Sclerotinia

Sclerotinia sclerotiorum

also called white mold, sclerotinia stem rot, cottony rot

A fluffy white cotton-wool mold on collapsing stems, with hard black pellets, like rat droppings, forming in and on the rotted tissue.

Fungus No cure Soil-borneAirborne sporesSpread by contact Spreads to neighbors

What you'll see

A dense, fluffy white mold, like cotton wool, on stems, crowns and fruit.
Hard black resting bodies, the size of small seeds or droppings, form within the white growth and inside hollow stems.
Stems develop a soft, watery, bleached rot and the plant above wilts and collapses.
Affected tissue is slimy at first, then dries pale and papery.
Whole plants keel over once the stem base is girdled.

Why it happens

A soil fungus with a very wide host range. It survives for years as hard black sclerotia in the soil; these germinate to release spores, or grow directly into the plant, rotting the stem and crown. Dying flowers and leaf debris caught in the plant often give the spores their first foothold.

Cool, damp, humid conditions favor it, especially in dense plantings where air moves poorly and the crown stays wet. Sheltered, over-watered beds and lush growth are most at risk.

Treating it

Dig up and destroy affected plants at once, taking as much surrounding soil as you can with them.
Never compost infected material, as the sclerotia survive to reinfect.
Improve airflow by thinning and spacing, and cut back on watering.
Clear away dead flowers and leaves that collect in the crowns of plants.
There is no cure once a plant is rotting, so removal is the only reliable response.

Keeping it away

Rotate susceptible crops widely, as the fungus attacks a great many plants.
Space plants for good air movement and avoid dense, sappy growth.
Water at the base and keep crowns and stems as dry as possible.
Remove crop debris promptly and don’t leave old stems standing.
Deep-dig badly infested beds so buried sclerotia are less likely to reach the surface.

Who gets it

A huge range of plants, including beans, lettuce, carrots, brassicas, sunflowers and many herbaceous ornamentals.