Why Are My Grapes Rotting with Grey Mould?

Grey mould caused by Botrytis cinerea is the most common and most destructive rot of ripe and ripening grapes. The fungus is ubiquitous in the environment and opportunistic — it enters through any wound, crack, or dead tissue and spreads rapidly from berry to berry within a tight bunch. A bunch that looks perfect one week can be consumed by grey mould the next if conditions turn wet and cool. Understanding how to prevent entry points and manage conditions is the key to avoiding it.

Recognising botrytis

Affected berries initially appear water-soaked and soft, then develop a covering of grey-brown dusty spore masses (the characteristic botrytis dust). The affected berry shrivels or collapses. In wet conditions, the infection spreads through the bunch by contact — berries touching an infected neighbour become infected themselves. In some seasons where botrytis affects very ripe fruit in moderate amounts, it produces noble rot (Botrytis cinerea in its desirable form), which wine makers use for sweet wines. For table grapes, any botrytis is unwelcome.

How botrytis enters the bunch

Dead flower caps that remain trapped between berries as the cluster tightens provide the primary entry point. This is why bunch hygiene at flowering — removing trapped debris — matters. Cracked skins from water irregularity, insect damage from wasps, powdery mildew scars, and physical abrasion against a wire or wall all create additional entry points. Botrytis cannot easily penetrate intact, healthy berry skin.

Canopy management to reduce humidity

Botrytis thrives in warm, humid, still conditions within a dense canopy. Leaf removal around the bunch zone — removing leaves that directly shade or physically touch the bunches — reduces humidity and allows air movement that dries the fruit after rain. This single practice, done in July, has more impact on botrytis incidence than fungicide spraying in most home garden situations.

Bunch thinning in tight-clustered varieties

Compact varieties with tightly packed berries (such as Muscat varieties) are far more susceptible to botrytis because there is no air movement between berries and wounds spread infection rapidly through direct contact. Thinning the tips of the bunch shoulders at fruit set creates space between berries, allowing air to circulate and skin damage to dry rather than staying moist and infected.

Fungicide treatment

Preventative sprays of a botryticide (fenhexamid, pyrimethanil, or cyprodinil are the main active ingredients) applied at four key timings — at flowering, just after fruit set, at bunch closure, and three to four weeks before harvest — provide the best chemical protection. Rotate between different modes of action to delay resistance. Organic growers can use Bacillus subtilis-based biocontrol products (such as Serenade) which have moderate effectiveness if applied preventatively.