Powdery mildew is probably the most common disease of grapevines in the home garden. The disease becomes conspicuous as a grey/white powdery growth on all green parts, including leaves, shoots and berries.
The berries may split and the pulp will often dry out. A fortnightly application of wettable sulfur or sulfur dust should be applied from mid September to late December. It is best to apply wettable sulfur early in the growing season and sulfur dust once the fruit has set. Do not apply sulfur on days when the maximum temperature is predicted to be 30°C or higher. Sprays containing bicarbonate of soda can also be used from early in the growing season to contain mildew.
This was was first detected in WA vineyards in 1998 but may not be a problem for the home gardener. The disease first appears on the upper surface of leaves as small yellow oilspots. The spots may enlarge to cover the leaf. A downy growth appears on the undersides of the oil spots.
As a preventive measure applications of copper oxychloride can be applied.
Bunch mites are not visible to the naked eye. Adults are less than 0.2mm long and are found on the leaf underside close to the bunches. The mites damage the berry stalks by interrupting the water supply which causes the berries to shrivel and fall off. Do not confuse this with wind-suck which may happen in hot, dry windy weather as the roots withdraw water back from the berries as a method of survival.
Grape leaf blister mite
These mites cause blistering of the leaf on the upper-surface. On the under-surface felt-like patches develop. Although the leaves become unsightly, fruit production is not affected. A spray of lime sulfur applied at bud-swell should give adequate control.
The sandy soils on the coastal plain near Perth are known to contain root-knot nematodes (Meloidogyne sp.) which can destroy the root system of the vine as the vines get older. Table grapes grown on a nematode-resistant rootstock will perform much better than vines growing on their own roots but are harder to obtain through garden centres.