By: Bonnie L. Grant, Certified Urban Agriculturist
Sweet, juicy, ripe tomatoes from the garden are a treat worth waiting for until summer. Unfortunately, that lusted for crop can be brought down low by numerous diseases and pests. Gray leaf spot on tomatoes is a classic example and is one of the many diseases that can strike plants in the nightshade family. Tomato gray leaf spot control is actually quite simple provided you practice good cultivation and hygienic routines.
You head out to inspect your bountiful tomato plants only to discover brown to gray lesions with a yellow halo. This is a common fungal disease that affects plants at any stage of their life. This is a fungal disease and doesn’t affect those marvelous fruits, but it can diminish the health of the plant and, therefore, the quality of fruit production.
Gray leaf spot on tomatoes is caused by the fungus Stemphylium solani. It causes the lesions on leaves which become glazed in the center and crack. This produces shot holes as the disease progresses. Lesions grow up to 1/8 (.31cm.) across. The affected leaves die and drop. Stems may also develop spots, primarily young stems and petioles. The consistently dropped leaves can lead to sunscald on fruit, which can make the tomato unpalatable.
Tomatoes grown in the southern states are primarily affected. The disease favors moist, warm conditions, especially when moisture on leaves has no time to dry before evening dew arrives.
Treating gray leaf spot on tomatoes isn’t as important as making sure the plants never get the disease in the first place. Prevention is always easier, so it is necessary to understand where this disease hides.
In the garden, it will overwinter in plant debris. Not only tomatoes but other nightshade leaves and stems that have fallen can harbor the disease. In heavy spring rains and wind, the disease spreads through rain splash and wind.
Good hygienic measures go a long way to preventing the disease. Sanitation of tools and equipment can also prevent this fungus from moving into other unaffected beds.
Some growers recommend treating gray leaf spot on tomatoes by using an early season fungicide. This can help prevent a variety of fungal diseases. There are also a few resistant tomato varieties if you can find them in your region.
The best tomato gray leaf spot control is crop rotation followed with seedbed sanitation and fungicide applications early in plant development. You can also hand pick off affected leaves to prevent the rapid spread of the fungus on the plant. Destroy any plant material rather than placing it in the compost pile.
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There are three leaf spot diseases commonly found on garden tomatoes in Minnesota: Septoria leaf spot, early blight and bacterial spot.
At the earliest stages of disease, they are difficult to tell apart but the management practices listed below will work for all three disease problems.
Most tomato leaf spot diseases overwinter in the soil and then splash on to the lower leaves of the plant. As a result, the first leaf spots can be found on the lowest leaves closest to the ground.
Look for brown to black round spots that are the size of a pencil tip or larger.
Other diseases (such as early blight, late blight, and gray leaf spot) can also be controlled by these biofungicides and fungicides, so application is multi-purpose.
Table 1. Fungicides to Control Gray Leaf Spot.
2 RTS = A ready-to-spray hose-end product.
If this document didn’t answer your questions, please contact HGIC at [email protected] or 1-888-656-9988.
Joey Williamson, PhD, HGIC Horticulture Extension Agent, Clemson University
This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.
If gray mold is a known problem in your region, preventative applications of a fungicide may help. Clemson University's Cooperative Extension program recommends applying a product that contains chlorothalonil. Follow package directions before applying the spray. For example, a typical product calls for mixing 6 teaspoons of the fungicidal concentrate with one gallon of water, assuming a tomato patch that is about 200 square feet. Spray the plants thoroughly, focusing on the stems and leaves. You can repeat the applications about once a week, with a total of no more than seven applications.
Ellen Douglas has written on food, gardening, education and the arts since 1992. Douglas has worked as a staff reporter for the Lakeville Journal newspaper group. Previously, she served as a communication specialist in the nonprofit field. She received her Bachelor of Arts from the University of Connecticut.