The powdery mildew of the vine, also known as white mold or fog, is a disease of the wine and table vine. It is a fungus, Uncinola necator, Latin name of the etiological agent, imported from America during the 19th century. Together with downy mildew, powdery mildew is among the most serious problems afflicting the vine, opening the way to subsequent infections of other pathogens, such as botrytis.
Symptoms and damage
The powdery mildew can infect all the green organs of the vine causing the greatest damage on shoots, bunches and single grapes. The first visible symptoms on the leaves consist in small translucent yellowish stains, which, with the passage of time, can totally or partially affect the leaf surface, preventing its development and causing brownish necrotic ripples: the flaps are curved upwards, the leaf turns yellow and falls.
In correspondence of these symptoms, the leaf is covered by the characteristic whitish and powdery patina. The affected tissues are bleached and translucent.
Sprouts can be affected by their development, due to the growth of the latent fungus in the buds; the mycelium can completely cover the bud that takes on a whitish color with the leaf flap folded upwards, the so-called “flag leaves”.
The buds that are being formed can be colonized by the mycelium, in which they will whiten in a latent form. The inflorescences, as well as the berries just attached, are very susceptible to oidic infections and are rapidly covered by the mycelium of the fungus.
Recent epidemiological studies have shown that the clusters are more susceptible to the fungus in the flowering phase, while starting from the 4-6 mm size, this susceptibility tends to decrease with the continuation of the vegetative season.
The early colonized grapes remain small and are covered by the characteristic white-greyish efflorescence that gives off a strong moldy smell. The blockage of the growth of the epidermis and the simultaneous development of the pulp cause longitudinal splits that often highlight the grape seeds, indirectly favoring the attacks of gray mold and other fungus and saprophytic bacteria. The berries can then shrivel, dry and fall. The outcomes of late infections of the berry are highlighted by brownish cross-linking of the skin.
Conditions of development
• Temperature: the mycelium is in motion at 5-6 ° C, thermal optimum 25 ° C
• Relative humidity: low 25-30%
• Absence of rain: the rain spreads the conidia
• High windiness: removes the water droplets from the plant by transporting the conidia
• Poor lighting: UV rays reduce the vitality of conidia
• High acidity of the grape: sugar content <12-15%
• Strong growth fabrics.
It is possible to fight against the powdery mildew both with agronomic and chemical control methods. As far as uninfected vineyards are concerned, obviously preventive methods are used.
In the already infected vineyards, on the other hand, we try to limit as much as possible the damage done by carrying out a series of chemical treatments, for which there is a greater risk of selection of resistant strains.
Many treatments are necessary for the Peronospora, therefore, usually, we try to make the treatments for the two pathogens coincide as much as possible, taking into consideration that the various treatments require opposite predisposing conditions. In this way the residues on the product and the cost of the treatments are reduced to a minimum.
Agronomic control methods
• Remove the infected leaves from the soil containing the wintering cleistotecium;
• Evaluate plant density, exposure and form of rearing;
• Arranging and tying shoots;
• Carry out slight peeling around the bunches;
• Balanced fertilizations and irrigations;
• Avoid the use of vigorous rootstocks;
• Avoid excessive nitrogen fertilization;
• Avoid long and rich pruning (bundling <light);
• Eliminate powdery mildew screws.
Chemical control methods
• Intervening with cover products until pre-flowering, and from pre-flowering to veraison with systemic and covering products;
• Sulfur, especially in pre-flowering, to be distributed in the morning or in the evening to avoid phyto-toxicity;
• IBE (ergosterol synthesis inhibitors): triazoles, pyrimidines, pyridines;
• Spiroxamine (inhibitor of pathogen enzymes);
• Quinoxifen (conidia and spore germination inhibitor) as a preventive treatment.