Tomato Leaf Diseases

Tomato plants susceptible to many diseases, disorders

Bill Sevier: Ask a Master Gardener

Tuesday, June 16, 2015

Q. My tomato leaves have large brown spots. What is this and what should I do? Leo, Tulsa

A. There are many diseases and disorders of our favorite vegetable, tomatoes. They may occur at different times of the growing season and simply may be incidental with little effect on the plants’ health and fruit production, or they can be devastating.

When tomato leaves develop disease patches, there are not only several fungal diseases to consider, but also a few caused by bacteria. Some of these leaf infections may also infect the fruit. There is a wide difference in disease susceptibility among the different tomato cultivars. These diseases often develop in similar situations, a fact that can be used in their control.

Many of the disease-causing organisms overwinter in the soil, in last season’s plant litter or nearby weeds. To lessen the chance of carry-over of disease from one year to the next, all garden trash and weeds should be removed in fall and the soil tilled to expose the microorganisms, as well as insects, to the winter weather.

Another strategy to reduce these problems is to rotate tomato planting sites. This will prevent disease buildup in the soil. If the size of the garden permits, you should rotate the planting site every three or four years. Also, it is best not to plant close relatives of tomatoes — potatoes, eggplant and peppers — in the same areas.

These infections need wet leaves to grow. If you eliminate overhead watering and water only the base of the plant, this will help. Also, watering should be done in the morning so the plants can dry by night.

Water splashing on the ground from irrigation or rainfall commonly carries soil organisms to leaves, causing infection. A generous layer of most any mulch will prevent splashing. Also, cages to keep plant leaves and fruit off the ground is effective in reducing infections.

If a disease has been a problem in the past or if you catch a disease early, treatments are available as a spray, usually applied every seven to 10 days. Most infections are due to fungi, so a fungicide such as chlorothalanil (Ortho Garden Disease Control) or myclobutanil (Immunox) may be used. Copper sulfate-based fungicides, such as those sold by Bonide, also treat bacterial infections and can be used in rotation with the other fungicides. This gives better coverage and prevents disease resistance.

Remember, no chemical pesticide will get rid of existing disease; at best it only prevents new disease. The non-chemical preventative measures mentioned above is the most effective tool.

For additional information about tomato disease, call the OSU Master Gardeners at 918-746-3701 or go to the vegetable section of tulsamastergardeners.org for links to several OSU fact sheets dealing with problems.

Garden tips

Renovate overgrown strawberry beds after the last harvest. Start by setting your lawnmower on its highest setting and mow off the foliage. Next thin crowns 12 to 24 inches apart. Apply recommended fertilizer, pre-emergent herbicide if needed and keep watered.

White grubs will soon emerge as adult June beetles. Watch for high populations that can indicate potential damage from grubs of future life cycle stages later in the summer.

Fertilize warm-season grasses at 1 pound of nitrogen per 1,000 square feet. Don’t fertilize fescue and other cool-season grasses during the summer. Because nitrogen is soluble in water, much of it may have been lost due percolation and runoff if you fertilized before recent rains.