In the spring, possible powdery mildew symptoms (Erysiphe necator) include a slowing down of growth, a shortening of the internodes and shrivelling of the leaves. On very sensitive varieties, a white felting can appear: these are "flag" symptoms.
The first powdery mildew symptoms are characterized by oily spots (quite similar to downy mildew), and a blackening of the veins on the underside of leaves corresponding to necrotic cells.
The next stage is the appearance of greyish and dusty felting (mycelial filaments and conidiophores) on the upper side of the spots (and on the underside for sensitive grape varieties), and shrivelling of the leaves' edges.
Powdery mildew on leaves (Source: Vigne Vin Occitanie)
The berries first appear to be covered with a grey, ashy-looking dust. Under these fruiting bodies, the organ remains green, then the cells die and the berries burst under the pressure of the developing cells.
This typically results in the appearance of seeds and is a prime opportunity for Botrytis to develop. The diseased bunches release a strong mushroom smell. After véraison, the vine is much less sensitive to powdery mildew.
Powdery mildew on bunches (Source Vigne Vin Occitanie)
Brown to black mycelium can be observed on shoots before lignification. After they harden, these brown spots become red and star shaped. In the autumn, black blisters appear on the affected shoots called cleistothecia.
Powdery mildew overwinters as:
Mycelia in the buds which are responsible for the early infection of shoots (flag form)
Cleistothecia, which form on affected organs (and are the most common form of conservation). The cleistothecia burst in spring releasing ascospores that are responsible for primary infection. The ascospores, and the conidia from the mycelia, colonize the various organs. Powdery mildew, unlike downy mildew, is an external vine parasite. The mycelium is thus on the surface of the organs and must develop suckers to attach itself and feed. The propagation of powdery mildew is ensured by conidiophores carrying conidia dispersed by the wind. Powdery mildew development is optimal between 25°C and 30°C, and 40% to 100% relative humidity. The presence of free water hinders germination and causes conidia to burst. The maximal point of vulnerability for the clusters is the "end of flowering - beginning of fruit set".
The previous sections are excerpts from Vigne Vin Occitanie.
Oidium lifecycle (Source: Vigne Vin Occitanie)
Several practices favour the appearance of powdery mildew and/or its development on leaves and bunches. However, even if powdery mildew appears very early and can be very subtle, certain prophylactic measures can be taken to limit its development:
Plant choice: certain grape varieties are more sensitive than others to powdery mildew, such as Carignan, Chardonnay and Pinot noir. Prophylactic solutions: Adapt the rootstock to the grape variety's vigour and more generally choose plant material suited to the plot's characteristics; also select the grape variety and/or a clone with loose bunches in order to reduce the risk of contamination by powdery mildew.
Management of the canopy and vigour: powdery mildew likes humidity and heat, a lack of canopy ventilation can increase the humidity in the microclimate around the bunches. Not carrying out certain operations, such as leaf thinning, can favour development of the fungus, which likes shade. Powdery mildew is a disease that spreads from localized outbreaks, with contamination resulting from close contact within the same area; high leaf density can thus lead to more contamination. Prophylactic solutions: High light levels dry out the mycelia and can be implemented by appropriate canopy management: ventilation of the vines and, if necessary, debudding, removal of secondary growth and/or leaf thinning. Regulate vigour by implementing grass cover that is kept short if necessary. Monitor the use of fertilizers, particularly nitrogenous ones, so as not to encourage vigour.
Treatment management: powdery mildew can appear very early and be very subtle, only treating when symptoms appear can be too late. It is a "historical" disease, i.e. it affects the same plots every year if conditions are favourable. The use of unsuitable treatment equipment, poorly adjusted nozzles and spray patterns can be ineffective. Prophylactic solutions: Adjust the equipment and spray nozzles appropriately and carry out regular spray tests. Reduce the inoculum for the following year: a copper-based treatment at véraison can help reduce cleistothecia.
The effectiveness of the protection depends on the quality of the spraying (nozzle and orientation adjustment towards the bunches) as well as on the choice of the optimal time for treatment. Sulphur is the product most used in the fight against powdery mildew. It can be in wettable or powder form:
Wettable sulphur is used preventively and repeatedly, and is the most commonly used formulation. It does not require additional equipment (whereas powdered sulphur must be applied with a powdering machine), has better adherence, longer residual time and better resistance to wash-off (risk of wash-off with more than 20 mm of rain), it exists in three forms:
Micronized: obtained by fine grinding, it comes in powder form to be dispersed in water. The particles are smaller than 5 micrometres.
Atomized micronized: obtained by grinding in the liquid phase and then formulated into small particles by passing through an atomizer, also to be dispersed into water. Its granulometry is very fine and regular, from 1 to 6 micrometres.
Concentrated or liquid suspensions: these are liquid forms, with a particle size of around 2 to 3 micrometres.
Powdered sulphur is used for shock action at the agglomerated flower bud stage (9-10 leaves) or on declared powdery mildew, it requires the use of a powdering machine, and has a higher risk of wash-off and product drift. Sulphur powder exists in two forms:
Ventilated sulphur obtained by simple grinding and then sieving using ventilation; the particles range in size from 15 to 100 micrometres.
Sublimated, obtained by distillation, is very pure and emits sulphur vapours more readily, the particle size ranges from 5 to 15 micrometres.
Other products can be used in combination with sulphur, for example, certain animal products such as skimmed milk or whey, diluted to 10-30% in water. Depending on the type of supervisory body and the origin of the products (organic milk), this practice is authorized in organic farming.
Certain preparations based on plant extracts can also be used (herbal teas or yarrow decoctions for example).
Because powdery mildew is difficult to detect immediately, early treatments at 2-3 leaves (the C-D-E stage) should be carried out on very sensitive plots "with flags". For plots with normal sensitivity, protection can begin around the separate flower bud stage.
Once the initial treatment has been applied, treatment rates should be based on weather conditions (wash-off), the development stage of the plant and the residual time of the applied product. Certain institutes, organizations and agricultural cooperatives provide predictive models which are very good tools for making decisions when applying the initial treatments.