Botrytis in South African Vineyards

Botrytis cinerea, commonly known as grey mould, is one of the most economically damaging diseases of grapevines worldwide. In South Africa, it is the primary cause of pre-harvest fruit loss in cool, humid growing regions and can reduce both yield and wine quality dramatically when conditions favour infection.

Botrytis has an unusual dual identity. On certain white grape varieties under specific conditions, it produces "noble rot" (pourriture noble) — the controlled dehydration that concentrates sugars and creates prized dessert wines. However, in the vast majority of SA viticulture contexts — particularly for table wine reds and whites — Botrytis is purely destructive. Infected clusters develop a grey, fuzzy sporulating mass that renders fruit unmarketable and taints wine with musty off-flavours. Losses in susceptible blocks can exceed 50% of harvestable fruit.

Favourable Conditions

Botrytis thrives in cool, wet conditions — especially when free moisture persists on berry surfaces for extended periods. Rain events, heavy dew, and fog during the ripening window create the greatest risk. Tight-clustered cultivars that trap moisture between berries are particularly vulnerable, as water retention inside the bunch creates a prolonged wet microenvironment.

Unlike powdery mildew, which can develop without rain, and downy mildew, which requires rain for initial infection, Botrytis needs sustained surface wetness on the fruit itself. This makes the véraison-to-harvest window — when berries are softening and sugar content rises — the critical risk period.

Disease Lifecycle

B. cinerea is a necrotrophic pathogen — it kills host tissue and feeds on the dead cells. It overwinters as sclerotia (hard, dark resting structures) and as mycelium on dead vine tissue, pruning debris, and mummified berries left on the vine or vineyard floor.

In spring, the fungus produces conidia (asexual spores) that infect flower caps, stamens, and other floral debris. These infected remnants become trapped inside developing grape clusters — particularly in tight-bunched varieties — establishing a latent infection that shows no visible symptoms for weeks or even months.

The latent phase is what makes Botrytis so insidious. The fungus remains dormant inside the berry or between berries until conditions shift in its favour: rising sugar levels after véraison weaken berry defences, skin micro-cracks from expansion or insect damage provide entry points, and prolonged wetness triggers explosive sporulation. Once active, the grey mould can spread rapidly through an entire cluster within days, and wind-dispersed conidia carry the infection to neighbouring bunches.

South African Context

Botrytis risk in South Africa is concentrated in cooler, humid coastal regions. Walker Bay and the Hemel-en-Aarde valley face the highest pressure due to maritime moisture, frequent fog, and cool temperatures during the ripening window. Parts of coastal Stellenbosch, Constantia, and Elgin also experience significant Botrytis seasons, particularly in years with late-summer rainfall.

Cultivar selection is a major risk factor. Sauvignon Blanc and Pinot Noir — both grown extensively in SA's cool-climate regions — are highly susceptible due to their tight cluster architecture that traps moisture. Chardonnay and Semillon also carry elevated risk. By contrast, loosely-clustered varieties like Syrah and Cabernet Sauvignon are less prone to bunch rot.

Canopy management is the first line of defence. Leaf removal in the bunch zone — especially on the morning-sun (eastern) side — improves air circulation, accelerates drying after rain or dew, and exposes clusters to UV light that suppresses fungal growth. In high-risk blocks, shoot thinning and selective bunch thinning to reduce cluster compactness are standard practice.

Chemical control centres on preventative applications at key phenological stages: early bloom, bunch closure, and pre-harvest. SA producers typically use a rotation of fludioxonil, cyprodinil, and pyrimethanil, with strict resistance management protocols. The pre-harvest withholding period is a critical constraint — growers must balance the need for late-season protection against the requirement to have clean fruit at harvest.