Downy Mildew of Grapes
A fungal disease that starts as oily spots on the edges of leaves that turn pale yellow and dries out often into a v-shaped area. A white furry growth appears on the underside of the leaf opposite the lesion. The leaves may become patterned in varying shades of brown and may fall early leaving the petiole attached to the vine. This may lead to poor fruit production and sunburn of the berries.
Infection of bunches at flowering results in irregular fruit set and grapes may become hard and brown and shrivel to red brown mummies. White furry hyphal growth may cover the bunch.
Species Affected:Grapes and Parthenocissus spp.
Biology:Favoured by warm humid conditions in spring and early summer.
Fruit is susceptible to infection until it changes colour.
Survives from year to year on dead leaves.
Zoospores are released in the soil.
Life Cycle:First and primary infections occur when temperatures rise above 100C for 24 hours after 10 mm or more rain and the foliage remains wet for 2-3 hours. Lesions or "oil spots" appear 5-15 days after infection depending on the temperature and age of the leaf.
Origin and History:Distribution:
Management and Control:
Avoid overhead irrigation.
Prune to produce and even open leaf canopy. In early summer there should be about 30% open area through the vine.
Grade vineyard to avoid the formation of puddles.
Burn fallen leaves and pruning residues.
Monitor vines for "oil spots" from when the shoots are 10 cm long until when the berries are pea sized.
Apply fungicides when conditions are favourable for the disease or when "oil spots appear. Very rapid epidemic spread may occur with secondary infections which are hard to control.
Rapid growth in early summer and rainfall that washes fungicides of the leaves may leave portions of the vine unprotected by early sprays.
Copper products provide protection and curative treatments include metalaxyl and phosphorous acid. (There may be some restrictions on phosphorous acid for Grapes used in wine making.)
Related and Similar Species:References:
Collated by HerbiGuide. Phone 08 98444064 or www.HerbiGuide.com.au for more information.