Gray Leaf Spot: How to Identify and Treat It in Tomatoes

Gray leaf spot is a disease that affects tomato leaves. It’s caused by three different fungi, Stemphylium solani, Stemphylium floridanum, and Stemphylium botryosum.

The fungus survives for a long time on plant debris, weeds, alternate hosts such as nightshade plants (peppers, eggplant, and potatoes), volunteer tomato plants, and gladiolas.

Spores are spread by wind and water. Wet weather, dew, and overhead watering help them germinate quickly.

Breeders have developed tomato varieties that are resistant to the disease. Look for disease-resistant code letters “S” or “St” (to indicate “Stemphylium”) on seed packages or plant labels.

What does gray leaf spot look like?

Brown to black specks appear first on older leaves, then newer growth.
Spots develop a gray center surrounded by a yellow halo. Leaves turn yellow, then brown, then drop.
The center of the spot may fall out leaving holes.
The entire plant may be affected.
Fruit is not affected directly, but the disease can stunt them. When plant loses its leaves, tomatoes are susceptible to sunscald.

When and where does it affect plants?

Photo: Vegetable MD Online

It often strikes in warm, wet weather.
Plants with heavy amounts of tomatoes are more easily infected.
It’s common in the southeast US.

How does it differ from Septoria leaf spot?

Symptoms between the two are similar, but there are no black spots in the center of gray leaf spot’s leaf damage.

How do you control and treat gray leaf spot?

The best control measure is prevention (see below).
Remove and destroy infected leaves, plants, plant debris, and volunteer tomato plants (be sure to wash your hands afterwards).
Treat as soon as possible and on a schedule.
Treat organically with a biofungicide like Serenade to lessen symptoms. Follow label instructions. Or apply a fungicide such as chlorothalonil, mancozeb, or maneb (all available under trade names) at the first sign of spot, when fruit sets (as a preventative measure) or when conditions indicate a strong potential for it to develop. Follow label directions. Re-apply every 7-10 days or after rain. Other diseases (such as early blight, Septoria leaf spot, and late blight) can be controlled by these biofungicides and fungicides, so application is multi-purpose.

How do you prevent gray leaf spot?

Plant disease-resistant hybrids to strengthen your plant’s chances of being disease-free.
Rotate crops. Fungus spores can remain in the soil and plant debris for several years.
Avoid planting tomatoes in the shade.
Avoid planting tomatoes where peppers, eggplants, or potatoes were grown the year before.
Plant tomatoes in a raised bed to improve drainage and prevent diseases from spreading.
Give tomato plants extra space (more than 24 inches) to let air to move among leaves.
Keep leaves as dry as possible. Water the soil – not the plants – to prevent splashing. Avoid overhead watering.
Stake tomato plants for better circulation.
Remove and destroy volunteer plants.
Remove and destroy affected plants or deep plow tomato plants into the ground at the end of the season.