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Leaf Diseases of Tomatoes

Source: NDSU Extension Service Horticulturist

There are several diseases of tomato leaves that are common in our area. Two of these cause spots on the leaves, usually beginning on the lower leaves and progressing upward. The spotted leaves turn yellow and drop off. Plants viewed at a distance may have an overall yellow appearance.

One of these leaf spot diseases is called Septoria. Septoria spots are about 1/8 inch in diameter but occasionally reach 1/4 inch. They are more or less circular with grey centers and dark brown borders. Small black spots, like finely ground pepper, develop in the centers of the spots. Septoria leaf spot usually occurs on the leaves but may also develop on the vines. Septoria leaf spot is a very common problem and often causes severe loss of leaves.

The other leaf spot disease is early blight. These spots are large, up to inch or even 1 inch in diameter. They are circular to irregular in shape and dark brown to black in color. Early blight spots have concentric rings arranged in a target pattern. These spots develop on both leaves and fruits and cause severe loss of both leaves and fruits.

Both early blight and Septoria leaf spot are most serious in damp, rainy weather. They live over the winter on old diseased tomato vines and can also be spread on transplants. Both can cause severe loss of leaves which can lead to sunscald of the fruits.

Early blight and Septoria leaf spot can be controlled by removal of old tomato vines in the fall, by careful selection of healthy transplants from a reliable nursery and by regular spraying with a fungicide. Spraying should begin by mid-July or when the first fruits are marble-sized. It should be done every 7 to 10 days. Fungicides to use include Captan (Orthocide) or Daconil 2787 (Ortho Multi-Purpose Fungicide). Follow label directions for amount of fungicide to use, how often to use it and how long before harvest to stop using it. Be sure to use a sprayer that has never been used for a weed killer such as 2,4-D. Tomatoes are very easily damaged by 2,4-D, and this chemical is very difficult to wash out of the tank.

Another leaf disorder sometimes seen in our area is leaf roll. The lower leaves of the plants roll upward and have a leathery texture. This is a non-parasitic disease. It may be caused by excess moisture, staking and pruning plants, heavy applications of nitrogen fertilizers and root pruning from close cultivation. Leaf roll is not contagious and usually does not have any great effect on plant growth or yield. Frequently plants will grow out of this after a period of time.

Another leaf problem sometimes seen is herbicide injury. This usually results from using 2,4-D or related weed killers too close to tomato plants, where the chemical drift or vapors can come in contact with the plants. Leaves of affected plants often become abnormally narrow or twisted, have parallel veins and are thick and leathery. Stems may split, and fruits may develop a cat-face pattern on them. Plants exposed to small amounts of 2,4-D usually recover, particularly if supplied with adequate water during the recovery period. Severely damaged plants do not recover.

For future reference, you may want a copy of Extension bulletin PP-659, "Disease Control in Home-Grown Tomatoes," which is available at your county office of the NDSU Extension Service.

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