Grapevine grey mould (botrytis): how to treat it

Botrytis or grey mould is a grapevine disease caused by the fungus Botrytis cinerea. It affects various plants and not just grapevines: in the garden it attacks cucumber, bean, aubergine, pepper and tomato, as well as strawberry and basil. It also attacks fruit trees (such as apricot, cherry, apple and peach), flowering plants including geraniums and roses, and even weeds.

The fungus grows quickly and is very adaptable to different environmental conditions, so it can easily overcome harsh spells of weather. What’s more, in addition to affecting a wide range of host species, it can thrive on both living and dead plant matter. Today we will talk in particular about grapevine botrytis: what it is, how it develops and how to protect the vineyard from it.

Symptoms of grapevine botrytis

Botrytis attacks all parts of a vine that are water-rich and green (rather than woody), namely the buds, shoots, leaves, flowers and grapes. It primarily affects grape bunches, significantly reducing the yield and quality of both table grapes and wine grapes.

Chlorotic spots form on the vine leaves, which then become necrotic. In high humidity, the affected areas develop a characteristic grey mould, hence the name. Infected buds and shoots dry up, while green shoots darken and become necrotic at the tips.

Leaves, buds and shoots are rarely affected by botrytis. By contrast, flowers are susceptible to repeated attacks: if it happens before flowering, the bunches dry up. If the bunches are affected after flowering, B. cinerea can remain latent, nestling within the floral residues and then becoming active later. Bunches can also be infected starting from the peduncle, stalk and pedicels. Botrytis also directly attacks grapes when the defence mechanisms of unripe grapes fail during veraison (the ripening stage, when they change colour) or afterwards, once they are already ripe. Infections generally appear from veraison to harvest or, less often, at flowering or post-flowering. In any case, the typical symptom of botrytis is grey mould covering the affected parts.

The biological cycle of grapevine grey mould

The botrytis fungus that causes grapevine grey mould persists in various forms in the environment. In particular, it survives the winter in the form of sclerotia and mycelium. Sclerotia are small, rounded bodies that commonly overwinter in infected crop debris (prunings, mummified fruits, etc.) on the soil of the vineyard. Mycelium, which is a network of fungal filaments called hyphae, overwinters in infected plant material and within cracks in the vine bark. In and around the vineyard, sclerotia and mycelium can also be found on other plants, including weeds and their remnants.

From spring onwards, sclerotia and mycelium produce spores (called conidia) that are transported by wind, rainwater, sprinkler irrigation and insects. When conidia reach a vine, they can germinate and infect the host if conditions are favourable, penetrating its tissues and, after an incubation period, causing a botrytis infection. During the growing season, between spring and autumn, the fungus is a constant presence in vineyards due to its ability to continuously produce and disperse spores.

It thrives in environments with mild temperatures – ideally around 20-25°C but as low as 16°C – and humidity of 90% or higher, conditions which are often found in areas with rain, fog and poor air circulation. Microlesions caused by insects, parasites, hail and other factors enable the fungus to infiltrate plant tissues, particularly in grapes. Other factors that create entry points are thinning of the skin after veraison and tight bunch structures in certain grape varieties.

As for the grey mould that develops on the parts of vines infected by botrytis, it is the mass of structures that enable B. cinerea to then reproduce and disperse spores that will infect other vines, creating a cycle of infections.

How to prevent grapevine botrytis

Combating grey mould depends largely on prevention. In essence, you can prevent it by promoting balanced vine growth with good air circulation within the canopy and around the bunches, so as to avoid conditions favourable to B. cinerea.

Before planting a vineyard, you can prevent grey mould and other vine diseases by carefully working out:

• Vineyard location: if possible, select a dry, ventilated site free from damp.

• Landscaping: so as to ensure good drainage and avoid buildup of standing water, taking into account row orientation, plant spacing and training systems.

• Vine varieties: choose cultivars that are less susceptible to disease (for example, those with loose bunches and widely spaced grapes) and avoid overly vigorous varieties that produce dense foliage.

• Rootstock: you can choose a weaker rootstock to reduce plant vigour and bunch compactness.

• Planting density (spacing of vines between rows and between plants within a row): arrange the plants so that they are not too close together.

• Training method (configuration of the vine’s structure on the row): in general, espalier systems allow for better air circulation within the canopy. Here you will find an overview of the main vine training systems and an in-depth look at how to plant a espalier vineyard.

Meanwhile as regards vineyard management, prevention means containing the vine vegetation (i.e. the foliage). You can do this through:

• Proper canopy management: with both winter and summer pruning, to ensure that the canopy does not clump together, thereby creating a humid microclimate and excessive shading of bunches (particularly towards the end of July/beginning of August, it is important to defoliate around bunches so as to improve their exposure to sunlight).

• Grass cover: competes with the vines for water and nutrients and thus limits their vigour, but to prevent moisture from stagnating, grass cover must be controlled by mowing under and between rows using a brushcutter, flail mower or garden tractor, depending on the case.

• Balanced nitrogen fertilisation: likewise to prevent the growth of an excessively dense canopy with overly large and compact clusters, potentially compromising fruit quality. You can learn more about this topic in our article on how to fertilise vineyards.

• Measured irrigation: to provide water only when it’s needed – from fruit set (the transition from flowers to fruits) through to veraison – and in a way that avoids waterlogging.

• Insect control, including against the grapevine moth: it can carry spores and create wounds that enable B. cinerea to infect plants more easily. Also guard against powdery mildew, a fungal disease that causes cracked or split grapes.

• Be careful when working in the vineyard: accidental damage can create lesions that provide entry points for the fungus, while disinfecting tools helps to prevent the spread of botrytis from one vine to another.

Combating grapevine botrytis

When should you take action against grey mould? The best times to intervene are during the most susceptible stages in the vine’s annual cycle:

• From the end of flowering to veraison, when conidia are very abundant in the vineyard, leading to a high inoculum potential and greater probability of latent infections (B. cinerea can actually exist in flowers without causing active disease, so doesn’t present visible symptoms).

• During maturation, when there is a risk of cyclical infections, which in practice are the most dangerous.

Intervene immediately after flowering if conditions favour fungal growth: for example, in the case of dampness, rainy seasons, compact grape bunches or visible lesions on grapes, or if you notice any symptoms of botrytis.

What should you use to combat grapevine botrytis? You can use treatments permitted in organic farming, which should be applied with a mistblower:

• Copper salts: should be distributed before rainfall.

• Substances that dry vegetation by absorbing surface moisture, thus preventing the formation of a damp microclimate: these include bentonite, especially when mixed with sulphur, and rock dust such as zeolite, which is also found in the natural fungicide lithothamnium.

• Microbiological products, specifically those containing antagonistic microorganisms like Bacillus subtilis or Bacillus amyloliquefaciens, which prevent B. cinerea from attacking vines by competing with it for nutrients, space, etc.

• Other active ingredients: potassium bicarbonate, a fungicide with both preventive and curative effects, or sodium bicarbonate, which has a similar action (but is less recommended due to the risk of sodium accumulation in the soil and the associated negative impact on plant health).

In addition to fungal diseases such as botrytis and powdery mildew, insects like the vine mealybug are a significant threat to grapevines – read our tips on how to effectively eradicate it.