There are many fungi that cause leaf and fruit spots on tomatoes in Connecticut. The four described here share the characteristic of having obvious fungal growth. In some diseases, this fungal growth is present at all times. In others, the growth can be seen when the leaves are wet or the humidity is very high. In these cases, it is best to look at the plants in the early morning, while the leaves are still wet from dew. A leaf may also be removed and put in a plastic bag with a piece of moist paper towel and left overnight, usually at a normal room temperature.
Gray Mold and Ghost Spot
Gray mold and ghost spot diseases of tomato are caused by the fungus Botrytis cinerea. Botrytis causes disease in practically all plants, including tomatoes. The fungus is everywhere, especially in greenhouses. Plants become more susceptible as they get older. Plants with dense or succulent foliage are most heavily damaged. The disease causes a loss of leaf area and fruit quality, including losses during storage and shipment.
Symptoms. Light tan or gray spots appear on leaves, which become covered with gray-brown fungal growth. The leaf later collapses and withers. The fungal growth is dense and may resemble felt.
Elliptical spots may form on the stem where an infected leaf meets the stem. These cankers are tan with concentric rings. They can girdle the entire stem and cause wilting of the stem above the canker.
Dying flower petals are very susceptible. The fungus can either kill the flower or, if fruit is set, may grow into the developing fruit and cause a soft rot to occur. Spots start as soft, water-soaked areas with irregular edges. The spots may be up to 1″ in diameter and are usually grayish or yellowish green with lighter edges. The skin of the fruit usually breaks over the decayed areas but remains intact over the rest of the fruit. A dark gray growth of fungus usually appears over the spot.
When a spore lands directly on the fruit, ghost spots may develop. A ghost spot is a 1/8″ to 3/16″ pale ring with a dark speck in the center. The spot will not rot, fruit quality is not affected. The spore is unable to cause a real infection and dies.
Identification of disease. Gray-tan fungal growth is present. The fungal growth is dense and may resemble felt.
Prevention. In the greenhouse, keep temperatures above 70oF and relative humidity below 90%. Keep leaves dry by avoiding overhead watering. Keep air moving at all times. Prune plants by breaking petioles close to the stem; do not leave stubs.
In the field, plant in well-drained soil. Allow plenty of room between plants for good air circulation. Keep weeds controlled in order to improve air circulation. Keeping soil pH at 6.3 has been shown to help keep disease levels low. Good nitrogen levels help keep this disease down. Liming the soil, if it is acidic, helps to increase the calcium content of plants and reduces the plants’ susceptibility to Boytrytis. The disease is favored by relatively cool weather. There are few resistant varieties. See current recommendations for chemical control measures.
Leaf mold is caused by the fungus, Fulvia fulva, also known as Cladosporium fulvum. This is a disease of tomatoes only. The disease occurs all over the world, but it is primarily a greenhouse disease in Connecticut. It causes tomato leaves to fall off, which will lower yield. High humidity is required for this fungus to grow successfully.
Symptoms. Generally, leaves are the only part of the plant that is affected. Lower leaves are affected first, then younger leaves. Pale-green or yellowish areas appear on the upper leaf surface. These later become distinctly yellow. The edges are indefinite, and spots may grow together. At the same time, the fungus begins to grow on the undersides of the leaves in areas corresponding to the pale upper surface areas. The growth is olive green to greyish purple and velvety. The mold is more deeply colored in the center of the areas. As the disease progresses, the leaf spots turn yellowish brown. The leaves curl, wither and drop prematurely. Defoliation starts at the bottom of the plant and works up.
Occasionally, flowers may be killed. The green or ripe fruit may have a black, leathery stem-end rot. This rot may cover up to 1/3 of the fruit surface and may cause blackened furrows to form in the fruit. The fruit may develop a lop-sided shape and the small side may ripen slower than the fully expanded side.
Identification of Disease. Fungal growth is seen on undersides of leaves only. The color of the fungal growth is unusual, especially the deeper color in center.
Prevention. Keep temperature at least 60o to 65°F throughout the season. At night, keep the greenhouse temperature warmer than the outside temperature. Ventilate when the humidity is above 85%. Avoid wetting leaves, and allow to dry before night. Use adequate plant spacing and fans to ensure good air circulation and leaf drying. Remove and destroy all plant debris after harvest. If possible, steam the greenhouses (135°F for at least 6 hours), when the houses are empty. This is easier on a sunny day. Although resistance is available, it may be difficult to ensure that the plants are resistant to all 12 races of the fungus, any of which may be present. See current recommendations for chemical control measures.
