Downy mildew can devastate individual vineyards and in some seasons, affect production from entire regions. It occurs sporadically according to the suitability of conditions for infection so that, in inland regions, severe disease occurs once in 9-10 years while in regions like the Hunter Valley, it is more frequent.
Downy mildew oil spot appearance on leaf surface
Downy mildew oil spot appearance underneath leaf
Previously considered a fungus, the downy mildew pathogen Plasmopara viticola, is now classified an algae, and like all algae, it required free moisture for most of its life cycle.
Downy mildew is recognised by its typical, circular, yellow oilspots on the younger leaves and, if the conditions have been suitable, the white down on the undersides of these spots. On older leaves, the symptoms are different. A tapestry pattern develops when the smallest veinlets become resistant to infection – this confines the disease to small, angular, interveinal patches of diseased cells. When infected, highly susceptible young bunches turn brown and die quickly. Later, 3-4 weeks after fruit set, berries gain resistance to infection. By pea-size (E-l 31), they are immune though the stems remain susceptible. Like powdery, downy is a ‘green’ disease. Mature tissue that has changed colour is no longer susceptible – for instance, green shoots are able to be infected whereas brown canes are not.
A primary infection occurs when the downy spores move from the soil to the vine. It produces a few oilspots in the foliage – usually about 1–3 oilspots in every 50 metres of canopy row. Secondary infection occurs when downy spreads from oilspots in the canopy. Spores move from leaf to leaf or leaf to bunch and, because each oilspot can produce many thousand of spores, this can lead to an explosive increase in disease levels causing many more new generation oilspots and bunch infections.