Fungal Diseases: Botrytis, Powdery Mildew, and Downy Mildew

Fungal Diseases: Botrytis, Powdery Mildew, and Downy Mildew

No forces test the grapevine grower's vigilance more relentlessly than fungal pathogens. In most wine-growing regions of the world, the three major fungal diseases — gray mold (Botrytis), powdery mildew (oidium), and downy mildew (peronospora) — collectively represent the most significant biological threat to crop health and wine quality. Understanding their biology is inseparable from understanding why wine is grown where it is, why certain varieties succeed in certain climates, and why organic and Biodynamic viticulture presents distinct challenges.

Botrytis cinerea: Destroyer and Creator

Botrytis cinerea is among the most economically significant plant pathogens in the world — and also, in specific circumstances, one of the most celebrated contributors to wine luxury. Its dual role in viticulture is unique in plant pathology.

The pathogen: B. cinerea is a necrotrophic fungus, meaning it kills plant tissue to feed on it (unlike biotrophic pathogens that require living tissue). It overwinters as sclerotia (compact masses of dormant mycelium) and as infected plant debris in the vineyard. In spring and summer, it produces conidia — asexual spores — that are released by the millions from sporulating lesions and dispersed by wind and splashing water.

The fungus infects wounded, senescent, or previously stressed plant tissue most easily. In vineyards, infection typically occurs through flower parts (the cap, or calyptra, that falls during flowering often carries Botrytis infection into the developing flower), through existing wounds (bird damage, insect feeding, sunburn), or through berry skin cracking caused by rain events during ripening.

Gray rot: Under humid conditions — persistent autumn rain, heavy morning dew, dense canopy that retards drying — B. cinerea spreads rapidly from berry to berry within a cluster and from cluster to cluster. This is gray rot: infected berries collapse, turn gray-brown, and become covered in the characteristic dusty gray sporulation (a mass of conidia). Gray rot in a white wine vineyard will, within days, produce grapes high in enzymes that oxidize the juice (particularly laccase), dramatically elevated acetic acid, and numerous off-flavor compounds. The result in the winery is oxidized, vinegary, moldy-tasting wine. Even a small proportion of heavily botrytized fruit can ruin an entire fermentation.

Noble rot: Under entirely different conditions — alternating morning fog or high humidity followed by warm, dry, sunny afternoons — the same B. cinerea produces a radically different outcome. In these conditions (which occur naturally in very few locations, most famously Sauternes in Bordeaux, the Mosel-Saar-Ruwer in Mosel, and Tokaj in Hungary), the fungus penetrates the berry skin but loses moisture rapidly in the afternoon heat. Rather than destroying the berry, this "benign" Botrytis infection concentrates the berry's sugars and acids while simultaneously modifying its flavor chemistry.

The key transformations in noble rot (pourriture noble in French, Edelfäule in German, muffa nobile in Italian) include:

• Sugar concentration: Berries lose 30–70% of their weight as water evaporates through the compromised skin, concentrating glucose and fructose to extraordinary levels (sometimes exceeding 500 g/L).
• Glycerol production: B. cinerea produces glycerol as a metabolic byproduct, contributing a distinctively viscous, oily texture to nobly rotted wines.
• Gluconic acid accumulation: Botrytis oxidizes glucose to gluconic acid, which is present at elevated levels in nobly rotted wines and serves as a diagnostic marker.
• Flavor compound synthesis: Most importantly, Botrytis produces sotolon — a fenugreek/curry-like compound — and modifies the grape's terpenoids, creating the distinctive honey, beeswax, and apricot aromas of great Sauternes and Botrytized Riesling from Mosel.

Riesling is particularly well suited to noble rot production because of its thin but tough skin (resisting burst while allowing water evaporation) and its naturally high acidity, which remains sufficient even after sugar concentration to balance the enormous sweetness of Late Harvest wines. Gewürztraminer and Chenin Blanc are also celebrated vehicles for noble rot.

Powdery Mildew (Uncinula necator / Erysiphe necator)

Powdery mildew, caused by Erysiphe necator (formerly Uncinula necator), arrived in Europe from North America in the 1840s — a decade before phylloxera — and caused immediate catastrophe. Unlike B. cinerea, powdery mildew is an obligate biotroph: it can only grow on living plant tissue, and it must penetrate the host cell to extract nutrients.

E. necator is an unusual fungus in its environmental requirements. Where most fungi require free water on plant surfaces (rain, dew) to germinate and infect, powdery mildew is inhibited by wet conditions and thrives in dry weather with moderate humidity. This makes it particularly problematic in Mediterranean climates with dry summers and also in enclosed or sheltered vineyard positions where air circulation is poor.

