Why Is Grey Mould Spreading Over My Bean Plants?

A grey, dusty, fuzzy mould developing on bean stems, leaves, and pods — spreading quickly in cool, humid conditions — is Botrytis cinerea, the grey mould fungus. It is one of the most common opportunistic pathogens on vegetables, attacking damaged, dying, or senescent tissue first and then progressing into healthy growth when conditions favour it. On beans, it most commonly establishes on spent flower petals that lodge between developing pods, on stems damaged by slugs or mechanical injury, and on the lower foliage in dense, poorly ventilated plantings.

How Botrytis establishes

Botrytis spores are present everywhere in garden air. They germinate and establish on tissue that is already weakened, wet, or dead — spent flowers, damaged stems, yellowing lower leaves — and then produce massive quantities of grey spores that spread by air movement to nearby healthy tissue. On a runner bean plant, the dense foliage and many flowers create a humid microclimate inside the canopy where Botrytis thrives in wet summers. Once established, it can kill entire sections of stem, cutting off pods and leaves above the point of infection.

Removing affected material

Cut out mouldy tissue immediately and bin it — do not compost it as this creates a Botrytis spore reservoir near the plot. Remove spent flower petals by running your fingers along the developing pods if you notice them accumulating — this removes the primary entry point. Prune out yellowing lower leaves before they collapse and form damp zones at the base of the plant. Work in dry weather if possible; handling infected material in wet conditions distributes spores widely.

Are Botrytis-affected pods edible?

Pods with grey mould on their outer surface should be trimmed of any visibly mouldy tissue, washed thoroughly, and the surface inspected. If the mould has penetrated the pod wall and the beans inside are grey or discoloured, discard that pod. Beans from neighbouring healthy pods on the same plant are perfectly edible. Harvest promptly once grey mould is visible on any part of the plant.