Powdery mildew is a fungal disease that affects grapevines. It is one of the most important and damaging diseases in viticulture. In this technical guide, we will examine the symptoms, biological cycle, control methods, and organic treatments for grapevine powdery mildew.
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Identification and damage: what Is wrapevine powdery mildew?
Powdery mildew, also known as grapevine powdery mildew, is caused by the fungus Uncinula necator (Schw.) Burr. (recently reclassified as Erysiphe necator (Schw.) Burr.), which represents the sexual stage of the pathogen. Oidium tuckeri is the name historically used for its asexual stage.
This pathogen infects grapevines and is an ectophytic fungus, meaning that it develops on the external tissues of the host plant. Together with grapevine downy mildew and grey mold (Botrytis cinerea), it forms the group of the three most important fungal diseases of grapevines.
The disease attacks all green parts of the plant, especially leaves and clusters, from spring through summer. On leaves, powdery mildew appears as white-grey patches with a characteristic powdery appearance (image 1). As the infection progresses, leaves may become distorted.
Grape clusters affected by powdery mildew also develop the characteristic white-grey fungal growth (image 2 and 3). Infected berries often become necrotic in the affected areas, resulting in deformation and reduced growth compared with healthy berries. Cracking may occur, predisposing the berries to secondary infections by Botrytis cinerea, Penicillium, Aspergillus, and other fungi.
Shoots infected during the herbaceous stage develop extensive dark necrotic lesions resembling irregular brown-black spots, resulting in weaker canes with poor lignification and increased susceptibility to mechanical damage (image 4).
When the fungus infects dormant buds, the shoots emerging from those buds are often stunted and deformed.
Biological cycle of grapevine powdery mildew
The pathogen can overwinter either as dormant mycelium inside infected buds or as cleistothecia (fruiting bodies containing sexual spores known as ascospores).
Infections originating from ascospores begin when overwintering cleistothecia release their spores from plant debris on the vineyard floor. This usually coincides with bud break and is strongly influenced by temperature and humidity.
Under favorable environmental conditions, wind-dispersed ascospores germinate after landing on grapevine leaves. The fungus then produces mycelium, visible as the characteristic white fungal growth, and begins feeding on host cells.
Subsequently, the mycelium develops conidiophores that produce large numbers of conidia (asexual spores), which are responsible for secondary infections throughout the growing season.
Infections arising from overwintering mycelium are largely independent of free moisture. Temperature, however, remains a critical factor, with maximum conidial germination occurring between 22 and 28°C.
You may also be interested in our technical guide: “How to Control Grapevine Downy Mildew in Organic Farming.”
Organic control and proper treatments
Organic control of powdery mildew relies primarily on sulfur-based treatments and several complementary products, including sodium bicarbonate, potassium bicarbonate, and the beneficial fungus Ampelomyces quisqualis, a well-known antagonist of powdery mildew pathogens.
When using bicarbonates, care must be taken to avoid excessive concentrations, which may cause leaf burn. These products should be considered supportive tools, as they are generally insufficient when used alone under high disease pressure.
Sodium and potassium silicates are also used in powdery mildew management. Their activity is mainly based on strengthening leaf tissues and creating a physical barrier that reduces pathogen penetration.
Among the most recent and effective biological tools for controlling grapevine powdery mildew are:
- COS-OGA (Chito-oligosaccharides and Oligogalacturonides)
Commonly referred to as chitosan-based elicitors, these products activate the plant’s natural defense mechanisms, making grapevines less susceptible to pathogen attack. In Italy, one formulation is currently registered as a plant protection product, while several others are marketed as biostimulants.
- Cerevisane
Cerevisane is a systemic resistance inducer obtained from cell wall extracts of a specific strain of Saccharomyces cerevisiae. Similar to chitosan, it stimulates the plant’s natural defense responses. In Italy, a registered organic plant protection product is available.
- Laminarin
Laminarin is a resistance-inducing oligosaccharide extracted from the brown seaweed Laminaria digitata. It activates the plant’s natural defense mechanisms and is available both as a registered plant protection product and as a component of various biostimulants.
- Sweet Orange Essential Oil
Sweet orange essential oil acts as both an insecticide and a fungicide. It works by direct contact, disrupting insect cuticles and damaging the external reproductive structures of fungal pathogens, including conidia and cleistothecia. A commercial formulation approved for organic farming is available.
Beneficial bacteria used against powdery mildew
- Bacillus amyloliquefaciens
This antagonistic bacterium helps suppress infections through competition and the production of antimicrobial compounds. Several formulations are approved for organic agriculture.
- Bacillus pumilus
This beneficial bacterium controls fungal diseases through three mechanisms:
- Competition for ecological niches
- Inhibition of fungal hyphal development
- Activation of the plant’s natural defense responses
Commercial formulations approved for organic farming are available.
Agronomic practices
Although not sufficient on their own, several cultural practices can help reduce powdery mildew damage.
Nitrogen fertilization should be carefully balanced, as excessive nitrogen promotes succulent growth that is more susceptible to fungal infection.
Training systems should ensure adequate canopy ventilation and spray penetration, minimizing poorly covered areas where disease can develop.
Additional measures should focus on reducing inoculum sources by removing infected shoots during summer pruning and eliminating diseased canes during winter pruning.
A sound organic management strategy includes sulfur applications combined, where appropriate, with silicates or bicarbonates. Sulfur treatments can also be alternated with applications of Ampelomyces quisqualis (which should not be applied in mixtures with sulfur or within five days of sulfur treatments), Bacillus amyloliquefaciens, or Bacillus pumilus.
Unlike Ampelomyces quisqualis, the two Bacillus species are compatible with sulfur-based programs.
Sulfur applications may also be alternated with treatments based on chitosan, Cerevisane, laminarin, or sweet orange essential oil.
Dusting sulfur remains highly effective against powdery mildew and should be preferred or alternated with wettable sulfur formulations.
Control of grapevine powdery mildew is rarely completely successful, especially when susceptible grape cultivars are grown in highly favorable disease environments. The use of tolerant or resistant cultivars remains the most effective long-term strategy for managing this disease in organic viticulture.
Most common fungicides available online
- Sulfur: https://amzn.to/44cYH7j
- potassium bicarbonate: https://amzn.to/4we0MMz
- Chitosan: https://amzn.to/43FYAB3