Grey rot

Grey rot is the destructive form of botrytis cinerea infection in vineyards, in contrast to the beneficial phenomenon of noble rot. It is one of the most widespread and damaging fungal diseases in viticulture, reducing both grape yield and wine quality when conditions favour its development. While noble rot can produce celebrated sweet wines, grey rot is regarded as a major wine fault that growers seek to prevent.

The pathogen

The causal agent of grey rot, botrytis cinerea, is a necrotrophic fungus that infects a wide range of plant species, including grapevines. Its spores are ubiquitous in vineyard environments, persisting in plant debris and spreading through wind, rain splash, and insects. Initial infection often occurs through natural openings in the berry skin or wounds caused by mechanical damage, insects, or other fungal diseases. In the absence of conducive weather, the fungus may remain latent, but high humidity and moderate temperatures activate its rapid spread.

Symptoms and impact on vines

Grey rot typically appears as a greyish-brown mould on infected grape clusters, accompanied by berry softening and collapse. Dense bunches are especially vulnerable as spores spread quickly between adjacent berries. Secondary bacterial and fungal infections often follow, further degrading grape quality. Severe outbreaks can destroy entire crops, particularly in compact white varieties such as Chardonnay or Semillon. Even low levels of infection reduce marketability, since visibly mouldy fruit cannot be vinified for quality wine production.

Effects on wine

In contrast to the concentration and complexity produced by noble rot, grey rot leads to dilution of grape sugars, increased volatile acidity, and the development of earthy or mouldy off-aromas. Wines made from affected grapes may display browning, premature oxidation, and instability during storage. Grey rot also increases the risk of spoilage organisms proliferating during fermentation, further diminishing wine quality. Its presence is therefore considered highly detrimental in both still and sparkling wine production.

Environmental factors

The expression of grey rot depends strongly on weather and vineyard conditions. Prolonged rainfall, morning fogs, and high humidity near harvest create an environment conducive to uncontrolled fungal growth. Canopies with excessive shading or poor airflow exacerbate infection, as do thin-skinned varieties and tightly packed bunches. Conversely, warm and dry weather following initial infection can transform the disease into noble rot. Thus, a subtle balance of climatic factors determines whether Botrytis is beneficial or destructive.

Control and prevention

Viticultural practices are central to limiting grey rot. Effective canopy management improves airflow and sunlight penetration, reducing humidity around grape bunches. The use of less compact varieties or appropriate rootstocks can reduce susceptibility. Fungicides may be applied at sensitive growth stages, although over-reliance can lead to resistance and is discouraged in integrated management systems. Alternative measures include biological control agents, cluster thinning, and timely harvest to prevent widespread rot. Preventive strategies are usually preferred, as curative treatments are limited once infection is established.

Distinction from noble rot

Although caused by the same organism, grey rot differs fundamentally from noble rot. In the latter, alternating wet and dry conditions allow berries to shrivel, concentrating sugars and flavours to produce celebrated wines such as Tokaji, Sauternes, and German Beerenauslese. By contrast, under persistently damp conditions, Botrytis spreads aggressively and decomposes berry tissue, leading to grey rot. Recognising and managing this distinction is critical in regions where both outcomes are possible.

Global significance

Grey rot occurs in vineyards worldwide, with outbreaks reported across Europe, the Americas, Australia, and New Zealand. It poses particular challenges in cool or humid regions, such as northern France and parts of Germany. The disease has significant economic impact, reducing yields, increasing vineyard management costs, and threatening the consistency of premium wine production. Ongoing research explores resistant clones, improved canopy design, and environmentally sustainable methods to mitigate its effects.

See also

• Noble rot
• Canopy management
• Vineyard
• Wine fault