The primary infection cycle begins in early spring from overwintered cleistothecia (sexual reproductive structures) in bark or crevices, which release ascospores that provide the primary inoculum for the season. Secondary spread through the season comes from asexual conidia produced prolifically on white powdery colonies on green tissue.

Powdery mildew infects all green plant parts — leaves, shoots, and most critically, developing berries. Infected berries develop a characteristic web-like scar pattern as their skins attempt to form a cuticle over the infection site. As the berry expands, these scarred skins crack, exposing the pulp and creating entry points for secondary infections including B. cinerea and bacterial rot.

Management: Sulfur — one of the oldest fungicides in agriculture — is the primary tool against powdery mildew and forms the backbone of programs in both conventional and Organic Wine production. Sulfur Dioxide (and elemental sulfur applied as dust or wettable powder) is fungistatic against powdery mildew and is approved for organic use. Applications must be timed carefully: sulfur applied in extreme heat (above 35°C) can phytotox the vine, and it must not be applied too close to harvest as it can interfere with fermentation.

Canopy Management — particularly leaf removal to improve air circulation and sun exposure around clusters — is a key cultural control. Dense canopies create the microclimate conditions (low air movement, moderate temperature, buffered humidity) that favor powdery mildew establishment.

Downy Mildew (Plasmopara viticola)

Downy mildew (Plasmopara viticola) was introduced to Europe from North America even more recently than powdery mildew, first documented in France in 1878, and spread across European vineyards within years with equally devastating effect. It is among the most destructive grapevine pathogens under conducive conditions.

Unlike powdery mildew, P. viticola is not a true fungus but an oomycete — an organism more closely related to brown algae than to fungi, though it behaves similarly from an agricultural perspective. It requires free water for spore germination and infection: the famous "10-10-10" rule (10mm of rain, 10°C minimum temperature, and green growth exceeding 10cm) approximately describes the conditions needed for primary infection. This makes it predominantly a threat in regions with wet spring and summer conditions.

P. viticola produces two types of spores: asexual zoospores (sporangia) that disperse in water splashed from oospore-contaminated soil to lower leaves, and sexual oospores that overwinter in infected plant debris. Primary infections on leaves produce the characteristic "oil spot" lesions on the upper leaf surface and white sporulation on the corresponding lower surface. The white, downy appearance of the spore masses gives the disease its common name.

Severely infected leaves and shoots drop prematurely, reducing the vine's photosynthetic capacity. Cluster infections produce the "brown rot" of downy mildew — desiccated, brown, leathery berries — distinct from B. cinerea gray rot but similarly devastating.

Management: Copper compounds (Bordeaux mixture and its descendants) have been the primary defense against downy mildew since the discovery of Bordeaux mixture in the 1880s. Like sulfur, copper is approved for organic and biodynamic use but is accumulative in soils at high application rates. EU regulations now limit copper applications to an average of 2 kg/ha/year (with a transitional maximum of 4 kg/ha in some years) — a restriction that significantly challenges growers in high-pressure mildew seasons.

Synthetic fungicides (systemic and contact materials including phenylamides, phosphonates, and carboxylic acid amides) are effective against downy mildew in conventional production but are subject to resistance development with repeated use. Integrated programs rotate chemical classes and combine cultural and biological control measures.

Integrated Disease Management

Modern viticulture's approach to fungal disease management has evolved from calendar-based spray programs (apply fungicide every X days regardless of disease risk) to risk-based, integrated programs informed by disease monitoring, weather modeling, and periodic resistance screening.

Disease models: Computer models like EPI (EuropePrognosis and Information) for powdery mildew and Vitimeteo for downy mildew integrate weather station data to calculate infection risk on a daily or hourly basis. Growers with access to these tools can make spray decisions based on actual risk rather than calendar intervals, reducing total fungicide use while maintaining effective control.

PIWI varieties: As noted in the genetic diversity chapter, resistant "PIWI" varieties bred to carry genomic resistance factors (Rpv genes for downy mildew, Ren genes for powdery mildew) can reduce or eliminate the need for fungicide applications. These varieties face regulatory and market acceptance challenges but represent the long-term solution to the chemical dependency of conventional disease management.

Biological control: Bacillus subtilis and Aureobasidium pullulans are commercially available biological agents with activity against Botrytis. Kaolin clay applications create a physical barrier against powdery mildew and have shown efficacy in trials. These tools are of particular interest to producers pursuing Organic Wine or low-intervention winemaking.

The ecological and regulatory constraints around fungicide use are tightening globally, driven by environmental concerns, resistance evolution, and consumer demand for fewer chemical residues. This pressure is accelerating innovation in disease-resistant varieties, precision application technology, and biological control — potentially reshaping the disease management landscape of viticulture within the coming decade.