Why is there grey fuzzy mould on my plant?

Grey mould is the commonest disease under glass and on winter houseplants, and almost every outbreak traces back to two things: dead tissue left lying, and air that is not moving. It is a saprophyte first and a pathogen second, which is why hygiene is a far more effective control than anything you could apply.

Step by step

1. Confirm what you are looking at.A soft grey-brown rot with a fuzzy, felty grey coating that puffs a cloud of dust when disturbed. The tissue beneath is soft and collapsing rather than dry. On fruit it starts as a soft brown patch that greys over.
2. Find where it started.Trace it back to a dead flower, a bruise, a broken stem, a stub left from a bad pruning cut, a fallen leaf lying on the compost surface, or a piece of fruit touching another. That entry point is the thing to remove, and it explains the outbreak.
3. Cut back well into clean tissue.Not to the edge of the visible rot — several centimetres beyond it, into firm healthy stem, cutting at a bud or a joint. Botrytis is already in tissue that still looks normal. A cut at the margin leaves the infection behind.
4. Bag the material as you cut.Disturbing an infected stem releases an enormous cloud of spores. Hold a bag under the cut, put everything straight in, and take it out of the greenhouse or room rather than dropping it on the floor.
5. Remove every piece of dead material.Fallen leaves on the compost surface, faded flowers still attached, dead lower leaves on tomatoes, spent bedding, and any prunings on the greenhouse floor. This is the single most effective thing you can do and it is free.
6. Get the air moving.Ventilate a greenhouse or conservatory even in cold weather — a little air movement matters more than warmth for this. Space plants so leaves are not touching, thin congested growth, and do not crowd seed trays.
7. Change how you water.Water at the base, in the morning, so that foliage and the compost surface dry before evening. Avoid splashing the crowns of plants, and avoid standing pots in saucers of water in cool weather.
8. Take extra care with the classic cases.Strawberries fruiting on damp soil, tomatoes with a botrytis lesion at a leaf scar on the main stem, cyclamen and African violets with water sitting in the crown, lettuce under cloches, and overwintering pelargoniums and fuchsias in a cold greenhouse. Each one is solved by ventilation and by removing dead tissue rather than by treatment.
9. Judge whether the plant is worth keeping.A botrytis lesion that has girdled the main stem of a tomato or a pelargonium usually finishes that plant, because everything above it is cut off. Remove it before it becomes the spore source for the rest of the greenhouse.

Tips

• Straw or mats under strawberries keep the fruit off wet soil and are the classic answer to grey mould on the crop.
• Deadheading is a disease control measure as much as a cosmetic one — a rotting flower on a plant is where botrytis begins.
• Do not water overwintering pelargoniums and fuchsias much at all. Cool and dry is what they need, and cool and damp is what kills them.
• A greenhouse fan running gently through winter prevents more disease than any product on the shelf.

Common mistakes

• Cutting back only to the edge of the visible rot — The fungus is already in tissue that still looks healthy, and the infection continues from the cut.
• Shaking or brushing the mould off — It releases millions of spores over everything nearby. Bag it in place instead.
• Closing the greenhouse up to keep plants warm in winter — Still damp air is the condition botrytis requires. Ventilation matters more than a few degrees.
• Leaving dead leaves lying on the compost surface of pots — That is where it establishes before moving onto the plant.

Questions people ask