Tomato Leaf and Fruit Diseases & Disorders

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By Megan Kennelly, Plant Pathologist

Several tomato diseases and disorders cause leaf spots and fruit rots. Typically, these diseases do not kill the plants, but they can lead to significant yield and quality losses. Many disease-causing microorganisms can survive in plant debris,on seed, or in the soil. Proper sanitation is one of the best ways to prevent or reduce losses. Gardeners should remove all tomato debris in the fall and avoid planting tomatoes in the same area year after year. This publication discusses some of the major leaf spot and fruit diseases and physiological disorders in Kansas. The first section describes fungal diseases, the second section describes bacterial diseases, and the third section describes physiological disorders.

Fungal Diseases

Septoria Leaf Spot and Early Blight

The fungal diseases Septoria leaf spot and early blight are two of the most common tomato diseases. Both diseases can occur anytime during the growing season,but they generally become more severe after blossom-set. These diseases result in the formation of leaf spots that typically develop first on the older leaves nearest the ground. During favorable conditions for disease development,these diseases can cause extensive defoliation, resulting in sunscalding of fruit and reduction in yield.

Septoria leaf spot and early blight have similarities, but they can be distinguished from one another in the field.

Figure 1. Septoria leaf spot lesions often have small black specks (fungal spore-producing structures). Photo courtesy Wendy Johnson, Kansas State University.

In Kansas, Septoria tends to be more common than early blight. Symptoms of Septoria leaf spot first appear as small, water-soaked spots on the lower leaves. Infection moves up the plant as the season progresses. The leaf spots (lesions) generally are smaller and more numerous than spots resulting from early blight. Eventually the center portion of the Septoria lesion turns light tan or gray, while the margin remains dark. Small black fruiting structures (pycnidia) of the fungus, readily visible with a 10X hand lens, form in the center portion of the lesion (Figure 1). Heavily infected leaves may scorch and wilt, giving the plant the appearance of a wilt disease.

Figure 2. Early blight lesions on tomato leaves usually have a target-like appearance. Photo courtesy Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org.

Early blight causes irregular, brown leaf spots (lesions) that range in size up to . inch in diameter. The most important diagnostic feature of early blight is the formation of dark, concentric rings within the lesion, giving the spots a target-like appearance (Figure 2). Often, several lesions coalesce, causing the leaf to turn yellow, dry up, and fall off the plant. Defoliation weakens the plant and exposes the fruit to sunscald injury. Although early blight is primarily a foliage disease, lesions may develop on both stems and fruit. Fruit lesions are tan to brown, leathery, and typically originate at the stem end of the fruit.

Cause

Septoria Leaf Spot: Septoria lycopersici

Early Blight: Alternaria solani
Both fungi survive the winter in plant debris, on seeds, or on weeds such as nightshade and horse nettle. In the spring and summer, spores of these fungi can be splashed or blown to tomato leaves. Relatively warm temperatures, abundant rainfall, and high relative humidity favor disease development.

Control

Sanitation measures in the fall reduce the amount of inoculum available for infection the following year. In the fall, deep plow tomato plots to bury tomato debris,or remove and destroy dead plants. Avoid planting tomatoes in the same area of the garden year after year. Clean seed and healthy transplants in the spring help control the disease. To reduce humidity and leaf wetness, avoid overhead irrigation. To improve airflow, use staking and appropriate plant spacing. To prevent rain splash, use mulch. Avoid composting diseased plant material.

Start with the cultural practices described above. However, there are control materials available. For best results, begin applications as soon as the symptoms become apparent.

Homeowners can refer to Table 1 …

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Article & photographs courtesy of www.plantpath.ksu.edu/