Issue 12, July 16, 2012
The tomato is one of the more popular vegetable grown in home gardens. With it, however, come a plethora of problems and this season has no shortage. Below are some of the problems that gardeners may be seeing.
Flower drop. Tomatoes do not set fruit when day temperatures are above 95°F., especially with hot, dry winds. Night temperatures above 70° also can reduce fruit set. The recent blast of hot weather most likely put tomato plants into a holding pattern. The advent of more seasonal weather should aid in more normal fruit set. There is nothing to do about this but continue to water the plants, fertilize as needed, and mulch.
Blossom-end rot. This disorder appears as a dry, sunken, leathery rot on the bottom, or blossom end of the tomato fruit. Typically, early maturing fruits are most affected. It is actually caused by a calcium deficiency exacerbated by lack of available moisture. Plants that are pruned (as in removing the suckers on staked tomatoes) have increased susceptibility to this condition as well as those where cultivation around the plants is too excessive, cutting off roots which restricts water uptake. It is most prevalent during prolonged dry periods that occur while the plant is making vigorous growth. The best way to prevent blossom-end rot is to provide uniform water to the plants once a week (twice a week during extremely hot weather) with 1 inch of water. Use a 2 to 4-inch mulch around the plants to retain moisture. If you are staking or caging the plants, avoid pruning severely. The fruit is typically not usable and should be discarded.
Cracking. Erratic watering (or sudden summer rains) after a dry period may cause fruit cracking. The cracks are generally concentric – located around the sides of the fruit. Fruits are still useable. Gardeners should water on a regular schedule when rainfall is lacking and use mulches to retain moisture in the soil.
- Early blight (Alternaria solani). This disease occurs on the leaves, stems, and fruits, though it is most noticeable on the lower leaves of the plant. Look for small, dark brown spots with concentric rings. Similar spots may appear on the stems. Spots on the fruits will be dark, sunken, and leathery. The disease is favored by humid weather and temperatures above 75°F. To control this disease, provide ample spacing around plants for good air circulation. Avoid watering the plants late in the day, and preferentially, keep water off the foliage by watering near the base of the plants. Regular application of fungicide sprays can be used for control. At the end of the season, be sure to rake up all plant residue and plan on rotating the tomato planting to a different area next year.
- Late blight (Phytophthora infestans). Tends to be more serious with cool, wet weather. Leaves infected with this disease first develop greenish-brown or black water-soaked spots starting at the tip. In moist weather, a white, downy mold will appear along the margins of the infected area and on the undersides of the leaves. Fruits of infected plants develop grayish-green, water-soaked spots. The spot becomes brown and often covers half the fruit. Infected plants eventually wither and die. Controls are same as Early blight.
- Septoria leaf spot (Septoria lycopersici). This is a serious foliar disease. Infected leaves develop circular water-soaked spots that have gray centers and blackish borders. Tiny dark specks develop in the centers of the spots. If spotting is severe, the leaves eventually die and fall, and fruits are subject to scald.
- Fusarium wilt (Fusarium oxysporum). A soil-borne disease that causes rapid wilting of the plants. Symptoms include a yellowing, wilting, and dying of the leaves progressing from the bottom of the plant upward. This usually happens about the time of fruit set but may occur any time during the growing season. The disease is most active during warm weather when soil temperatures are between 75 and 85°F. It can be prevented by growing resistant varieties but many old-time favorites lack resistance. Infected plants should be removed immediately. Gardeners should choose resistant varieties when selecting plants in the spring and rotate to another garden location next year.
- Verticillium wilt (Verticillium sp.). Another soil-borne fungus with symptoms similar to Fusarium wilt, though it tends to be more serious with cooler weather. The leaves wilt, gradually wither, die and drop off. Eventually only the tip leaves appear green and alive, and the fruit may be small. Defoliation exposes the fruit to sunscald. As with Fusarium wilt, selecting resistant varieties and planting in a different location the next year are recommended practices.
(Jim Schmidt, Extension Specialist, Horticulture)