Name for two very dangerous vine diseases caused by fungi. They originate from North America and were only introduced into Europe in the second half of the 19th century with contaminated vine material. Both fungal species are biotrophic parasites, which means that they feed on living cells of the infected host. The two powdery mildew diseases are often confused, not so much because of the disease symptoms, which are quite clear, but because of the confusingly similar names. There are plant-specific mildew fungi, for example for apples, peas, cucumbers, roses, spinach and vines. The fungi are strictly host-specific, which means that they can live exclusively on their host.
The two diseases are combated in the classical way with sulphur (powdery mildew) and copper sulphate or Bordeaux broth (downy mildew). Increasingly, however, special fungicides or plant strengthening agents are also being used. Control must often be carried out several times during the growing season. When crossing new breeds, importance is nowadays also attached to resistance to both types of fungi. It should be noted that some species of ladybird, which are among the most important beneficial insects in viticulture, feed exclusively on mildew. However, this is of no importance when it comes to control in the vineyard.
Powdery mildew is also called "Oidium" or "Oidium tuckeri" after the gardener William Tucker, who first discovered the fungus in England in 1845. The pathogen causing the disease belongs to the tubular fungi (Ascomycota), the botanical name is "Erysiphe necator var. necator" or also "Uncinula necator var. necator". The fungus was identified and described in North America as early as 1834. It was probably introduced to Europe via England in the early 1840s and subsequently spread rapidly across the entire continent. This, together with phylloxera, which also originated from North America a few years later, led to a real catastrophe in European viticulture. Large parts of the vineyards were destroyed in many countries. In 1854, the damage caused by powdery mildew in France meant that only a tenth of the normal quantity could be harvested. Later, two other plagues originating in North America were added, namely downy mildew and black rot.
Powdery mildew attacks all green parts of the vine, especially on hot days with cool nights. It prefers dry conditions, which is why it is also known as a "fair weather fungus". However, it does not like a sunny location, so a dense canopy of leaves promotes its development. The transparent, cobweb-like network (mycelium = sum of all hyphae) covers young shoots, leaf surfaces and, depending on the time of infestation, also the petals and the still green, unripe berries. After about two weeks, grey-white, flour-like spores appear. The leaves look as if they have been dusted with flour or ashes. This is why the disease is also called "Aeskerich".
The fine white threads (hyphae) send suction organs into the epidermal cells of the vine. Here the nutrients are absorbed which the fungus needs for its nutrition. This results in growth disturbances and curvature of the infested parts, which in extreme cases can cause the leaf to die prematurely. In the case of severe infestation, the entire shoot is discoloured violet. If this occurs before flowering, fruit set and yield are severely impaired. Berry development is slowed down, the berries burst open and dry out. The spores are blown away by the wind and the disease spreads quickly. The wintering of the fungus takes place as mycelium between the bud scales. Wine made from infested grapes has a typical unpleasant smell and mould taste, so selection is recommended.
It took more than ten years before sulphur pollination was recognised as an effective antidote. It is not clear who had the groundbreaking discovery, as there are several people who are claimed by their countries to be "inventors". One of them is the Frenchman Comte de la Vigne, whose vineyards in the Médoc were heavily infested. He developed the solution in lengthy experiments. From 1857 onwards, sulphurization was then generally used in Bordeaux. A second is the Austrian nobleman Ludwig von Comini (1814-1869), who completed his research with the same result in 1865 and was therefore aptly nicknamed "sulphur apostle". In dry climate sulphur dust is used, in rainy, so-called net sulphur. For preventive control, the vines are given an airy education.
Certain species of American vines are largely resistant, but many European vines are susceptible to this (without grafting with American rootstock). In particular, these are the grape varieties Cabernet Franc, Colombard, Chardonnay, Chenin Blanc, Elbling, Kerner, Mazuelo, Müller-Thurgau, Blauer Portugieser, Scheurebe, St. Laurent, Silvaner and Trollinger. On the other hand, the varieties Aramon Noir, Cabernet Sauvignon, Cot, Merlot, Syrah, Tempranillo, Pinot Noir and Riesling possess a certain resistance.
Thirty years later, downy mildew, introduced from America to southern France around 1878, followed. Other names are "peronospora" or "vine peronospora", or occasionally also "leaf fall disease", "drought ring" or "leather berry disease". The pathogen belonging to the egg fungi (Oomycota) is called "Plasmopara viticola". The fungal lawn is similar to powdery mildew, which is why, after initial confusion with the "real" one, it was called "false". The fungus was identified in 1878 by the French botanist Jules Émile Planchon (1823-1888). In only ten years it spread throughout Europe.
In contrast to the real one, downy mildew ideally needs humid conditions. The appearance and growth is particularly favoured by spring thunderstorms with heavy rain or warm and humid weather. The fungus therefore occurs mainly in Northern European countries with such a climate. On the upper side of the leaves, circular yellowish spots resembling oil stains first appear. The fungus breaks down the chlorophyll, causing the leaves to lose their green colour, become yellowish and translucent and wither (hence the name Dürring). In contrast to powdery mildew, the mouse-grey-blueish, downy mildew only forms on the underside of the leaves in compact areas.
It consists of many spores, which are attached to a small stem like a tree. These continue the cycle, which can be repeated about eight times a year. The aggressive spores penetrate very deeply into the host tissue with their flagella via the stomata. The mycelium spreads through the tissue. The inflorescences (glumes) and fruits can also be attacked. This causes a complete loss of foliage (hence leaf fall disease) as well as small, shrivelled and leathery berries (hence leather berry disease) and impairs the wood ripeness of the shoots. The spore containers (sporangia) are carried by wind and rain on other plants, so that the fungi spread very quickly in a vineyard.
The fungus hibernates as a winter spore in the fallen leaves and leather berries and ripens in spring. In 1885 it was first controlled using the copper lime broth developed by the botanist Alexis Millardet (1838-1902). The agent was first used successfully on a large scale in Bordeaux, from which the popular term Bordeaux broth was derived. Many European vines are susceptible to downy mildew, especially Chasselas, Müller-Thurgau and Blauer Portugieser. The American species Vitis cinerea var. helleri, Vitis rotundifolia and Vitis rupestris have a good resistance to complete resistance.
Powdery mildew on the left: © André Mégroz
Powdery mildew on the right: © Christoph Hoyer
Downy mildew: © Christoph Hoyer