Grey leaf spot doesn’t exactly live up to its name. Rather than gray spots on leaves, this fungus causes small brown dead areas that coalesce into large dead areas, frequently seen with yellow.
Grey leaf spot is commonly caused by Stemphylium solani fungi. S. floridanum and S. botryosum may also be involved, but the symptoms are pretty much the same. These pathogens prefer members of the nightshade family, including tomatoes, potatoes, and peppers. Gray leaf spot can also occur on amaranth, broad beans, coriander, peas, sorghum, spinach, and wheat. When seen on members of the allium family, it is called leaf blight.
Grey leaf spot symptoms
Older leaves tend to show symptoms of gray leaf spot before new growth. My guess is it takes time for the fungi to grow enough to become a problem. It starts out 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 be able to 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, or other diseases, 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 spots in the middle, the way Septoria leaf spot infections do.
Grey 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 surfaces as well as on twigs and stems. Each infected plant part becomes a new point of infection. As infected leaves rub against their neighbors’ leaves, the infection can spread.
Grey leaf spot prevention
Gray leaf spot is one of those diseases you’d rather prevent than have to try to cure. Once an infection begins, you can remove infected leaves to slow the spread of the disease. Just make sure to sanitize your garden tools between each and every cut. You can use a bathroom cleaner to disinfect your tools. And it’s a good idea to give them (and your hands) a good soap and water washing, drying, and oiling after the ordeal. And those infected leaves should be tossed in the trash.
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 tomato plants out enough that they are not touching their neighbors. This will slow the spread of disease and improve airflow which will help leaves dry faster. Avoid overhead watering and water in the morning, which will give leaves time to dry during the day.
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 copper sprays or Bt (Bacillus thuringiensis). These treatments may be able to 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.
You can grow a surprising amount of food in your own yard. Ask me how!
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