Gray Leaf Spot

Gray leaf spot doesn’t exactly live up to its name. Rather than gray spots on leaves, this fungus causes small brown or yellow dead areas that merge to make larger necrotic areas.

Gray leaf spot on tomato (Clemson University, USDA Cooperative Extension, Bugwood.org) CC BY 3.0 US

​Gray leaf spot is a fungal disease commonly caused by Stemphylium solani fungi. S. floridanum and S. botryosum may also be involved, but the symptoms are the same. These pathogens favor tomatoes, potatoes, peppers, and other nightshade family plants. Gray leaf spot can also occur on amaranth, broad beans, coriander, peas, sorghum, spinach, and wheat. When seen in the onion family, it is called leaf blight.

Gray leaf spot symptoms

Older leaves tend to show symptoms of gray leaf spot before new growth. It starts as small spots on both sides of the leaves. Yellow halos may also be visible. As these lesions grow and mature, they become dry and brittle, ultimately causing leaf drop. Some people say these dead areas look gray. To me, they’re brown. [If you have a hand lens, you may see gray fungal growth in the center of areas damaged by gray leaf spot.] Whatever you call it, these infections can lead to secondary infections, creating a domino effect of destruction in your garden.

Symptoms of gray leaf spot look a lot like Septoria leaf spot. The only difference is that the damaged areas of gray leaf spot do not have black specks in the middle. Septoria leaf spot infections do have those black specks.

Gray leaf spot of tomato (Clemson University, USDA Cooperative Extension, Bugwood.org) CC BY 3.0 US

Gray leaf spot lifecycle

The fungi responsible for gray leaf spot can travel through the air, in rainwater, and on infected seeds. In areas with high humidity, gray leaf spot can easily take hold on leaf, twig, and stem surfaces. Each infected plant part becomes a new point of infection. Infected leaves can spread the disease to neighboring plants just by touching them.

Gray leaf spot prevention

Preventing gray leaf spot is a lot easier than dealing with an infection. Start with resistant varieties and certified disease-free seeds and seedlings. Then, space your plants so that they are not touching their neighbors. The gap slows the spread of disease and improves airflow for faster drying times. Avoid overhead watering and water in the morning to give leaves time to dry during the day.

Once an infection begins, remove infected leaves to slow the spread of the disease. Just make sure to sanitize your garden tools between every cut. You can use bathroom cleaner. And it’s a good idea to give them (and your hands) a good soap and water washing, drying, and oiling after the ordeal. Throw infected leaves in the trash. Since this pathogen can overwinter in life and plant litter, it is a good idea to clean up at the end of each growing season and practice crop rotation.

Commercial growers use fungicides to prevent and manage gray leaf spot. You can help prevent this disease with fixed fixed copper sprays or Bt (Bacillus thuringiensis). These treatments may halt an infection if they are applied early enough. Otherwise, get rid of infected leaves, harvest what you can, and give your tomato plant an umbrella if it loses too many leaves to prevent sunburn