Nonparasitic Disorders of Tomato

F. L. Pfleger, Professor
S. L. Gould, Assistant Scientist
Department of Plant Pathology

Copyright ©  2002  Regents of the University of Minnesota. All rights reserved.

Tomato plants can develop several nonparasitic disorders that distort plants and blemish fruits. Unlike the "parasitic" diseases caused by fungal, bacterial, or viral organisims (see Parasitic Diseases of Tomatoes, FS-1155, these disorders are not infectious or a threat to your entire garden.

Nonparasitic, or physiological, disorders are the result of plant responses to abnormal environmental conditions. In some cases these environmental factors are unknown. In other cases, the causes are known, but are difficult to correct. Generally, good cultural practices that ensure consistent plant growth will reduce the occurrence of nonparasitic disorders.

Tomato varieties also differ in susceptibility to these disorders. By trial and error you can find the best varities for your location or seed catalogs give varietal information that may help avoid problems.

The purpose of this fact sheet is to point out some common nonparsitic diseases of tomatoes and current knowledge about them.

Physiologic Leaf Roll

When certain tomato varieties are grown for maximum fruit production they often develop a physiological disorder called leaf roll. Symptoms are most prominent on lower leaves. Leaf margins roll upward until they touch or overlap and leaves are firm and leathery to the touch.

Factors often associated with this disorder are heavy applications of nitrogenous fertilizers, root pruning (due to close cultivation) and supporting and pruning plants. Leaf roll does not noticeably check plant growth or yield.

Blossom-end Rot

Blossom-end rot can appear on fruit in any stage of development, but it is most common when fruit are one-third to one-half grown. The initial symptom is a watersoaked spot at the blossom end of the fruit. This spot enlarges and becomes brown and leathery. Discolored tissue shrinks until the affected area is flat or concave and tomato flesh is conspicuously rotted (see figure 1).

Figure 1. Tomatoes showing various stages of blossom-end rot and secondary invasion by decay-causing organisms.

Blossom-end rot is associated with a calcium deficiency, fluctuations in moisture, heavy applications of nitrogen fertilizer, and root pruning.

The amount of calcium salt available to the plant decreases rapidly in the presence of excessive salts such as potassium, magnesium, ammonium, and sodium. Extreme fluctuation in moisture can also reduce the availability of calcium salts needed by the plant. Heavy applications of nitrogen fertilizers and abundant rain cause rapid and luxuriant plant growth and predispose the fruit to blossom-end rot, especially during periods of dry, hot weather.

Blossom-end rot can be minimized by maintaining a uniform supply of moisture through irrigation and soil mulches, incorporating fertilizers high in superphosphates and low in nitrogen prior to planting, and avoiding root pruning by not cultivating within 1 foot of the plants.

Sunscald

Sunscald is most commonly seen on green fruit (see figure 2). A light spot on the side of the fruit facing the sun is the first symptom. Later there is blistering and finally a large, flattened, grayish-white spot with a dry paperlike surface forms. Such spots are frequently invaded by decay-causing organisms.

Figure 2. Tomatoes showing advanced stages of sunscald injury and secondary invasion by decay-causing organisms.

Any factor causing a loss of leaves, such as disease, will expose fruits to sunlight and increase chances for sunscald. Maintaining a disease control program will lessen chances of foliage loss. Covering exposed fruits with straw, if plants are not supported, should be used as a secondary control measure for preventing sunscald.

Growth Cracks

Cracks of varying depth radiating from or encircling the stem end of tomato fruits are typical of growth cracks. They are fruit blemishes and possible entrance points for decay-causing organisms.

Growth cracks result from extremely rapid fruit growth brought on by periods of abundant rain and high temperatures, especially when such weather conditions follow drought.

Control methods mentioned for blossom-end rot may be beneficial in preventing growth cracks. Varieties differ in susceptibility to cracking, and variety descriptions often include this information.

Catface

Catface is a condition involving malformation and scarring of fruits, particularly at blossom ends. Affected fruits are puckered with swollen protuberances and can have cavities extending deep into the flesh.

The causes of catfacing are not definitely known, but it is generally agreed that any disturbance to flowers can lead to abnormally shaped fruits. Extreme heat, drought, low temperature, and contact with hormone-type herbicide sprays may be causes of flower injury. Other than keeping herbicides away from flowers, the only control for catface is planting less susceptible tomato varieties.

Herbicide Injury

Tomatoes are very sensitive to injury from herbicide chemicals. The most common injury symptoms observed are caused by phenoxy herbicides such as 2,4-D (2,4-dichlorophenoxyacetic acid) and dicamba (substituted benzoic acids). These chemicals are growth regulator, hormone-type weed control chemicals. Tomato plants usually come in contact with the chemical through spray drift or the use of a sprayer that once contained the herbicide.

Contaminated plants show one or more of the following symptoms depending on the degree of exposure and age of plant at exposure. Older leaves are excessively pointed, down-curved, or rolled with prominent light-colored veins; young leaves do not fully expand and are narrow and elongated with parallel veins; stems are split, distorted, or brittle; and fruits are catfaced or irregularly shaped.

Plants exposed to small amounts of phenoxy herbicides will outgrow the symptoms without seriously reducing yield or fruit quality. Harvest might be delayed, however. Plants do not recover from severe damage by herbicides.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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