Q: My tomato plant has dark spots on the leaves and the stems. An un-ripened tomato on the plant also has spots and the plant is showing signs of wilt and seems to be dying. What is wrong with it?

A: There can be a confusing array of symptoms that tomatoes display this time of year, making it difficult to diagnose tomato diseases. Often tomatoes have more than one problem, further complicating diagnosis.

Dark spots on leaves could possibly be a symptom of early blight, caused by the fungus Alternaria solani. Early blight is best controlled by good sanitation. Remove all diseased plant debris and do not plant tomatoes in the same location next year.

Because your plant has dark spots on the stems and fruit, the disease could also be bacterial canker. Bacterial canker is not common in Colorado, but can be confirmed with a laboratory test. If it is bacterial canker, destroy the plant and do not compost the plant material. Do not plant tomatoes in the same location for two to three years, as the bacteria will remain in the soil.

Your tomato could also have impatiens necrotic spot virus (INSV) or tomato spotted wilt virus (TSWV), both spread by the western flower thrip. Since thrips are extremely difficult to control, the only way to manage the disease is to remove and destroy the affected plant. INSV and TSWV can be confirmed by a laboratory test.

Q: I have a tomato plant that is stunted and pale. The leaves have curled but do not look wilted. I have noticed that the leaf veins are purple. The tomatoes are planted next to an open field that contains a lot of Lambsquarter. What’s wrong?

There are a couple options for this problem. It is possible that your tomato is suffering from psyllid damage. Psyllids are common insects in eastern Colorado. They inject toxic saliva while feeding that causes the plant to yellow and veins to turn purple. Check the undersides of the leaves for the nymphs, and look for a sugary-like substance that the psyllids secrete. For control, dust the undersides of the leaves with sulfur or use insecticidal soaps.

It’s also possible that your tomato is suffering from curly top virus. Curly top virus is spread by the beet leaf hopper, usually found on the western slope of Colorado. However, the leafhoppers can make an appearance on the eastern slopes with warm, windy, dry springs like the one we just experienced. A favored host of the beet leaf hopper is Lambsquarter. No chemical controls are effective. You can use row covers to protect your tomato plants from the leaf hopper, and currently remove all infected plants from the garden.

For more information on Tomatoes, read Fact Sheets #2.949 - Recognizing Tomato Problems; #5.540 - Potato and Tomato Psyllid; #7.846 - Plant Health Care: Growing Tomatoes and 7.847 - Plant Health Care: Tomato Early Blight. These Fact Sheets, and many more, are available at www.ext.colostate.edu

Gardening and Insect Fact Sheets are available on-line by clicking HERE.

Return to Master Gardener Articles

This page updated:  September 28, 2006