Controlling Tomato Leaf Spot Diseases

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Controlling Tomato Leaf Spot Diseases
Sarah Browning, Nebraska Extension Educator
Controlling Tomato Leaf Spot Diseases
Early blight, Alternaria solani, lesions on tomato leaves. Image from Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

Our tomatoes are growing strong, and it looks like this will be a much better year for vegetable gardens than we had last year. Dry spring conditions have slowed the development of leaf spot diseases, but even with good weather we know that they inevitably appear. So now is a great time to inspect your plants for the beginning of leaf spot symptoms and prepare to manage problems at the first sign of symptoms.

Tomatoes are attacked by both fungi and bacterial diseases that affect the leaves, petioles and stems, and cause blemishes on the fruits. Foliage diseases weaken infected plants by killing the leaves, which are the plant's factories for carbohydrate and energy production. Loss of foliage causes tomato plants to be less productive and vigorous, affecting tomato yield and quality. If severe enough foliage loss affects tomato flavor and pH, which is an important factor for safe canning, and can lead to sunscald on developing tomatoes due to suddenly exposure to more intense sunlight. If foliage diseases are not controlled, they can lead to death of the plant.

Common diseases of tomato include septoria leaf spot, early blight, bacterial speck and bacterial spot. All of these diseases overwinter in the vegetable garden on infected plant debris. The spores are spread during the growing season by wind, water and human activity.

  • Septoria leaf spot begins as tiny black dots on the leaves, enlarging to small circular spots with a dark margin and gray center. Infected leaves turn yellow and die. Elongated lesions develop on stems and petioles.
  • Early blight appears as irregular, dark brown areas on the leaves with concentric, black rings developing in a target-like pattern as the spots enlarge. Dark brown, sunken lesions form on stems and petioles. These symptoms appear about 10 days after infection. Early blight occurs in midsummer during warm, humid periods and can spread very rapidly.
  • Bacterial speck and spot are both spread by infected plant debris during periods of humid, wet weather. Bacterial speck appears as tiny, pinhead sized, raised black specks on tomato leaves and fruits. Bacterial spot is very similar to bacterial speck, but the leaf and fruit spots are slightly larger. On tomato fruits, bacterial spot results in slightly raised, brown, scabby lesions.

Sanitation is very important for reducing disease pressure in your garden each year. Remove all plant debris that is left in the garden from last season before tilling and planting. Establish a 3-4 year rotation schedule in your garden, by moving those plants most affected by disease to containers or new plots of ground. Or choose not to grow heavily affected plants for a few years to reduce populations of disease organisms in the soil.

One of the most common methods of tomato leaf infection is through rain splashing on bare soil. All of the diseases mentioned above overwinter on infected plant debris in the soil. During a rainstorm, water droplets hit the soil surface, splashing water and soil up onto the lowest tomato leaves. Prevent rain splash in your garden by covering the soil with mulch. Apply a 2-3 inch layer of mulch, using clean straw, black plastic, newspapers topped with wood chips, or any other coarse organic material. Mulch also helps suppresses weed growth, moderates soil temperature extremes and helps retain soil moisture.

Keep tomato leaves as dry as possible by applying water to the base of plants through soaker hoses, instead of using an overhead sprinkler, since water on the leaf surface promotes germination of fungal spores and leaf infection.

Suppression of leaf spot diseases, once plants have been infected, can be accomplished through sanitation and the application of fungicides. Remove and discard heavily infected plants. Infections may be slowed by removing diseased leaves as they appear.

Fungicides are protective; they keep healthy leaves from becoming infected. Fungicides are not curative. This means that infected foliage will remain diseased and may die. Fungicides must be applied on a regular basis to provide continued protection for the healthy leaves. Fungicides for use on garden vegetables, such as liquid copper or Bordeaux mixture, are readily available at most garden centers. Read the fungicide label carefully to determine the number of days you must wait after the final fungicide application before fruits can be harvested.

For additional disease management, remove debris from tomato and pepper plants in the fall after harvest is completed or till debris into the soil.

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Associated Video

Identifying Tomato Problems

Nebraska Extension Educator John Porter talks about how to differentiate between disease, insect and environmental issues with tomatoes