Stemphylium Gray Leaf Spot: A “New” Tomato Disease for Maine

January 15, 2025

By Caleb P. Goossen, Ph.D., MOFGA’s Crop Specialist

I have been getting more and more photos sent to me from tomato growers showing symptoms of a foliar disease that never used to be present in Maine. Though the disease was only formally confirmed to be Stemphylium gray leaf spot in the 2024 growing season, I suspect that we have been seeing smaller outbreaks of it, with less progression of disease symptoms, for several years now.

Gray leaf spot can be caused by several Stemphylium species. Stemphylium solani and S. lycopersici are most common in North America, but S. botrysum also causes the disease. Gray leaf spot is typically considered to be most prevalent in humid tropical and subtropical regions. In fact, I was not even aware of the disease until several years ago when Andy Radin, the former research associate/ag extension agent for the University of Rhode Island, alerted a group of New England crop specialists that he was beginning to see the disease on farms in Connecticut and Rhode Island, with dramatic crop losses in some instances. Within a year or two of that warning, I started to receive photos from Maine farmers who were noticing symptoms similar to gray leaf spot.

The arrival of gray leaf spot is likely an aspect of the new climate reality that growers will continue to need to adapt to. Though it likely can’t be tested scientifically, it seems probable that recent summers’ prolonged bouts of higher heat and higher relative humidity — which can also cause elevated temperatures to persist overnight to a greater extent — have allowed this disease to expand the range in which it effectively infects tomato plants and spreads. While we’ve been having summers with these conditions more frequently, the disease can probably only spread so far in a given year, and we may now be seeing the result of overwintering populations having gradually established themselves further and further north over many seasons.

As gray leaf spot is a new concern for growers here in Maine, it is important to both recognize what the symptoms look like and to understand the disease organism’s lifecycle. In that way, we can best understand where we might be able to interfere with that lifecycle, using organic production methods. Because of the way gray leaf spot presents itself, I’ve started to think of it as “acting like early blight but infecting like botrytis.” One fortunate difference is that gray leaf spot does not affect tomato fruit the way that early blight or botrytis can. However, that’s likely to be cold comfort if your plant has lost all of its leaves.

gray leaf spot infection
Dead tomato leaves showing progression of gray leaf spot infection. Photo by Andy Radin

Stemphylium infections can start from nearby infected plants (tomatoes, peppers, and other solanaceous weeds, though I’ve only heard of it confirmed on tomatoes in Maine so far); can be seedborne; or, more likely in its gradual spread into Maine, can start from infected crop debris from prior years. Fungal spores produced on living or dead plant tissues spread via splashing rain or irrigation water, or go airborne and travel on the wind in favorable conditions. The disease survives and infects most successfully in relatively hot and wet weather conditions: requiring liquid water or simply high humidity to infect leaf surfaces, and developing most successfully at 77 degrees Fahrenheit. Though petioles and stems can become infected, the disease is mostly noticed on leaves. Fruit do not become infected.

Initial symptoms show up as small dark brown specks, which turn lighter — eventually turning tan to gray in their centers as they enlarge. The centers of these lesions will commonly crack as they begin to dry out. As the disease progresses, the entire leaf may turn gray, dry, and crispy. This defoliation is the primary damage of this disease, reducing the plant’s photosynthetic potential and also diminishing the affected leaves’ ability to shade fruit and protect it from sun scalding.

The leaf symptoms of Stemphylium gray leaf spot can be easily confused for bacterial leaf spot or Septoria leaf spot. The disease development and symptoms of Stemphylium seem to me, however, to also have many similarities with Alternaria linariae, the disease which causes “early blight” (not to be confused with the more devastating “late blight” caused by Phytophthora infestans and responsible for the great potato famine). Both gray leaf spot and early blight overwinter on infected crop debris in the soil, spread via splashing water or from windborne spores from active infections, and tend to progress up an infected plant from the lower leaves towards the upper leaves. The big difference is that early blight requires liquid water to successfully infect leaves, while gray leaf spot can do the same simply with high humidity. Because of this, my first interactions with this disease were photos sent to me from Maine farmers noticing symptoms that looked like early blight but showed up in covered production (i.e., greenhouses or high tunnels) the way that botrytis might — but early blight typically does not.

The best strategies for management of gray leaf spot are likely to be multi-faceted for many growers. As with many tomato foliar diseases, any growing practices that facilitate rapid drying and reduce trapped air around the plants will be helpful in limiting initial infection and subsequent spread (like greater plant spacing, and sucker, stem, and leaf pruning). When conditions are still favorable for the disease (i.e., prolonged high humidity or leaf wetness during warmer periods) it will likely still be able to infect susceptible plants, unless a preventative fungicide has been applied. Copper is likely to be the most effective organic control for home and commercial growers, though I suspect commercial growers may also see some control with Oso (polyoxin D zinc). Similarly, other common best practices for organic methods of disease management are also important. Because the disease overwinters on infected crop debris, it will be important to practice good sanitation and crop rotation. Remove or destroy infected tomato crop debris, or at least incorporate it into the soil to speed its decomposition before tomatoes are grown again in the same area.

Leaf spots caused by Stemphylium
Leaf spots caused by Stemphylium (first leaflet in image above) resemble those caused by Septoria (second leaflet). Note the difference in spot color and presence of spore tendrils (tiny bumps) in the spots caused by Septoria. Photo and caption by Meg McGrath

Luckily, resistance to gray leaf spot can be found in tomatoes that contain a single gene (Sm), which has been traditionally bred from a wild tomato relative into many modern varieties. This means that anyone willing to grow these varieties likely won’t see gray leaf spot infect their crop in the first place. I recommend growing one or more resistant varieties for at least an “insurance portion” of your overall tomato plants, especially if you have already struggled with gray leaf spot showing up in your growing area.

Cornell Cooperative Extension maintains a spreadsheet of crop varieties with disease resistances noted, available here: vegetables.cornell.edu/pest-management/disease-factsheets/disease-resistant-vegetable-varieties/disease-resistant-tomato-varieties. Additionally, I took a quick look at several commonly used seed companies and compiled the following list of currently available tomato varieties that have a resistance to gray leaf spot. This is likely not a complete list, and availabilities may change or improve for the coming growing seasons.

  • From Fedco: Rutgers 250 Schermerhorn, Sweet Treats.
  • From High Mowing Seeds: Enroza, Galahad, Mountain Vineyard.
  • From Johnny’s Selected Seeds: Amai, Big Beef, Big Beef Plus, Celebrity Plus, Enroza, Galahad, Grand Marshall, Hot Streak, Lemon Boy Plus, Mountain Merit, PinkID, RuBee Dawn.
  • From Seedway: Better Boy Plus, Big Beef Plus, Camaro, Celebrity Plus, Delicious, Jolene, Kuzco, Picus, Pink, Red Bounty, Roadster, Rubee Dawn, Ruby Crush, STM 2255, Sunfresh.

Please note: This information is for educational purposes. Any reference to commercial products and trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production, and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

This article originally appeared in the winter 2024-2025 issue of The Maine Organic Farmer & Gardener.

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