Powdery mildew outbreaks
In southern regions late December to early January is the likely time for outbreaks to occur. In northern regions (Gascoyne) late September to early October outbreaks may occur. To achieve better spray coverage and prevent further disease development the following practices are recommended:
- trim shoot growth to allow shoots to become more erect and expose bunches;
- lift wires to expose bunches;
- leaf pluck two to three leaves immediately above and below bunches;
- adjust spray nozzles and direct air flow of ducted spray machines to ensure most spray is deposited into fruit zone;
- use high spray volumes and highest rate of wetting agent (as leaves and bunches infected with powdery mildew are difficult to wet); and
- apply two to three sprays each seven to ten days apart to ensure maximum coverage of leaves and bunches. Spray in the opposite direction with the final spray.
Caution: Exposed bunches are susceptible to sunburn. Application of fungicides late in the season may give rise to chemical residues in the fruit or visual residue problems on berries. Visual residue on berries at harvest is undesirable for table grapes. Refer to product labels, AWRI 'Dog Book' and the viticulture spray guide for general comments and restrictions on usage.
Postharvest sprays are of limited value except for young vines. Buds will already be infected and most chasmothecia will have already lodged into bark crevices and other sheltered places. The cost, amount of chemical and the chemical resistance consideration to control the survival structures of the fungus usually outways any benefit achieved by the spray.
Young vines may require postharvest sprays to ensure continued shoot growth and to prevent premature defoliation so that the young vine can establish its vine framework and can lay down its carbohydrate reserves for the following season.