Oidium and downy mildew are two dangerous diseases threatening grapevines in various regions of the world. Since the 19th century and to this day, they have caused many problems for grape growers on both large and small plantations.
These diseases—true and false powdery mildew—manifest with similar symptoms, although the reproduction and spread of their pathogens occur under different conditions: oidium develops most rapidly in dry hot weather and during sharp fluctuations in temperature and humidity, while downy mildew "operates" in shady areas with high humidity and feels best at a comfortable 25°C.
The diseases come in waves, which may alternate with changes in the weather. The first symptoms of infection often appear as early as late spring to early summer.
Despite the differences, both diseases are equally dangerous for vineyards, as they can destroy most of the crop in just a few weeks and, in the near future, the plants themselves. Therefore, upon detecting the first symptoms, one should immediately begin treatment and restoration of the plantings.
How oidium spreads
Oidium, or true powdery mildew, is a specific disease that affects only grapevines. It is caused by the pathogenic fungus Erysiphe necator, which by its nature harms only grapes.
The spores of the pathogen overwinter between the scales of grape buds, on the surface of the vine, and in fallen leaves. It is believed that extreme frosts in the range of –20 to –25°C destroy the pathogen, although cases have been described where the fungus survived even at –30°C.
With the arrival of spring, when grape buds begin to awaken and grow actively, primary infection of young leaves and shoots occurs. Favorable conditions for the development of the pathogen are a combination of high temperatures and low air humidity. During hot weather, when the temperature reaches 25–35°C, the foliage and shoots of the grapevine slightly wilt, losing turgor, which facilitates the penetration of the pathogen into the tissues.
The disease spreads quite quickly thanks to the wind, which carries spores between plants, leading to large-scale infection of the entire vineyard.
Signs of oidium on grapes
Oidium spares no part of the grapevine, damaging young shoots and leaves, flowering inflorescences, and developing berries, manifesting differently on each. To detect infection in time, it is important to regularly inspect grape bushes.
Grape leaves are the first to signal a problem. A characteristic white coating appears on their upper surface, resembling flour. Gradually, it spreads to the lower part of the leaf, eventually acquiring a grayish tint. In severe cases, the leaf blades turn yellow, lose vitality, but do not fall off.
Young shoots also suffer from powdery mildew: they are covered with a white coating, which, as the disease progresses, is supplemented by dark spots. These gradually merge, leading to the death of affected tissues. Normal shoot growth is disrupted, and the lignification process occurs unevenly. Such weakened branches slow down in growth, form unhealthy "curly" leaves, and in winter these shoots are especially vulnerable to frost.
Grape flowers under the influence of oidium stop developing. Inflorescences are covered with a white coating, gradually dry out and fall off, not having had time to produce future fruits.
On berries, the disease manifests differently depending on the stage of ripening. Young fruit sets the size of a pea, containing little sugar, quickly become covered with a coating and dry out. In more mature berries, the disease causes deformation and cracking of the skin, exposing the seeds. During ripening, other symptoms may also be noticed: the appearance of dark spots under the skin, the formation of a net-like pattern, and the development of corky areas during natural coloring of the berries.
Causes and conditions of downy mildew infection
Downy mildew, or false powdery mildew, is one of the most dangerous diseases of grapevines. The pathogen is a special fungus-like organism Plasmopara viticola, which belongs to the Peronosporaceae family and is a close relative of blue mold.
This pathogen appeared in European vineyards in the 19th century when spores of the fungus were brought from North America along with imported vines. In a favorable climate, the pathogen reproduced rapidly, leading to widespread disease.
Today, downy mildew can be found in almost all regions where grapevines are cultivated. The only exception is Central Asia with its dry climate, which is not suitable for the pathogen's reproduction.
The mechanism of vine infection is deceptively simple. Spores overwinter in fallen leaves and the upper layers of soil. As soon as warm days with high humidity arrive, the pathogens become active and begin attacking the above-ground parts of the plant, destroying the chlorophyll contained in them, which leads to the gradual death of tissues.
