Grey mould

What you are looking at

A dense coat of grey-brown fuzz, longer and looser than the flat film of powdery mildew, sitting on a fruit, a flower, a stem or the base of a seedling. Underneath it the tissue is soft and brown.

Knock it, or breathe on it, and a cloud of fine dust rises. Those are the spores, and there are millions of them. That puff is the identifying test.

In old dead stems you may find hard black lumps a few millimetres across, like grains of coal embedded in the tissue. Those are sclerotia, the resting bodies, and they survive in debris and soil for years waiting for conditions to return.

It needs a wound, or something already dying

Grey mould is a necrotroph. It kills tissue and eats what it has killed, and it is poor at attacking healthy living cells directly. What it does instead is establish on something already dead and use that as a base.

A faded petal fallen onto a leaf. A yellowing lower leaf on a tomato. A pruning snag dying back. Frost-damaged tissue. A slug bite. A bruise on a fruit. A dead flower still attached to a young strawberry. From any of those it builds enough mass to push into living tissue next door, and by then it is unstoppable on that shoot.

Everything on the control list follows from this. Removing dead material is the control measure. Handling plants gently is the control measure. Cutting to a bud rather than leaving a stub is the control measure.

Why it spreads in winter greenhouses

Grey mould spreads in humidity near saturation, still air, and cool rather than warm conditions. It works well between about 15 and 20 degrees C and carries on slowly close to freezing.

A British greenhouse in October has all of that. The heating comes on, the vents stay shut against the cold, the air holds moisture it cannot lose, and every surface goes damp overnight. That is why an overwintering fuchsia or pelargonium collection can be ruined between November and February while the same plants were untouched all summer.

Heat and ventilation have to go together. Warming a closed greenhouse raises the amount of moisture the air can carry and then traps it, which makes matters worse rather than better.

Noble rot on grapes

The same fungus on ripe grapes, under dry autumn conditions that let the fruit shrivel rather than rot, is noble rot. It concentrates the sugars and produces Sauternes, Tokaji and the German Trockenbeerenauslese wines.

The difference between the great sweet wines of Europe and a punnet of ruined raspberries is humidity and timing.

What to do about it

Open the vents. In a greenhouse, a conservatory or a propagator, moving air is the intervention, and it is free. Ventilate on every dry day through the winter, and if you heat the space, ventilate it as well.

Go over your plants weekly and take off everything dead or dying. Faded flowers, old leaves, fallen petals, spent fruit. On cyclamen, begonias and primulas, pull dead flower stalks out from the base rather than snapping them halfway.

When you find an infection, cut it out well past the visible edge, on a dry day, without shaking the plant. Bag what you remove. Compost heaps in a British garden do not reliably kill this.

Pick fruit often and sort it as you go. One mouldy strawberry in a punnet takes the rest within two days.

Clear the greenhouse right out in autumn, wash it down, and do not leave last year’s stems under the staging.

Why there are no products listed here

The biological tier is empty because nothing available to a gardener can be introduced to control a fungus growing through dead plant tissue.

The chemical tier is empty because grey mould develops fungicide resistance readily, and commercial crops can carry resistance to several chemical groups at once. Amateur approvals in Great Britain have also narrowed to a handful of protectant products that cannot reach an infection already inside a stem. Air, hygiene and spacing are the available tools for gardeners, and they work.

What it gets mistaken for

Powdery mildew is a flat white film across a leaf surface that wipes away in a clear track. Grey mould is fuzzy, three-dimensional and grey-brown, and it grows on dying tissue rather than on a healthy leaf blade.

Downy mildew produces a fine greyish or purplish growth confined to the underside of a leaf, with angular yellow patches above bounded by the veins. Grey mould is not confined to the underside and does not respect veins.

Damping off kills seedlings at soil level with a dark pinched stem and often no visible growth at all, and it is caused by different organisms in the compost. Where a collapsed seedling carries a grey fuzzy coat, this is the one.

White fluffy growth on the surface of compost is usually a harmless saprophytic fungus feeding on the compost itself. It is white rather than grey, it sits on the compost rather than the plant, and it does no damage.

Sclerotinia produces a white cottony growth with much larger black sclerotia inside hollow stems, and it collapses whole plants rather than working in from dead flowers.