Grapevine botrytis: what it is and how to treat it

Grapevine botrytis is one of the most widespread and studied diseases in viticulture and is caused by the fungus Botrytis cinerea, which tends to strike close to harvest time, when the clusters are swollen, sugary, and apparently ready. In the most severe cases, it can compromise the quality of the must, altering the aromatic profile and color of the wine, and make an entire batch of grapes unusable. 

In this article, we will delve into the biological cycle of the fungus, the periods of greatest risk, the symptoms to recognize, the preventive strategies and the available treatments to effectively manage the problem.

Biological cycle of botrytis

Botrytis cinerea is a necrotrophic fungus, meaning it feeds on dead or weakened plant tissues. This means it does not need to attack a healthy plant in full strength, but small entry points such as a hail wound, a moth attack scar, or a cracked berry are enough for it to proliferate.

The fungus overwinters mainly in the soil and on infected plant debris, where it forms resistance structures called sclerotia, capable of surviving even unfavorable conditions for long periods. In spring, with the return of mild temperatures and humidity, the sclerotia germinate and produce conidia, which are asexual spores, dispersed by the wind and insects onto all the surrounding vegetation. 

Primary contamination often occurs as early as flowering, when the fungus colonizes dry flowers and organic debris that remain trapped within the forming cluster. From that moment, the disease can remain latent for weeks without showing obvious symptoms, waiting for the right conditions to progress.

What are the most critical periods

The risk of botrytis is concentrated mainly in three seasonal windows:

• Flowering: anthers and floral caps, once fallen, often remain trapped between the forming berries. These organic residues are the ideal substrate for the primary colonization of the fungus. A prolonged and humid flowering period significantly increases the risk.
• Bunch closure: when the berries swell and the clusters compact, internal ventilation is reduced. In this phase, even a rain of moderate duration can be sufficient to create the humidity conditions that the fungus exploits to progress.
• Veraison and pre-harvest phase: this is the moment of maximum vulnerability. The berries are turgid, the skin thins, and the pressure of internal juices increases. Any event (rains, hailstorms, or moth attacks) capable of causing micro-cracks opens the doors wide for the proliferation of the fungus. In this phase, going from a localized colonization to a general attack on the grapevine can take just 48-72 hours.

Causes and symptoms of botrytis

The environmental conditions that favor botrytis are: temperatures between 15°C and 20°C and humidity above 85-90% maintained for at least 15-18 consecutive hours. Higher temperatures (above 25°C) slow down the fungus but do not stop it. This means that even in the hottest years, the risk remains.

At the variety level, grapes with dense berries, tight clusters, and thin skins are the most at risk. Nebbiolo, Pinot Noir, Sangiovese, and Gewürztraminer tend to pay the heaviest consequences of the disease compared to varieties with loose clusters and tougher skins.

Symptoms are quite recognizable once the fungus is in an active phase: on the berries, the first sign is a localized brownish coloration, often matching small wounds or the contact point between two adjacent berries. Following this, and in conditions of high humidity even within a few hours, the characteristic gray-ashy efflorescence tends to develop on the necrotic area. Affected berries rot and, if the environment remains humid, the infection spreads like a cascade throughout the entire cluster. In the most severe cases, the rachises blacken and the clusters completely lose consistency. On leaves and shoots, irregular brown spots surrounded by a pale halo can instead be observed, which are also covered by the gray efflorescence under favorable conditions.

Prevention and control

In viticulture, botrytis prevention is a job that starts in winter and extends until grape harvest, with a set of practices that significantly reduce the probability and severity of attacks.

• Correct canopy management: leaf removal in the cluster zone, performed between fruit set and bunch closure, is one of the most recommended operations. It increases ventilation, reduces humidity around the clusters, favors the penetration of plant protection products, and accelerates drying after rains.
• Cluster thinning: on varieties with very compact clusters, partial thinning improves the structure and reduces the contact points between berries.
• Winter pruning and debris management: removing and destroying debris affected by botrytis reduces the possibility of it returning in the following season, and the same applies to shoots left on the ground.
• Vigor control: plants with excessively lush vegetation develop tender tissues and dense canopies, conditions that favor botrytis.
• Management of other pathogens: any attack that causes wounds on the berries or weakens the tissues is a factor that increases the risk.

Effective treatments

Unfortunately, the botrytis fungus has demonstrated over time a particularly high and fast capacity to adapt and develop resistance to fungicides. For an effective treatment, the rotation between active ingredients with different mechanisms of action is therefore crucial.

• Fenpyrazamine, fludioxonil, cyprodinil: they are among the reference active ingredients for anti-botrytis defense in conventional viticulture. Fludioxonil and cyprodinil are often found in a combined formulation, providing a complementary action.
• Boscalid and isofetamid: they belong to the SDHI class (succinate dehydrogenase inhibitors) and represent a valid alternative to be included in rotation with the previously mentioned molecules.
• Organic and low-impact products: for those operating under an organic regime or looking for solutions with a lower environmental impact, formulations based on Bacillus subtilis and Trichoderma asperellum/atroviride offer an interesting defense, especially as a preventive measure and as a component of an integrated strategy. Potassium bicarbonate has a contact action and must be repeated with greater frequency.
• Timing of interventions: the periods in which to concentrate treatments are: flowering (to reduce primary inoculum), bunch closure, and veraison. In rainy years or on high-risk varieties, a pre-harvest intervention may also be recommended.
• Quality of application: product penetration inside the cluster is one of the most critical technical factors in the treatment. A compact cluster, if treated with a superficial spraying, does not receive adequate coverage in the internal areas, where the infection often originates. The use of sprayers and mist blowers can make a clear difference.

Conclusion

Grapevine botrytis is a disease that develops quickly, strikes at the worst moment, and leaves little room for maneuver if one intervenes late. The good news is that it can be managed through correct agronomic setups, careful monitoring during critical phases, and a treatment strategy tailored to the variety, seasonal trends, and the risk level of the vineyard.

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