The past couple of weeks I have had several calls and questions on tomato plant problems. It is kind of surprising to me due to the weather conditions we’ve had that we’ve had this many tomato fungus problems. The hot, dry weather usually is not favorable for environmental caused tomato disease.
The most important practice to remember is to keep a consistent water supply to your tomato plants. Just as drought stress is hard on tomatoes, so is an over abundance of water or over watering followed by not enough water repeatedly causes undue stress on your plants and sets you up for several tomato fungus problems.
It is also recommended to grow your tomatoes in a cage or trellis to keep the fruit off of the ground and allow air movement under the tomato foliage.
Tomato foliage can be attacked by a number of fungal diseases. Approved fungicide usually give satisfactory control of the disease when used in combination with good cultivar practices, such as ample spacing, caging, proper fertilization and watering.
Late Blight (fungus - Phytophthora infestans): Lesions produced on the leaves are at first irregular, rather large, greenish-black and water-soaked. These areas enlarge rapidly, becoming brown, and under humid conditions, develop a white moldy growth near the margins of the diseased area on the lower surface of the leaves or on stems. The disease spreads rapidly under humid conditions, destroying quickly large areas of tissue. Fruit lesions occur as large, green to dark brown lesions, mostly on the upper half of the fruit, but they may also occur on other parts. White moldy growth may also appear on fruits under humid conditions. The fungus produces abundant numbers of spores which may be splashed by rains or be airborne. These spores infect healthy leaves, stems and fruit readily if climatic conditions are optimum.
Early Blight (fungus - Alternaria solani): Early blight is first observed on the plants as small, black lesions mostly on the older foliage. Spots enlarge, and by the time they are one-fourth inch in diameter or larger, concentric rings in a bull’s eye pattern can be seen in the center of the diseased area. Tissue surrounding the spots may turn yellow.
Gray Leaf Spot (fungus - Stemphylium solani): First infection appears as minute, brownish-black specks on the lower leaves that extend through to the undersurface of the leaf. These spots usually remain small, but may enlarge until they are about one-eighth inch in diameter. They become glazed and at times the centers crack and tear across. Infected leaves usually die and drop. Spots also form on the stems.
Leaf Mold (fungus - Fulvia (Cladosporium) fulvum): Leaf mold is usually first observed on older leaves near the soil where air movement is poor and humidity is high. At first, diffuse whitish spots appear on the upper surfaces of older leaves; these rapidly enlarge and become yellow. The best control of this disease is by using a preventative fungicide program at 7 to 10 day intervals, the same as used for late and early blight control.
Fusarium Wilt (fungus - Fusarium oxysporum f. sp. lycopersici): The first indication of disease in small plants is a drooping and wilting of lower leaves with a loss of green color followed by wilting and death of the plant. Often leaves on only one side of the stem turn yellow at first; yellowed leaves gradually wilt and die. The stem of wilted plants shows no soft decay, but when cut lengthwise, the woody part next to the green outer cortex shows a dark brown discoloration of the water conducting vessels. The fungus is soilborne, passes upward into the xylem of the stem. Blocking of the water-conducting vessels is the main reason for wilting. Control can be obtained by growing plants in disease-free soil, using disease-free transplants, and growing only resistant varieties.
Verticillium Wilt (fungus - Verticillium albo-atrum): The first symptom is yellowing of the older leaves, followed by a slight wilting of the tips of the shoots during the day. Older yellowed leaves gradually wither and drop, and eventually the plant is defoliated. Verticillium wilt does not show the one-sided effect as does Fusarium wilt. Leaves from Verticillium infected plants sometimes show brown dead spots that may be confused with those caused by other fungi. However, they are lighter in color and do not show concentric zones as in early blight. In late stages of the disease, only the leaves near the tips of the branch remain alive. When the stem is cut lengthwise, the base shows a discoloration of the woody tissue similar to Fusarium, but is usually darker, and generally it occurs only in the lower part of the stem. The fungus lives in the soil for a long time and it is exclusively the source of infection. Using resistant varieties are the most effective means of controlling the disease.
Septoria Leaf Spot (fungus - Septoria lycopersici): Infection usually occurs on the lower leaves near the ground, after plants begin to set fruit. At first, small watersoaked spots are observed, which under ideal conditions will become numerous. Large areas of the leaves may be affected but the individual spots can be recognized. The watersoaked spots become roughly circular, with dark margins surrounding a light gray center. With time, black specks which are spore producing bodies can be seen in the center of the spots. If the spots are numerous, the lower leaves will turn yellow, die and progressively drop from the plant until only a few leaves remain on the top of the plant. The fungus is most active when temperatures range from 60 to 80 degrees F., and humidity conditions are high. The disease is usually not serious during periods of hot, dry weather. Repeated fungicide applications will keep the disease in check.