DISEASE

NAME
Downy Mildew in Grapes
Plasmorpara viticola


Description
Pathogenic, Fungus

Symptoms
All parts of the vine are affected, leaves, shoots and fruit. The leaves first form rounded translucent yellow to pale green spots that have an oily appearance and are up to 20mm across. These become blotches that dry out, turning brown then killing the leaf. During humid weather furry growth (fruiting bodies) appears on the underside of infected leaves.

     Vitisspecies

Source and Dispersal
The spores overwinter in leaf litter as thick walled winter spores (oospores) which can lay dormant for up to 2 years then germinate, producing a sporangia that release motile spores (zoospores). These are dispersed by wind or splashing water and by the movement of plant material then enter the leaf through the stomates. In Australia it is found in all the eastern states.

Favoured Conditions
The infection occurs when the temperature is above 10 deg C and at least 10mm of rain has fallen. The formation of spores occurs during humid conditions with temperatures above 13 deg C. The secondary infection occurs on the leaves when they are wet in the morning and the temperature is above 11 deg C.

Affected Plants
Grapes (Vitis vinifera) and its cultivars and Parthenocissusspecies.

Non-chemical Control
Remove and destroy all infected fallen leaves or pruned plant material. Improve air circulation around the plant with selective pruning and avoid watering over head and creating a humid environment.

Chemical Control
Preventative spraying should be carried out on the new shoots after the fourth week of growth and followed every 2 to 3 weeks thereafter. They must be thoroughly covered around the fruit and on both sides of the leaf. Follow up spraying later in the season can be carried out if the weather conditions are favourable for infection.
Fungicides such as Bordeux mixture, copper oxychloride, mancozeb, cupric hydroxide and dithianon are used as a protectant. There are fungicides that eradicate Downey Mildew if applied within 3 to 5 days of infection.
Note
Always read the label for registration details and direction of use prior to application of any chemicals.