The second attack of mildew on plants, including vines, comes from the sporangia formed by the mycelium inside the leaf, which come out on the reverse of the leaves. They can produce millions of spores that can be easily dispersed by air flows far away from the plant where they are produced. Mildew can attack the tender parts of the plant, specially grapevine buds, leaves and during the flowering season. The attack can be devastating and can destroy the entire production of the vineyard. The symptoms on the leaves are identical to those caused by the first attack. It appears as a characteristic downy white growth spots in the underside and a oily yellowish patch in the obverse side of the leaf, resembling an oil stain on paper sheet when looked against the light.
The intensity of the attack also depends on the weather: spring temperatures and high humidity caused by rainfalls plus the plant’s own moisture that allows the formation of wàter droplets on the leaves are key factor to perpetuate the attack in the vine. Mildew is a common spring disease that ceases with the high temperatures that come along with the summer time, temperatures that go beyond 28-30ºC, when the mildew attack ceases.
Within the defence strategies adopted to fight against mildew attack, we find differences between ecological and traditional agriculture. In the former one, a preventive defence can be implemented by preventing the spores from germinating and penetrating the leaves; while in the latter one allows the use of certain systemic compound when the attack has already occurred and can penetrate into the plant to eliminate the mycelium that has been formed. It should be noted that most vineyards, even if they are not registered as ecological, prefer a preventive defence. When the weather conditions are optimal for a fungi attack, the vineyards are immediately treated with Bordeaux broth, an active copper sulphate compound that leaves a blue-green discoloration on the vine leaves when treated. The leaves remain protected from the mildew attack as long as they are covered by the blue-greenish dust, so it is necessary to spread the treatment all over the vineyard again when they are washed off after a rainfall. These compounds cost around 6 euros per kilo and it takes over three hours to treat one hectare.
Fortunately, there have been only few major mildew attacks in recent years. This year we have yet to see what the spring season will be like. For time being, despite the recent rains and the increasing temperatures consistent with the current season, and in the meantime the constant air flows that dry the plants and the lack of heavy rains, it is quite possible that the attacks won´t be very significant.