Tomato Leaf Mold: Diagnosing the Greenhouse and High-Tunnel Disease

Telling leaf mold apart from blight

The two-sided symptom is the key. University of Minnesota and Cornell Extension describe pale greenish-yellow spots, usually less than a quarter inch and without sharp margins, forming on the upper leaf surface, with olive-green to grayish-brown velvety mold, the mass of spores, developing on the lower surface directly below each spot. The oldest, lowest leaves are infected first, and infected leaves wither and die, which reduces yield indirectly rather than rotting the fruit.

This is what separates it from the blights. Early blight makes concentric-ring target spots, and late blight makes greasy gray-green blotches with white sporulation, both patterns differ from the yellow-spot-plus-olive-underside-mold of leaf mold. Cornell also cautions that magnesium deficiency and the early stages of other diseases can mimic the top-surface yellowing, so it is the velvety mold on the underside that confirms leaf mold. When in doubt, compare against the early blight and late blight guides, or send a sample to Extension.

Why it thrives under cover

Leaf mold is a humidity disease. Cornell notes that relative humidity greater than 85 percent triggers germination of the spores, and Minnesota puts the optimum temperature in the low 70s Fahrenheit, both conditions that greenhouses, high tunnels, and hoophouses create and hold, especially overnight when the air is still and leaves stay wet. Field tomatoes in open, breezy air get it far less often.

The pathogen persists and travels efficiently in that environment. Cornell and Minnesota note it spreads by wind and water splash and can be carried on tools and by workers, and it survives on infected crop debris (and possibly soil) for at least six months to a year, with infected seed another route in. That combination, a humid enclosure, spores surviving on last year's debris, and easy splash and handling spread, is why leaf mold can establish quickly under cover and return season to season.

Managing tomato leaf mold

The first and most effective moves lower humidity and leaf wetness. Michigan State and Minnesota Extension advise increasing air circulation through venting, wider plant spacing, pruning and training, and removing weeds, and using drip irrigation, or if watering overhead, watering in the morning so foliage dries fast. In a greenhouse or tunnel, actively venting to drop nighttime humidity below the 85 percent threshold is often the single biggest lever.

Back that with sanitation and resistance. Remove and destroy diseased plants and crop debris, sterilize tools and equipment, and where the problem recurs choose resistant cultivars, which are widely available. Cornell and Michigan State list fungicide options, organic copper products and conventional materials such as mancozeb and chlorothalonil-class products, applied per label; treat them as a supplement to the environmental controls, not a substitute. Because the top-surface yellowing overlaps with nutrient issues and other diseases, confirm with your Extension diagnostic clinic if you are not sure before committing to a spray program.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• University of Minnesota Extension - Tomato Leaf Mold — Leaf mold is caused by Passalora fulva (previously Fulvia fulva or Cladosporium fulvum) and is typically only an issue on greenhouse and high-tunnel tomatoes; pale greenish-yellow spots under a quarter inch form on upper leaf surfaces with olive-green to brown velvety mold on the lower surface below, oldest leaves infected first; it is driven by relative humidity greater than 85 percent with optimal temperature in the low 70s; spores can survive six months to a year and the fungus overwinters on crop debris; management is to increase air circulation through ventilation, spacing, and pruning, use drip irrigation or water overhead in the morning, and remove debris.
• Cornell University - Tomato Leaf Mold (Vegetables) — The disease is caused by Passalora fulva (syn. Cladosporium fulvum); symptoms are light-green to yellow spots on leaves with olive-green spores on the underside, appearing first on lower leaves and leading to defoliation; relative humidity greater than 85 percent triggers germination of the spores, favoring greenhouses and high tunnels; the pathogen spreads by wind and water splash and survives on infected plant debris and possibly soil, with infected seed and tools spreading it; management is to remove diseased plants, dispose of infected material, sterilize equipment, and use resistant varieties, plus listed fungicides; magnesium deficiency can mimic symptoms.
• Michigan State University Extension - Tomato Leaf Mold in Hoophouse Tomatoes — Tomato leaf mold (Passalora fulva) produces yellow spots on the upper leaf surface with discrete masses of olive-green spores on the underside; infection begins on older foliage which dies prematurely; it thrives with extended leaf wetness and relative humidity greater than 85 percent, optimal in the low 70s Fahrenheit; management is to promote air circulation through venting, increase spacing and remove weeds, use drip irrigation to minimize leaf wetness, irrigate in the morning, select resistant varieties, and apply listed copper or conventional fungicides.

Keep going