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  • Jimmie Cassity

Tomato Diseases: Identification, Treatment and Prevention

Knowing how to identify and treat common tomato diseases is critical for growing successful crops. Tomato plants are vulnerable to fungi, bacteria, mildew, viruses and fruit problems like sunscald and blossom end rot. The first line of defense is to provide adequate conditions for growing healthy plants with good disease resistance.

Most common diseases are caused by fungus that favor certain weather conditions. Excessive rain during cool or warm periods creates an ideal environment. When adverse growing conditions persist, treating your plants proactively gives you a head start on problems that spread and can be difficult to eradicate.

Good garden practices including crop rotation, debris removal, weed and pest control, and proper irrigation go a long way to discourage infections. Here are symptoms to watch for and ways to keep your tomatoes healthy.

Early Blight

Early blight is the most common of several leaf spot diseases on tomatoes. It is caused by Alternaria fungus. Dark brown spots encircled with rings start on the lowest leaves and move up, eventually causing foliage to shrivel, dry, and fall. Lesions can also develop on stems and fruits with defoliation causing sunscald.

Early blight is more prevalent in hot, humid weather and remains in the soil for one year.

Treat early blight by removing lower leaves, including up to one-third of infected foliage. Apply a fungicide at the first sign of infection.

Prevent early blight by watering at the soil level and mulching. Keep adequate space between plants and rows; use cages and practice good weed control. Prune bottom leaves from plants and rotate tomato plants every year.

Fusarium Wilt

Caused by Fusarium oxysporum f. sp. Lycopersici which enters tomato plants through roots. In warm weather

the entire plant wilts during the day, often recovering at night. Eventually leaves on one side turn yellow, dry up and fall off. Inner tissue of the lower stem turns red or black. Plants may die quickly or succumb in a week or more.

There is no fungicide or fusarium wilt, so you need to remove and dispose of infected plants. Fungus spores can remain in the soil for many years, but this disease does not spread among plants grown in the same season.

Prevent fusarium wilt by planting varieties labeled VFN or FN which indicate resistance. Keep tools clean and practice crop rotation. Fusarium wilt is hosted by pigweed and crabgrass so weed control is important. Avoid excessive nitrogen fertilizer.


Verticilliumm wilt, although caused by different fungus, shows symptoms almost identical to fusarium wilt. It develops in cool temperatures. Treatment and prevention are the same for both types of wilt.

Powdery Mildew

Powdery mildew spreads through three different types of airborne fungi. The type of spore differs according to temperature but high humidity levels increase disease occurrence. Yellow spots appear on leaves turning to white powdery lesions coating the entire leaf and appearing on stems. Fruits do not develop powdery mildew but defoliation leads to sunscald and crop loss.

Treat powdery mildew by stopping its spread with sulfur dust, fungicides, biofungicides and horticultural oils. Remove infected leaves and buds. Treat entire crop.

Prevent powdery mildew by allowing adequate space between plants. Prune for good air circulation. Provide regular consistent hydration at soil level and avoid wetting leaves.


A disease infecting mostly fruits caused by Colletotrichum coccodes, a fungus that favors warm temperatures and wet conditions. Small sunken, water-soaked spots appear on fruit and increase in concentric circles causing tomatoes to rot. Leaves may develop small, round spots with yellow halos. Infection starts in small immature fruits but symptoms don’t appear until ripening. Dozens of weeds and other plants host this fungus which overwinters in soil and plant debris.

Fungicides are more effective used as a preventative. Apply to the entire crop at the first sign of infection or when weather conditions are favorable for disease to take hold. Avoid letting tomatoes over-ripen on the vine.

Prevent anthracnose by caging plants, mulching, and watering at soil level. Use certified seed and plant in well-draining soil. Remove and dispose of rotten fruit and debris. Practice good wee control and rotate crops every year.

Septoria Leaf Spot

Caused by the Septoria fungus and appears on leaves as multiple small, dark, round circles. Symptoms are similar to early blight, but septoria more often appears at the first fruit set. It spreads rapidly causing loss of older leaves, then infects new foliage, and can quickly move through an entire crop. Insects, tools, and water all spread fungus spores which remain in the soil for up to two years. Early leaf drop leads to fruit loss and sunscald.

The most effective treatment is to control the spread which requires repeated applications with a fungicide for the entire crop. This fungus thrives in warm, wet weather so watch for symptoms and act immediately.

Good garden sanitation is critical for preventing septoria leaf spot. Remove fallen leaves and debris from the garden immediately. Clean tools before and after working with plants. Water at ground level and control insect pests. Rotate tomato crops every year.

Botrytis Gray Mold

Botrytis presents as brown lesions on leaves and stems and a whitish soft rot in fruits. Infections takes root in damaged stems or pruning cuts and can lie dormant for up to 12 weeks. The mold can cause tomatoes to rot after harvest. Leaves die and fall off and stem girdling leads to wilt. Tomato flowers are also susceptible. Spores are spread by wind and water and are most prevalent in cooler temperatures.

Botrytis often dies back when temperatures rise. Treat widespread or persistent infection with a fungicide with a specific application for gray mold.

Prevent botrytis by pruning plants in the early afternoon allowing cuts to dry quickly. Avoid overhead watering and working with wet plants. Leave adequate spacing between plants and rows for good air circulation.

Bacterial Speck

Infects tomato plants with Pseudomonas syringae during persistent cool wet weather. Yellow tissue surrounds small, irregular, dark brown to black spots, close to leaf margins. It spreads by splashing water on leaves.

Bacterial speck can’t be cured, however, preventative steps include treating plants with a copper fungicide during periods of cool, wet weather. Hot weather stops the pathogen form spreading.

Prevent bacterial speck by delaying planting until weather conditions are warmer and drier. Apply a protective fungicide if cool, wet weather persists. Avoid overhead irrigation and rotate tomatoes every year.

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