Powdery Mildew

This disease is caused by a parasitic fungus (called Oidium tuckeri or Erisyphe necator), which is specific to the grapevine. Practically all species of vines are vulnerable to this disease, though the American varieties and their hybrids are usually less vulnerable than Vitis vinifera. The fungus was introduced into Europe through the USA. It was first identified in Europe by a Mr Tucker of Margate in 1847. By 1851 it had spread throughout the continent. It was the first grapevine disease to require regular spray applications, and the use of resistant American varieties and their hybrids may have lead to the introduction of Phylloxera and downy mildew into Europe.

It is probably the most widespread vine disease, and has a significant effect on vine vigour, even causing crop loss in extreme cases. However, although difficult to cure when well established, it responds well to pesticides and so is quite easily prevented.

Symptoms

Powdery mildew attacks all green parts of the vine, but is mostly a parasite of young growing organs.
Young leaves curl in a characteristic way, and develop dull grey patches that become cobwebby when the organism sporulates. These cobwebby patches are easily differentiated from downy mildew as they appear on both sides of the leaf, and are easily rubbed off. When the infection is advanced, a musty odour emanates from the canopy. Petioles and tendrils suffer the same symptoms, but the damage caused is less serious.
Green shoots will often get infected, and this may cause them to grow in a curved fashion as the disease slows down growth. As the canes ripen, characteristic black patches appear.
Flower and flower stem infections are rare, but very serious, as they will cause crop reduction.
Young berries can be infected before veraison. They become covered with a grey-white velvet, and the small berries drop off. Even if the berries are subsequently cured of the disease, they are permanently scarred, and will split all the way down to the pips at veraison. These berries never ripen, and are prime targets for Botrytis (grey rot) attacks.

Biology

This fungus is made up of a mass of long tubular filaments that live on the surface of the plant and feed through suckers that pierce the outer cell layer. It is this activity that weakens the plant. Powdery mildew reproduces mostly asexually by putting out small hair-like structures called conidia, which shed enormous numbers of spores from their tips. The spores are spread by the wind, and will germinate to form new filaments under the right conditions. It does have a sexual method of reproduction, in which tough 'perithecia' are produced to enable the fungus to survive adverse conditions, but these are found very rarely in Europe.

The spread of the disease

In general, the organism overwinters inside dormant buds and so if there has been an infection the previous year, it will be present at budburst. Dormant buds are infected as they are formed, and so the earlier the previous year's infection, the greater the risk of disease. However, as the spores can be carried by the wind over very long distances, the disease may spread from another vineyard, as long as it is upwind.

The rate of growth of the fungus is largely determined by the temperature of the canopy. Growth starts at 5ºC, but it is very slow. The best temperatures for the fungus are between 21 and 30ºC, but it is killed by bright sunlight or temperatures above 35ºC.

The spores do not require rain to germinate; the slight humidity from mists and dews is enough. Trees casting shade, thick canopies, and rapid, vigorous growth will promote the development of the organism. Powdery mildew is therefore usually associated with warm, cloudy, but not necessarily rainy summers, and vines growing vigorously in a culture method that encourages leaf-bunching.

Control

The risk of powdery mildew can be reduced by any technique that reduces:

  • Leaf bunching
  • Humidity
  • Vigour

Sulphur sprays are very effective against the disease, they have a preventative, stopping and curing action. They work when the sulphur vaporises and so can form a 'cloud' of sulphur inside the canopy. The minimum temperature for sulphur to vaporise is 18ºC, while it is most effective above 25ºC. Sulphur will scorch vegetation above 35ºC. The smaller the sulphur particles, the more active they are, but the shorter they last. Sulphur is also active against Phomopsis (dead-arm), and red spider mite, and is a valuable plant nutrient.

Since the 1970s, new chemicals called DMIs (Demethylation Inhibitors) have been introduced, and they have proved very popular. Their main advantages are that they penetrate into the green tissue and so cannot be washed off by rain. They do not vaporise at high temperatures, and are not carried around in the plant, and so they must be applied carefully. Some resistance to these chemicals has been shown, and it is not recommended to apply more than three applications in one season.

The timing of spray applications is very important:

  • Because the disease is difficult to eradicate once it is established, it is best not to wait for symptoms to appear, but treat preventively.
  • Early applications are beneficial, particularly if the disease had appeared the previous year. Start at three leaves apparent, and repeat after each heavy rainfall, or every 10 days, if the risk is high. Use sulphur in hot weather, and DMIs in cold.