The fungus causes ash-grey to white powdery growth on green tissue of the grapevine. In particular, the upper and lower surfaces of young leaves, shoots or clusters are highly susceptible.
The chains of conidia that develop from the powdery mildew hyphae give the infected vine tissue the characteristic powdery or dusty appearance. Severely infected vines emit a musty odour mid to late season.
Flag shoots are stunted shoots covered partly or wholly with ash-grey to white powdery growth with distorted leaves that curl upwards. These shoots become evident two to eight weeks after budburst (Figure 1).
Leaves are most susceptible when they are expanding. Infections result in small yellow-green blotches 2-10mm in diameter with an irregular outline on the upper surface of leaves in spring (Figure 2). The blotches form an ash-grey to white powdery growth of hyphae which develops conidia on both sides of the leaf surface.
Web-like hyphae and chains of conidia are clearly visible with a 10x hand lens (Figure 3). In the field this fungal growth is flat, as the conidia chains are constantly broken. The blotches enlarge and may merge to cover the whole leaf (Figure 4). Smaller veins on the underside of the infected leaves may turn brown (Figure 5).
The earliest infected leaves become distorted and discoloured (Figure 6), sometimes giving the vines a wilted appearance. Severely diseased leaves blacken, dry out and fall prematurely in hot weather. Leaves become more resistant to infection with age, but are never completely resistant.
Shoots – ash-grey to white powdery growth develops in patches until the whole shoot is covered (Figure 7). Severely diseased shoots are weakened, stunted and can die.
Bunches of most cultivars are susceptible between flowering and up to five weeks later. Although berries develop resistance with age the bunch stalk and stems remain susceptible. Ash-grey to white powdery growth develops on immature berries and bunch stalks (Figure 8).
Severely infected berries may develop irregular shapes, crack or split and rot. Red varieties may colour unevenly. Post-veraison, berries develop a brown web-like pattern on the surface, very noticeable on white varieties (Figure 9).
Over time berries become resistant to powdery mildew infection and thus once this occurs the fungus is killed inside the berry leaving scar tissue on the berry. When the berry expands these scars can lead to berry split and a site for Botrytis cinerea infection.
Black patches on green immature shoots develop into reddish-brown patches on mature canes. This is evidence of a powdery mildew infection earlier in the season.