Sclerotinia

What you are looking at

A plant that has collapsed from one point on its stem. Above that point everything has wilted; below it the plant looks untouched. The stem itself, at the place it failed, has gone pale, soft and water-soaked, as though it had been soaked and then wrung out.

Over that patch, in damp weather, grows white mould. Not a dusting or a bloom but a dense mass like wet cotton wool, thick enough to pull off in a piece.

Split the stem open. Inside the hollow are hard black lumps, irregular, five to ten millimetres long, which people almost always describe as mouse droppings. Those are the sclerotia, and they are what makes this disease a long-term problem rather than a bad season.

On lettuce the plant rots into a slimy heap at the base while the outer leaves are still standing. On stored carrots it is a white fluffy mat spreading between roots in the box.

The black lumps, and what they mean

A sclerotium is a survival package. Dense, dark-walled, packed with food reserves, and capable of sitting in soil for five to eight years, sometimes longer, waiting.

When conditions are right, in a cool damp spell during the growing season, a sclerotium near the soil surface germinates. What comes up looks like a tiny mushroom rather than a thread of mould, a buff-coloured cup on a stalk a few millimetres across, standing just above the ground. Those cups fire spores into the air, and the spores land on the crop.

Two things follow. Any infected plant left standing is manufacturing more of these packages, which is why removal has to be immediate rather than at the weekend. And any infested soil stays infested for years, whatever is done to it, which is why this page carries the flag saying there is no cure.

Why dead petals matter

An airborne spore landing on a healthy green leaf usually fails. The fungus is a poor invader of living tissue and needs something dead to establish on first.

Spent flower petals are what it uses. A petal falls off, lodges in a leaf joint or against a stem, and stays damp. The spore germinates on the dead petal, builds up a colony there, and then moves from the petal into the living stem underneath.

That explains almost everything about how the disease behaves. Beans get it shortly after flowering. Dahlias and chrysanthemums get it where the old blooms have been left to rot in the foliage. Rapeseed gets it in wet weather at petal fall, which is when growers spray for it.

For a gardener the useful consequence is that deadheading and regular picking are disease control rather than tidiness, and that a wet fortnight during flowering is the moment to pay attention.

What to do about it

Pull out any affected plant on the day you find it, bagged, with the soil around its base.

Bin or burn it. Nothing goes on the compost heap.

Deadhead dahlias and chrysanthemums and pick beans regularly so that spent petals do not sit in the foliage.

Space, support and thin the crop so that air moves through it, and take the lower leaves off beans and dahlias.

Water the soil in the morning rather than the foliage in the evening.

Keep the ground weed-free, since chickweed, sow thistle and dandelion carry it too.

Rotate onto sweetcorn, onions, cereals or grass, which it does not attack, and give affected ground five years or more.

Sort carrots carefully into store and keep them cold and dry.

What it gets mistaken for

Grey mould is dusty grey-brown and releases a visible cloud of spores when disturbed, where sclerotinia stays a dense white mat.

Onion white rot looks very similar and takes only onions, garlic and leeks, working up from the roots, and its black bodies are the size of pinheads rather than lumps.

Blight blackens the foliage from the top down in a wet July and moves through a crop in days.

Root rot rots from below in waterlogged ground and leaves no black bodies.

Damping off flattens seedlings within days of germination.

Bacterial soft rot turns tissue to a foul-smelling slime with no mould over it and nothing hard inside.