Tomato leaf mold moves fast once conditions turn favorable. A grower can see pale spots on Monday and a fully mold covered leaf underside by the weekend.
Greenhouses and high tunnels see this disease far more than open field plantings do. That is not a coincidence.
This guide covers what the fungus actually is, the specific humidity threshold that triggers it, and the controls with real research behind them.
The short version
Tomato leaf mold comes from a fungus, Fulvia fulva, previously known as Cladosporium fulvum. It only infects tomatoes, unlike many other foliar diseases.
Relative humidity above 85 percent triggers spore germination. That threshold is the single biggest factor separating an outbreak from a quiet season.
Pale spots appear on top of leaves first, with velvety tan mold developing underneath. Airflow, humidity control, and prompt removal of infected material make up the real defense.
What tomato leaf mold actually is
University of Massachusetts Extension identifies the pathogen as Fulvia fulva. The fungus was formerly classified as Cladosporium fulvum, and some older references still use that name.
This disease is host specific to tomatoes alone. That sets it apart from Botrytis cinerea, gray mold, which attacks a far wider range of plant species.
Light green or pale yellow spots appear on the leaf’s upper surface within about a week of infection. University of Connecticut Extension describes the underside symptom that gives the disease its name.
Velvety, tan colored mold develops on the underside directly beneath those pale spots. That patch darkens as the infection matures.
The humidity threshold that drives outbreaks
University of Massachusetts Extension pinpoints the exact trigger. Relative humidity above 85 percent activates spore germination on the leaf surface.
That threshold explains why greenhouses and high tunnels see this disease so much more than open field tomatoes. Enclosed structures trap humidity in a way open air rarely allows.
University of Connecticut Extension adds a specific, practical detail growers often miss. Outside rows near roll up sides left open overnight see disproportionate infection.
Night air cools faster than day air, and moisture condenses on leaves once temperatures drop. Leaving those sides open invites exactly the wet leaf conditions the fungus needs.
How the disease spreads
Wind carries spores between plants once an infection is established. Water splash does the same job at closer range.
UMass Extension lists several other routes too. Infected debris, contaminated soil, infected seed, tools, and worker movement between plants all move the fungus around a growing space.
Symptoms start low on the plant and climb upward as spores spread. Left unchecked, advanced infection causes leaf curling, wilting, and premature leaf drop.
Severe cases go beyond the foliage entirely. UConn Extension notes infected plants can develop stem end rot on fruit, along with lopsided, unevenly shaped tomatoes.
Cultural controls that actually work
Temperature and humidity management sit at the center of real prevention. UConn Extension recommends keeping greenhouse temperatures between 60 and 65 degrees Fahrenheit where practical.
Applying heat or ventilation at dusk helps prevent the dew formation that follows a temperature drop. Keeping relative humidity below 90 percent denies the fungus the moisture it needs to germinate.
Avoiding overhead irrigation matters here just as it does with other foliar tomato diseases. Wetting leaves directly recreates the exact conditions this fungus favors.
Spacing plants properly and pruning for airflow reduce humidity right around the foliage. UMass Extension adds that removing symptomatic plants promptly, disposing of infected material, and disinfecting tools and structures after harvest all cut down on carryover into the next season.
Sourcing clean seed from a reputable supplier closes one more entry point. Crop rotation and relocating high tunnel structures when possible help as well, since the fungus can persist in soil and debris between seasons.
Resistant varieties worth knowing
UMass Extension lists real options across different growth habits. Determinate varieties with resistance include Red Mountain, Primo Red, Mountain Spring, and Florida 97.
Indeterminate resistant options include Geronimo, Trust, and Boa. Some heirloom varieties carry documented resistance too, including Cherokee Purple, Amish Paste, and Prudens Purple.
Choosing a resistant variety does not guarantee a mold free season on its own. It does meaningfully lower the odds compared with a fully susceptible variety grown in the same humid conditions.
Fungicides with real registrations
UMass Extension lists both organic and conventional options with current labels. Among organic choices, a copper hydroxide based fungicide offers one option, and a rhamnolipid biosurfactant based biofungicide offers another.
Conventional growers have additional choices. A combination fungicide containing azoxystrobin and difenoconazole is one labeled option, and a chlorothalonil based fungicide is another.
Checking a current regional vegetable management guide for what remains labeled in your specific area is worth the extra step before buying any product. Registrations and restrictions change over time and vary by state.
Frequently asked questions
What causes tomato leaf mold?
A fungus called Fulvia fulva, formerly known as Cladosporium fulvum. It infects only tomatoes and requires relative humidity above 85 percent to germinate on the leaf surface.
What does tomato leaf mold look like?
Pale green or yellow spots appear on the upper leaf surface first. Velvety, tan colored mold develops on the underside directly beneath those spots, darkening as the infection progresses.
Why does tomato leaf mold mainly affect greenhouses?
Enclosed structures like greenhouses and high tunnels trap humidity far more than open field plantings do. That trapped moisture crosses the 85 percent relative humidity threshold the fungus needs to spread.
How do I stop tomato leaf mold from spreading?
Lower humidity below 90 percent, avoid overhead irrigation, and improve airflow through spacing and pruning. Remove and dispose of infected material promptly, and disinfect tools and structures between seasons.
Are there tomato varieties resistant to leaf mold?
Yes. Options include Red Mountain, Primo Red, Mountain Spring, and Florida 97 among determinate types, plus Geronimo, Trust, and Boa among indeterminate types. Some heirlooms, including Cherokee Purple, also carry documented resistance.
The bottom line: tomato leaf mold spreads once relative humidity crosses 85 percent, which is exactly why greenhouses and high tunnels see it so much more than open field plantings. Pale spots on top and velvety tan mold underneath confirm the diagnosis. Humidity control, airflow, prompt debris removal, and resistant varieties offer real, combined defense. For the full disease identification picture, see our tomato diseases guide. For the closely related fungus that shares similar humidity triggers, see our powdery mildew on tomatoes guide.
SOURCES
University of Massachusetts Amherst Extension, Center for Agriculture, Food, and the Environment: Greenhouse Tomato, Leaf Mold (Fulvia fulva). https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/photos/greenhouse-tomato-leaf-mold-fulvia-fulva
University of Massachusetts Amherst Extension, Center for Agriculture, Food, and the Environment: Tomato, Leaf Mold. https://www.umass.edu/agriculture-food-environment/vegetable/fact-sheets/tomato-leaf-mold
University of Connecticut Extension, Integrated Pest Management: Greenhouse Tomato Foliar Diseases. https://ipm.cahnr.uconn.edu/wp-content/uploads/sites/3216/2022/12/2017greenhousetomatofoliardiseases.pdf
United States Environmental Protection Agency: Zonix Biofungicide Product Label. https://www3.epa.gov/pesticides/chem_search/ppls/072431-00001-20150521.pdf