Grey Fuzz on Flowers and Fruit

Botrytis is the disease of enclosed spaces — greenhouses, conservatories, propagators, and any windowsill with poor air movement.

What it looks like

A soft grey-brown mould with a distinctly fuzzy texture, on flowers, fruit, or the base of soft stems. Disturbed, it releases a visible puff of spores.

The tissue underneath collapses into a wet rot rather than drying out. That distinguishes it from most leaf-spot diseases, which leave dead but firm tissue behind.

The entry point

Botrytis is a weak pathogen. It rarely attacks healthy undamaged tissue, and it establishes on something already dead or dying — a faded petal, a bruised fruit, a pruning wound, a leaf killed by something else.

From that base it grows into adjacent healthy tissue, which is what makes it destructive.

The practical consequence is unusual among plant diseases: tidying is the treatment. Removing spent flowers and dead leaves takes away the entry points, and a plant with no dying tissue on it is largely safe.

Conditions

The temperature range is worth noting because it covers exactly the conditions of an unheated greenhouse in autumn, which is when the disease does most of its damage.

Where it hits hardest

Seedlings, where it merges with damping off — the same still, humid, crowded conditions produce both, as damping off in seedlings describes.

Soft fruit, particularly strawberries, where fruit touching wet soil or foliage rots before ripening.

Tomatoes under glass, entering through leaf scars left by pruning. Pruning in dry conditions in the morning, so the wound seals before evening, measurably reduces it.

Cuttings under cover, where the propagator's humidity is the point and also the risk.

Controlling it

Ventilate. More than feels necessary. Air movement is the single most effective control, and a small fan in a greenhouse does more than any spray.

Space plants. Crowding creates still humid air in the canopy regardless of what the greenhouse as a whole is doing.

Water the soil, in the morning. Wet foliage overnight is what the fungus needs.

Dead-head and clear debris continuously. Not as tidiness but as control.

Avoid damage. Bruises and tears are entry points, and rough handling during picking is a common source.

Fungicides exist and resistance is widespread, partly because the fungus reproduces quickly and has been sprayed heavily in commercial production. On a home scale the cultural measures do more.

In storage

Botrytis is why soft fruit goes mouldy in the fridge, and why one affected fruit takes the punnet with it. It continues growing at refrigerator temperatures, slowly.

Sorting fruit before storing, and removing any that is damaged, is worth the two minutes — the same principle as removing dying tissue on the plant, applied after picking.

Distinguishing it from the other moulds

White powdery growth on the upper leaf surface is mildew, not botrytis, and the distinction matters because the conditions differ — powdery mildew tolerates dry air where botrytis needs it humid. The comparison is in powdery and downy mildew.

Blue-green mould on fruit is usually Penicillium, a storage rot that arrives after picking rather than in the garden.

White fluffy growth on the soil surface is generally a harmless saprophyte living on organic matter in the mix, and it indicates a wet surface rather than a diseased plant.

The unusual exception

Botrytis on grapes late in the season, under the right alternating wet and dry conditions, concentrates sugars and is deliberately encouraged to make sweet wines.

That is a narrow and carefully managed case. Everywhere else in a garden it is a straightforward loss, and the conditions that produce the desirable form are not the ones present in a damp greenhouse.