Looks like
Powdery mildew is often confused with downy mildew. Downy mildew fungal growth occurs only on the underside of the leaf while powdery mildew grows on both sides of the leaf surface. Downy mildew fungal growth is white and raised while powdery mildew is ash-grey to white and flat.
Young distorted leaves and flag shoots can be confused with bud mite damage. Late in the growing season distorted and discoloured powdery mildew leaf damage can resemble rust mite damage.
Powdery mildew of grapevines is specific to grapevines and although powdery mildew of other horticultural or ornamental crops look similar they will not infect a grapevine. If powdery mildew of other plants is occurring close to your vines it indicates conditions have been suitable for grapevine powdery mildew infection.
Damage and loss
Powdery mildew is considered the most costly disease to grapevine production in Australia with the combined effect of increased costs and reduced income totalling A$76 million annually.
Powdery mildew infections around flowering and up to five weeks later pose the greatest risk of damage and loss. Early infections lead to a greater number of diseased buds and fungal resting bodies to carry the disease over to the next season. Leaf, shoot and stalk damage interferes with vine metabolism and fruit quality. Infected flowers have poor fruit set and reduced yield.
Berries with cracks or splits predispose them to attack by other fungi. Infected bunches can cause off flavours and may be down graded or rejected by wineries. Severe powdery mildew infections in subsequent seasons can reduce the vigour and productivity of the vine.
Varietal susceptibility
All Vitis vinifera varieties are susceptible, in varying degrees. The more susceptible wine varieties include chardonnay, chenin blanc, riesling, semillon, verdelho and cabernet sauvignon. Shiraz and grenache are less susceptible. The most susceptible table grape varieties include flame seedless, red globe, cardinal, italia and crimson seedless.