Symptoms of downy mildew on grapes
Downy mildew manifests on grape leaves with characteristic signs that are important to notice at an early stage of the disease. Large spots appear on the leaf blades, resembling oil stains. Their color ranges from yellowish to reddish, and the shape of the spots depends on the age of the leaf: on young leaves they are round, and on mature ones – elongated with uneven edges.
The speed of disease spread directly depends on weather conditions. In warm, humid weather (temperature 25°C and humidity 85%), the pathogen reproduces most rapidly. The underside of the leaf gradually becomes covered with mycelium in the form of a silvery-white coating, which is clearly visible after the foliage gets wet.
Deprived of normal nutrition, the leaves first turn brown, then dry out and fall off. The fungus actively develops, covering young shoots, buds, inflorescences, and unripe fruits with a coating (interestingly, on large berries the coating does not appear, but they become mummified and unsuitable for consumption).
The threat to the vineyard from downy mildew is extremely high. Under favorable conditions, the fungus reproduces at an astonishing rate – up to 15 generations of the pathogen can form in one season.
The consequences of infection go far beyond "cosmetic" defects:
- the disease weakens the grapevine, which negatively affects fruit quality – sugar content decreases, and the fruits acquire an unpleasant taste;
- yield is significantly reduced – losses can reach 50%;
- affected leaves become easy prey for pests;
- infected shoots cannot be used for cuttings, which creates problems for propagation;
- weakened bushes lose winter hardiness and often die during the winter.
Treatment and prevention of fungal diseases of grapes
The main method of preventing grapevine infection with true and false powdery mildew is strict adherence to agricultural practices when cultivating this crop:
- purchasing healthy seedlings from trusted nurseries;
- planting vines in suitable areas – elevated, well-lit, without water stagnation;
- sparse planting and soil mulching;
- observing watering and fertilization schedules;
- balanced application of nitrogen and potassium-phosphorus fertilizers;
- timely pruning and pinching of the vine;
- tying plants to trellises.
Preventive measures also include regular site maintenance: timely removal of weeds and plant debris, deep digging of the soil in the trunk zone in autumn or early spring.
Preventive treatment of the vineyard against oidium and downy mildew is carried out in early spring – after the leaves reach a length of 2.5–3 cm, but strictly before flowering. It is important to act before optimal conditions for the development and reproduction of the fungus occur. For spraying, copper-containing fungicides can be used: Bordeaux mixture, OxyHOM. Systemic preparations such as Chorus are also suitable.
To maintain the result, it is recommended to carry out at least one repeat preventive treatment of the vineyard in summer, after flowering. For spraying during fruiting, it is safer to use biological pesticides (e.g., Fitosporin-M).
If the first signs of fungal infection are detected in the vineyard, urgent measures should be taken to protect the vine. At the initial stage of infection, a solution of potassium permanganate (5–7 g of substance per 10 liters of water) can be used, thoroughly spraying the grape leaves. If the first spraying does not yield results, a repeat treatment is allowed after a few days with an increased dosage: 10–14 g of potassium permanganate per 10 liters of water.
If more noticeable signs of powdery mildew appear, the bushes should be treated with contact fungicides containing copper (1% Bordeaux liquid, copper oxychloride, Abiga-Peak). It should be remembered that copper preparations are quickly washed off by precipitation, so repeated treatments are required: 7–10 days after the first, then every 7–10 days until symptoms disappear completely.
Instead of copper-containing products, it is possible (and even better!) to use combined agents based on triazoles. Good results in the fight against fungal diseases of grapes have been shown by preparations containing penconazole (Topaz), strobilurins (Strobi, Provizor). And of course, the classic treatment for oidium is the sulfur-containing fungicide Thiovit Jet. The working solution is prepared by diluting 30–50 g of the substance in 10 liters of water, and spraying the plantings at a rate of 10 liters per 100 sq.m. Since the waiting period for the product is only one day, it can be used for treatment throughout the season up to harvest time (usually 4–6 treatments are required).
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