Powdery mildew, a very common disease, is caused by the fungus Oidiopsis taurica, also known as Leveillula taurica. The fungus has been known in the United States only since 1978. This particular fungus affects tomato, potato, eggplant, onion, cotton and artichoke. There are many other fungi that cause powdery mildew on other plants. The disease can affect outdoor tomatoes, especially those grown in shady areas with poor air circulation, but the disease is more common in the greenhouse. The shriveling of the leaves causes a loss of yield.
Symptoms. Older leaves are usually affected first. Bright yellowish spots, approximately ½” in diameter, develop and may grow together. The centers of the spots may die and turn brown. On the upper surface of the leaves, a fine white powdery fungal growth appears. The fungal growth may appear on the lower leaf surface later. Whole leaf blades may turn brown and dry up with a heavy infection. Blades seldom drop from the plant; fruits are not affected.
Identification of Disease. Characteristic white powdery growth on either leaf surface is observed.
Prevention. Good air circulation is the best way to prevent this disease. Avoid excess nitrogen fertilizer. There are no tomatoes resistant to this disease. See current recommendations for chemical control measures.
Late Blight is a very devastating disease of tomato, potato, and eggplant. It is caused by the fungus Phytophthora infestans. This disease was the cause of the potato famine in Ireland in 1845 and 1846, which caused a million people to starve and a million and a half more to emigrate. The disease can progress very quickly, destroying plants and rotting fruit in 2 weeks in favorable weather.
Growth Stages Affected/ Time of Season. Leaves, green and ripe fruit and stems are affected.
Symptoms. Fields should be scouted frequently in the early morning when the leaves are still wet. Dark green water-soaked spots appear on leaves. The spots sometimes have a purplish tinge and are an indefinite shape. They enlarge rapidly to green or brown spots which can cover most of the leaf. Fluffy gray to white moldy growth appears on undersides of the small leaf spots when lesions and a ring of moldy growth can be seen on undersides of larger spots. This growth can be seen in early morning when leaves are still wet. Dark brown to black spots form on stems, which can cause the portions of the plant beyond the stem spot to dry up rapidly. This disease causes a rot of green and ripe fruit in which dark greenish-brown greasy areas form and may enlarge until the entire fruit is covered. Generally the fruit remains firm, although, secondary soft rot often sets in. Favorable weather for this disease is cool nights, with warm days. High humidity for 24 to 48 hours allowing leaves to remain wet after rain.
Identification of Disease. Patchy spots and fluffy while growth on the undersides of leaves are present in the early morning. There are stem spots. This affects green and ripe fruit.
Prevention. This disease progresses quickly, so it is important to prevent it, and to look for it early and often. The fungus overwinters in cull piles, volunteer plants, and potato seed tubers. Make sure that cull piles are as far away as possible, and no volunteers of tomatoes or potatoes are left in the fields. Use certified disease-free transplants or seeds, and disease-free tubers when planting potatoes. Plant tomatoes as far as possible from potatoes. Scout frequently and in the early morning when leaves are still wet. Resistance is available for this disease. See current recommendations for chemical control measures.
- Hausbeck, M.. 1997. Proceeding of the New England Vegetable and Berry Conference. Cooperative Extension System.
- Jones, J.P. and J.B. Jones. 1991. Leaf Mold in Compendium of Tomato Diseases. APS Press. St. Paul, MN. p. 18. J.B. Jones, J. P. Jones, R.E. Stall, T. A. Zitter, eds.
- Sherf, A.F. and A. A. MacNab. 1986. Vegetable Diseases and Their Control. John Wiley and Sons, New York..
- Paulus, A.O. and J.C. Correll. 1991. Powdery Mildew in Compendium of Tomato Diseases. APS Press. St. Paul, MN. p. 19. J.B. Jones, J. P. Jones, R.E. Stall, T. A. Zitter, eds.
- Stall, R.E. 1991. Grey Mold in Compendium of Tomato Diseases. APS Press. St. Paul, MN. pp. 16-17. J.B. Jones, J. P. Jones, R.E. Stall, T. A. Zitter, eds.
- Stevenson, W.R. 1991. Late Blight in Compendium of Tomato Diseases. APS Press. St. Paul, MN. pp.17-18. J.B. Jones, J. P. Jones, R.E. Stall, T. A. Zitter, eds.
By Pamela S. Mercure, IPM Program Assistant, University of Connecticut
Reviewed by: T. Jude Boucher, IPM, University of Connecticut. 2012
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