Tomato leaf mold: yellow patches on top, olive fuzz underneath

If the top of a tomato leaf has a yellow blotch and the underside of that same spot carries an olive-green, velvety mold, it is leaf mold. The cause is a fungus, Passalora fulva (also called Fulvia fulva), and it thrives where the air is humid and still. The fix is mostly about air, not sprays.

This is the tomato disease of closed spaces. For outdoor tomatoes with brown spots, rings or halos, brown spots on tomato leaves is the better page. If you grow on an enclosed balcony, a lean-to or a windowsill with a crowded plant, read on.

Turn the leaf over

NC State Extension describes leaf mold as starting with pale-green or yellowish spots on the upper leaf surface, which later turn yellow. On the underside, an olive-green mold sits under the spots, denser and deeper in colour in the centre. Purdue Extension calls the top lesions bright yellow and blotchy and the underside growth an olive-green fuzz.

Two more details help:

• Older leaves go first. NC State says older leaves are affected, then younger ones. A plant with healthy new growth and spotted bottom leaves fits.
• It gets worse as spots join. NC State says spots can coalesce and kill the foliage when infection is severe, and leaves may curl, wither and drop.

On fruit, the two extension sources differ. Purdue says lesions do not appear on stems or fruit. NC State says leaf mold may spread in some cases to petioles, stems, blossoms and fruit, and that green and mature fruit may develop a black, leathery rot at the stem end.

Purdue also notes the disease is usually not serious, depending on how soon in the season it arrives. A plant hit late, with fruit already set, often carries on.

Telling it from the look-alikes

NC State warns that leaf mold may be confused with Botrytis gray mold, late blight and powdery mildew. The table above separates the first three; the notes below add the rest.

• Powdery mildew. NC State describes one type as light-green to bright yellow spots on top with powdery growth underneath, and another as white, powdery growth on the upper surface. White powder on top is not leaf mold. For mildew on herbs in the same space, see powdery mildew on container herbs, and for the grey-brown fuzz of gray mold on dead tissue, gray mold on herbs.
• Late blight. It is a separate disease with its own page: late blight on tomatoes. If the damage is spreading fast on leaves and stems with no olive fuzz underneath, go there first.
• Plain yellowing. If you turn the leaf over and find nothing, the yellowing is probably something else: tomato leaves turning yellow covers the pattern.

Why a glazed balcony or indoor spot invites it

NC State says high tunnels and greenhouses are protected from heavy rain, but that they are prone to high humidity and tend to be wind free, which encourages problems uncommon in field production, leaf mold among them. A glazed balcony behaves the same way. The fungus, NC State says, needs excess water on the foliage or high humidity above 85%, and can cause disease at any temperature from 40 to 94 °F, with development optimal at 72-75 °F. Purdue gives the same 72-75 °F optimum.

A closed balcony on a still, sunny afternoon, or a crowded indoor plant overnight, can sit in exactly that warm, humid range. Spores spread easily through the air, Purdue says, and NC State adds that rain and wind move them, as do tools and clothing. If you grow in a glazed space, growing on a glazed balcony covers venting the space itself.

What to do

The extension advice for leaf mold is mostly about air and sanitation. Purdue says the options are measures to reduce humidity, increase airflow and improve sanitation.

1. Remove the worst leaves. NC State says to remove infected seedlings and diseased tissue. Bag them. Do not compost them: the fungus survives on crop residue.
2. Thin the lower plant. NC State recommends removing lower leaves as the plant ages to improve air movement, and wider plant spacing and better ventilation. In pots that means pulling pots apart, not clustering them against a wall.
3. Move the air. NC State suggests fans that replace the air several times a day, especially in rainy or cloudy periods.
4. Keep leaves dry. NC State says to avoid overhead irrigation and water at the base. It also says to avoid handling wet plants, since that moves spores on hands and tools.
5. Cut condensation. NC State advises managing temperatures to reduce condensation. On a glazed balcony, that means venting before the evening drop in temperature rather than closing everything up warm and wet.
6. Wash what touched the plant. NC State says trays, tools, stakes, twine and structures should be washed and sanitized between crops.

None of this needs a spray. If you are considering one on a plant you will eat, the label decides what is allowed, as set out in choosing a pest treatment for edible plants.

Resistant varieties help, up to a point

Purdue calls resistant varieties an excellent management strategy, with a warning: some resistant varieties are not listed as such, and variability in the fungus population may overcome the plant’s resistance. NC State puts numbers on it: there are currently 12 races of the fungus, and each resistant variety protects against only one to a few. NC State names Trust, Geronimo and Starbuck as leaf mold resistant varieties for use as part of an integrated programme.

Before the next crop

NC State says the fungus survives for at least a year on crop residue and in the soil. Purdue says it survives season to season in crop residue. At the end of the season:

• Pull every leaf, including those under the pot.
• Wash the pots and any trellis or cage.
• Replace or refresh the potting mix rather than reusing it for another tomato.

For what is safe to keep, see reusing potting mix.

Sources

• NC State Extension, Foliar fungal diseases on high tunnel and greenhouse tomatoes
• Purdue Extension and University of Kentucky, Tomato disease management in greenhouses (BP-197-W)