2024 Author: Gavin MacAdam | [email protected]. Last modified: 2023-12-16 13:38
White rot of grapes, also called hail disease, attacks mainly the ridges and berries of this delicious crop. True, sometimes it can affect shoots with leaves. Affected berries have a bitter taste and an unpleasant odor and often crack. And their surface is almost always covered with very unpleasant bumps of off-white shades. Gradually, the berries turn reddish-brown, wrinkle and dry out quickly. If white rot appears on grapes at the very beginning of berry ripening, then yield losses can reach 70%
A few words about the disease
The main symptoms of white rot develop on vines and berries. This usually occurs a few days after heavy rainfall has fallen. The ridges attacked by white rot begin to dry out slowly. The berries first turn yellow, and after some time they acquire a pinkish-bluish tint, the manifestation of which usually begins from the side of the peduncles. Infected berries always lose their turgor, but if they dry up or remain juicy - that's how lucky. Harmful pycnidia, ripening under the cuticle, slightly raise it above the surface of the berry epidermis. The result of such transformations is the appearance of tiny cavities between the epidermis and the cuticle. And the air penetrating into such cavities creates an optical effect that gives the infected berries a whitish color.
At the end of the growing season, the grape clusters attacked by the disease fall off along with the berries, turning into a source of infection that remains in the soil.
And if the upper parts of the main axes of the flowering shoots are attacked by a harmful misfortune, then the development of atypical symptoms of a destructive ailment may begin. The areas of the hands located below the infected zones begin to dry out rather quickly. And the berries in their lower parts first turn pale, and then turn brown, becoming very lethargic. At the same time, it is far from always possible to identify the pathogen on them. Pycnidia are not formed on such berries, since they begin to dry out even before the fungus penetrates into them. Often, such symptoms are mistaken for physiological drying out, which often occurs due to a deficiency of magnesium or calcium, as well as due to an imbalance in water balance.
Grape leaves are rarely affected by white rot. However, with a particularly severe lesion, they acquire dark green shades and dry out quickly. However, dried leaves do not fall off.
On non-lignified shoots, the causative agent fungus is capable of causing necrosis, but they can also be observed quite rarely. Basically, necrosis is formed if the grapes spread freely over the soil surface. At the same time, on the affected branches, you can observe ring-shaped specks of dark tones, sores with influxes and numerous longitudinal cracks.
The causative agent of white rot of grapes is considered to be the harmful fungus Coniothyrium diplodiella. It is especially active at temperatures from twenty-four to twenty-seven degrees and at a fairly high humidity. The pathogen spreads at lightning speed even after showers. In general, the entire life cycle of this fungus is divided into two main phases - a parasitic short phase, in which a mycelium forms on the plants, and a dormant long one, when the pathogen remains in the soil of vineyards in the form of stroma and pycnidia of the pathogen.
Overwintering of the harmful fungus takes place on fallen infected ridges and berries, as well as in cracks in the bark and on affected vines.
How to fight
The most important preventive measure against white rot of grapes is the complete exclusion of contact of the berry clusters with the soil. Good lighting is important for growing grapes. Also, the bunches should be well ventilated, and the affected areas of the bushes should be cut and burned.
Before flowering, grape plantings are recommended to be treated with a solution of potassium iodide or "Immunocytophyte". Such treatments after hail are especially important.
Contact fungicides, the composition of which is copper, can significantly reduce the spread of harmful scourge. Only the treatment with these drugs should be carried out no later than 18 - 24 hours after the first symptoms are detected.
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