves as brown spots which enlarge into concentric rings like a bull’s eye. Eventually they get bigger and run together.

The lower leaves turn yellow and drop off, usually without affecting the fruit.

Sometimes dark patches will appear on the plant stems and on the stem end of the fruit. We have one ‘Early Goliath’ plant that shows signs of early blight but it soldiers on, has plenty of tomatoes forming (only one of those had stem end rot), and it keeps on blossoming. I keep plucking off the infected lower leaves and it continues to grow so for now it stays. I may regret that decision later.

Late Blight

Late blight is terminal.

One year we got it early in the season and watched helplessly as it turned all the plants and fruits to disgusting mush, practically overnight. You can track the spread of late blight across the country at the website usablight.org.

Avoid blight by giving your tomatoes good air flow and water at the base of plants to avoid wetting the leaves. Many gardeners even remove the lowest leaves specifically to improve airflow and minimize splashback when watering. Keep tomatoes off the ground. Laying a mulch of clean, dry organic matter such as straw can also reduce splashback. 

Also, choose blight-resistant tomatoes next year! Though most of the varieties we grow are heirlooms, we also grow some hybrid tomatoes for their disease resistance. We are trying ‘Mountain Magic’ this year which is bred to resist both early and late blight.

Anthracnose And Fungal Diseases

Anthracnose damages just the fruit with its 1/4 to 1/2 inch spots. Septoria is another fungus causing small brown spots with black centers to appear on the older leaves. Eventually they turn yellow and fall off. To prevent all fungal diseases be scrupulous when cleaning up plant debris in the fall. All old leaves and fruit, especially those that were affected by disease should be removed from the garden and disposed of in the trash rather going to the compost pile.

Wilts

Many tomato varieties are bred for disease resistance. Verticillium and fusarium are two wilt-causing diseases that have no cure. When shopping for tomato seed, look for the letters V and F after the variety name indicating resistance to those diseases.

Other letters are code for tolerance to other diseases: an A means the plant is resistant to alternaria, LB stands for late blight, EB early blight, N is for nematodes, T is tobacco mosaic virus, St is stemphylium leaf spot, Tswv is tomato spotted wilt virus, and Tylc is tomato yellow leaf curl virus.

Watering and Feeding Tomatoes

You may have noticed that many diseases come back to watering properly. With tomatoes, aim for consistent moisture as plants are establishing.

But once tomatoes begin to set fruit, let the soil just-about dry out between waterings. It’s okay for the foliage to show early signs of wilting before watering, but don’t push it too far. Inconsistent watering (seesawing between dust-dry then sodden soil) encourages water to rush into the fruits when it’s applied, causing them to split. The best time to water is in the morning, when plants are at their most receptive to moisture.

Of course, the tastiest tomatoes are gleaned from plants that have access to all the nutrients they need. Add slow-release organic fertilizer to the soil at planting time, or apply regular liquid feeds using a product specifically formulated for tomatoes. Feeding tomatoes should also avoid problems with blossom end rot,.

With all that can befall a tomato plant from the time of germination to the picking of the first ripe tomato, you might think that it’s a miracle that we get any fruit at all! However, growing tomatoes is all about avoiding some common pitfalls that can trip you up along the way. Knowing what to expect and what to do about it will greatly improve your chances of a truly terrific crop of tomatoes.

See our video on troubleshooting MORE tomato problems.
The Almanac’s Tomato Growing Guide has more information about pests and problems.