Tomato Problems Common in Montana

From a plant disease's point of view, years with cool, wet weather are very good years. Some of the more common tomato diseases we see in Montana include septoria leaf spot, early blight, and a variety of symptoms that are caused by environmental factors rather than a disease organism.

SEPTORIA LEAF SPOT, also known as SEPTORIA BLIGHT, is caused by a fungus that produces brown, somewhat circular spots that appear first on the lower leaves, then begin moving up the plant. The spots are quite small, ranging in size from 1/16" to 1/8" in diameter. As the spots mature, they sometimes become gray or white, surrounded by a dark margin.

To help control septoria leaf spot, stake plants to improve air circulation and to encourage quick drying of moisture on plant surfaces. Water only at the base of the plant, to avoid wetting the foliage. Remove weeds, which can harbor the disease, and remove badly infected leaves from the plant and destroy them. Clean up and remove all plant debris in the fall to minimize re-infection next year. Little is known about varieties that are resistant to this disease.

After discovering the disease, apply fungicides containing chlorothalonil, such as Ortho's Multi-Purpose Fungicide or Daconil 2787. Fungicides with the active ingredient maneb will also help, but do not apply within 5 days of harvest.

Another problem we see is EARLY BLIGHT, another fungal disease. This disease will also attack potatoes. Symptoms of early blight can be found on leaves, stems, and fruit. Small, irregular brown spots appear on older leaves first, enlarging until they are 1/4 to ½ inch in diameter. Dark rings of velvety spores commonly form in a bull's eye pattern inside the lesions. The surrounding tissue turns yellow. Often, the entire leaf turns yellow if the infection is severe. Dark, leathery spots may appear on the fruit.

High soil fertility reduces the severity of early blight, so keep plants properly fertilized and growing vigorously, and control weeds that might harbor the disease. When purchasing
tomato plants, ask for varieties that are resistant.

If you find early blight, begin applying fungicides that contain the active ingredient chlorothalonil, such as Ortho Multi-Purpose Fungicide or Bravo. Fungicides that contain mancozeb or maneb will also work, such as Dithane or Maneb.

Most of the non-pest problems we see in tomatoes are caused by environmental factors such as COLD, WET WEATHER, symptoms of which include rolled and deformed leaves, especially the lower ones, and yellowing leaves with reddish veins. There is not much to be done about these problems, obviously. Another condition, called OEDEMA, is caused by water-logged soil and high humidity. This can result in whitish-tan raised areas that have a corky texture. Improving drainage, if possible, might help. SUNSCALD and SCORCH are two other examples of environmentally caused symptoms.

Because tomatoes can also spread late blight, a very serious disease of potatoes, it is extremely important to plant only Montana-grown or certified late blight-free tomatoes. The Great Potato Famine in Ireland in the 1850s was caused by late blight. For more information on late blight in potatoes, see the fact sheet in this series "Potato Diseases".


Controlling Vegetable Pests; Environmentally Friendly Gardening. 1991. C. Putnam, Projec t Ed. Ortho Books, Chevron Chem. Co., San Ramon, CA 94583. 160 pp.
Flint, M.L. 1990. Pests of the Garden and Small Farm. University of California. Oakland, CA 94608- 1239. Pub. #3332. 276 pp.
Howard, R., J. Garland, and W. Seaman. 1994. Diseases and Pests of Vegetable Crops in Canada, An Illustrated Compendium. The Canadian Phytopathological Society and the Entomological Society of Canada, Ottawa, Ont. Canada K2A 1Y8. 554 pp.
Pacific Northwest Plant Disease Control Handbooks. 1997. Extension Services of Oregon State University. Corvallis, OR 97331-0817.


Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997


Categories: Disease, Tomato


Date: 04/02/02