Leaf mold on tomatoes — symptoms and treatment

Severity: medium

What is Leaf mold

Leaf mold of tomato is caused by the fungus Passalora fulva. It is above all a disease of protected cropping: it primarily occurs in greenhouse environments, where high humidity sits around the plants and never quite lifts. Outdoors it is uncommon; under glass or plastic it can be the defining problem of the season. It works on the leaves rather than the fruit, and the harm is indirect — a plant whose foliage is yellowed and killed cannot ripen the crop it carries.

Symptoms

Leaf mold gives two pictures depending on which side of the leaf you look at. From above, the leaf shows pale green to yellow spots, diffuse and without a sharp edge. Turn the same leaf over and beneath each pale patch sits a velvety growth, olive-green to brown — the fungus itself, fruiting in the humid air trapped under the canopy. As patches merge the leaf yellows and dies. It usually begins on the older, lower foliage where the air is stillest, and moves upward.

• Early: pale green to yellow spots on the upper leaf surface, with vague margins.
• Progressing: olive-green to brown velvety growth on the underside, beneath the pale spots.
• Advanced: patches merge; leaves yellow, wither and die; defoliation works up the plant.

The velvety underside is the deciding feature: yellow blotching alone can be nutrient deficiency or ordinary ageing, but blotching with a felt of olive mould underneath is Leaf mold.

Causes and conditions

This fungus is driven by humidity more than anything else. Spores are produced in great numbers on the leaf undersides and move on air currents, on hands, clothing and tools; they also persist on crop debris and greenhouse structures between crops. To infect, they need humid, still air and leaf surfaces that stay damp — precisely the microclimate of a closed greenhouse: warm, moist, poorly ventilated, with condensation forming overnight. Which is why cure and prevention are the same thing: move the air and dry the leaves.

Treatment

Ventilation and spacing — cultural

Timing: throughout the growing season. Increase greenhouse ventilation — the primary control, not an afterthought, because the disease lives on trapped humidity. Space plants adequately so air can move between them. Remove the lower leaves. Avoid wetting the foliage.

Chlorothalonil — chemical

Timing: at first symptoms. Apply the fungicide at the first signs, and pair it with improved greenhouse ventilation to reduce humidity — spraying into a closed, humid house treats the symptom and leaves the cause running. Pre-harvest interval: 7 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Ventilate protected structures generously, especially in the evening, so humidity does not sit on the leaves overnight.
• Water at the base and never over the leaves; keep the foliage dry.
• Grow resistant varieties, worth seeking out for greenhouse cropping.
• Clear crop debris and clean the structure before replanting.

Frequently asked questions

Can I eat tomatoes from a plant with Leaf mold? Yes. The disease is on the foliage and the fruit is safe to eat, though it may ripen poorly if the plant has lost much of its canopy. Observe the 7-day pre-harvest interval if you have sprayed chlorothalonil.

Why has it appeared in my greenhouse but not on my outdoor plants? Because your greenhouse supplies the humid, still air the fungus needs and the open garden does not. Outdoors, wind and sun dry the leaves; under cover, moisture lingers.

Does it spread to other crops? It is a tomato disease and will not move into unrelated crops. But it moves very efficiently from tomato to tomato — including on hands and clothing, so work through clean plants first.