We’ve had unusual weather this year with much more rain that normal. It means we have to be extra vigilant against two of the major dreaded vine diseases; downy mildew and powdery mildew.
Downy mildew is a highly destructive grapevine disease which has spread to all grape-growing areas where there is spring/ summer rainfall at temperatures above 10ºC . To begin with, Bordeaux mixture (a blue mix of copper sulphate and lime) was used to control downy mildew in the vineyards of France, beginning the chemical era of disease control. Botany professor Pierre-Marie-Alexis Millardet of the University of Bordeaux is credited with its discovery. He noted that vines closest to the roads did not show mildew, while all other vines were affected and he found out the roadside vines had been sprayed with a mixture of copper sulphate and lime to deter passersby from eating the grapes.
Initial leaf symptoms of downy mildew are light green- yellow spots called ‘oil spots’ because they may appear greasy. Under humid conditions, white, downy spore masses can be seen on the lower leaf surface, hence the name downy mildew. These spores are wind dispersed so it spreads quickly. The lesions eventually turn brown as the infected tissue dies and severely infected leaves drop prematurely, which reduces the hardiness of the vine. Berries become resistant to infection within three weeks of bloom but if they are infected earlier they readily fall from the cluster.
Downy mildew didn’t affect vineyards in the Robertson area until the early 1970s and now it occurs at Excelsior roughly once every five years, in exceptionally wet weather. We look out for the ratio 10:10:24 – if there is 10mm of rain, the temperature is above 10C and the leaves are wet for longer than 24 hours, it is very likely downy mildew will occur and we treat it by spraying copper sulphate – about 3.5 litres per hectare.
Powdery mildew, also known as oidium, is a much more formidable foe than downy mildew and can be considered to be endemic in our area. Initially from Eastern North America, it is caused by the fungus Uncinula necator. Nearly all the cultivars grown at Excelsior are susceptible (Cabernet Sauvignon, Shiraz, Chardonnay, Viognier and Sauvignon Blanc), only the Merlot is slightly more hardy.
Powdery mildew usually appears as white – green powdery patches on the undersides of basal leaves. It may cause mottling or distortion of severely infected leaves, as well as leaf curling and withering. Berries remain highly susceptible to infection during the first three to four weeks after bloom and infected berries may crack open and dry up or never ripen at all.
Powdery mildew is prevented and treated at Excelsior by dusting twice with sulphur. The sulphur dust is blown out over the vines very early on a windless morning allowing it to stick to the leaves. The heat of the sun (if it goes over 25C) will then allow the sulphur to react with oxygen in the air, forming sulphur dioxide and fumigating the vineyard.
The idea is to use as few chemicals as possible and to keep the chemicals we do use as simple as we can. Copper has been used for hundreds of years in viticulture and powdered sulphur can even be used in organic farming.
Sources and further reading…