Fruit and Vegetables

Vine downy mildew

What is the vine blight

This is a disease caused by the plasmora viticola fungus, native to North America and widespread in Europe since the 19th century. The microorganism belongs to the category of oomycetes, which generate spores that are formed during the vegetative cycle of the plant. The germination of these spores remains dormant during the winter period and then completes as soon as the weather conditions permit. Once their development is completed, the oospores emit zoospores that are responsible for the actual infestation of the vines. As mushrooms, the zoospores proliferate in climatic conditions of humid heat or at least when there are abundant rains. The shoots must be at least 10 centimeters long to allow their displacement on the film of water through a couple of flagella. If the shoots are smaller they do not have the space necessary to reach the stomatal openings of the plant where they generally nest. Through the emission of a tube, the pathogen sucks the lifeblood of the plant it uses as nourishment, causing its death.

Downy mildew attack on vine leaves

The first symptoms of the development of this type of pathology are evident in the upper part of the leaf. These in fact have spots, three centimeters in diameter, with irregular contours. They are located on the margins of the leaf and have a yellowish color, hence the name oil stains. These bleached parts tend to become black because necrosis begins, unless the environment is particularly wet. In this case, in fact, the spots will be whitish, a condition that highlights the formation of the reproductive structures of the fungus. If the climatic condition is optimal, it will be possible to have both necrosis and whitish efflorescence, without the formation of oil stains, which signifies the very advanced state of the disease. In this case the leaves fall early, unless they are old leaves or plants particularly resistant to pathogenic attacks. The branches instead suffer attacks on the nodes, becoming a purplish color. At this point, there are slits from which the whitish efflorescence emerges, and the branches take an unusual S-shape.

Downy mildew symptoms and damage on vine bunches

Bunches are also interested in infestation of vine blight. In fact, after having attached the branches, the spore is transferred to the berries, causing rather noticeable bruises. Subsequently whitish efflorescences develop as on the leaf and the fruit dries up. After the proliferation of the fungus, the bunches know two different syndromes: the gray rot and the brown rot. The first is typical of still unripe clusters, with small grapes and stalks still in a vegetative state. It begins with a dark coloration of the berries to which the whitish fructification of the fungus takes over, causing the typical gray shade of the disease. The older bunches are instead affected by brown rot, which takes its name from the dark color that the bunch takes and from the lack of fructification of the fungus. Since the stomata from which the fungus proliferates have lost part of their functionality with the age, the fungus remains trapped within them, and is not able to produce fruiting. However, the bunch does not receive nourishment and takes on a brown color.

Vine downy mildew: How to defend yourself

The causes that contribute to the proliferation of vine blight they can be the most diverse: the exposure and the place of the vineyard, the altitude, the presence or absence of humidity. A careful evaluation of the composition of the soil, the choice of notoriously more resistant rootstocks, the correct execution of cultivation techniques can be actions able to prevent the onset of the disease. Furthermore the development or not of the infestation is strictly connected to the type of climate in which the vineyard is located. A constant meteorological survey ensures that precautions can be taken in time. The method that is generally used to prevent vine blight It is the so-called three 10 rule. The outside temperature must be at least 10 degrees, the gems must not exceed 10 centimeters, the rainfall in the last 24 hours must not exceed 10 mm. The greatest guarantee is given by a constant treatment with chemical products able to create an unfavorable environment for the fungus, agents easily available in specialized shops.