How to treat the vine Black-rot

Grape Black-rot is a fungal disease that can persist for several years if left untreated.

It initially manifests itself as circular, yellowish lesions on young leaves. As these spread, they turn brown and give rise to fungal bodies resembling tiny peppercorns. As the infection progresses, the lesions may envelop the petiole of individual leaves, suffocating them. Finally, the fungus affects the shoots causing large, dark-coloured, elliptical lesions.

Originating in North America and arriving in France at the end of the 19th century, Black-rot then spread to the rest of Europe. In Italy it is not particularly frequent but, where it has been found, it has caused varying degrees of damage: in some cases it has led to a significant loss of production, in others it has not caused any negative consequences on either the quantity or the quality of the product. However, since in wetlands it is not so rare and can catch farmers unprepared, it is important to recognise it in order to know how to deal with it. 

Black-rot symptoms can be detected on the leaves, berries and the whole cluster.

Although the leaf symptoms are the most obvious, the real damage comes from the fruit symptoms. Often, in fact, the berries grow halfway through before they begin to show signs of infection: small brown lesions of the leaves begin to appear on the grapes, giving rise to soft areas that rot in a few days. What remains of the berry withers, turning into a tiny, hard fruit that resembles sultanas and is covered with fungal fruiting bodies. 

Depending on the part of the vine affected, Black Rot presents the following symptoms:

• Round necrotic spots appear on the leaves: from a few millimetres to a few centimetres in size, they have a brown body and purple-tinged edges. To understand whether it is indeed black-rot, you can look for the small dark pustules typical of the disease;
• The grapes are the part most affected by black rot: they dehydrate and mummify, taking on a light brownish hue. The typical black pustules also appear on them;
• Initially, the grapes affected by black-rot are isolated: however, they gradually grow in number, occupying the entire grape cluster.

Black-rot is not a disease that goes away quickly. On the contrary, both the black pustules (pycnidia) and the withered berries act as hotbeds and spread the infection, with the help of moisture, even light rain and watering. From the budding in spring until mid-July, the rain opens the asci contained in the perithecia, releasing ascospores which, carried by the wind, arrive on the leaves and fruit, giving rise to a primary infection (the incubation period can last from 8 to 28 days). Spots then appear on which the contagious pycnidia form, whose conidia are responsible for the secondary infections that originate throughout the warm season when it rains. 

The ideal conditions for the spread of black rot are temperatures between 20 and 26°C: 6-7 hours of wetting the leaves is enough for infection to occur.

But how is grapevine Black-rot treated? First of all, before treatment, it is necessary to talk about prevention. Aerating the foliage, removing infected grapes and bunches, as well as pruning residues, is essential. An anti-peronosporic (generally with copper) or anti-oidic (generally with sulphur) control strategy can then be implemented. Finally, products with difenoconazole, fenbuconazole, myclobutanil, penconazole, tetraconazole and trifloxystrobin are useful in combating ongoing infections.

The main issue, however, is hygiene. Although numerous fungicides are available for the management of Black Rot, the importance of hygiene conditions cannot be underestimated: proper cleaning can limit the amount of Black Rot inoculum in the vineyard. In particular, the removal of infected canes through pruning during the dormant season will reduce the level of inoculum during overwintering. However, the most infectious part of the vine is the grapes and clusters withered grapes, which may either fall to the ground in the vineyard or be retained by the canopy. Removing the mummies from the canopy, and the affected shoots, is a key step. Other preventive control tools are soil tillage before the vegetation regains and spring tamping after the first treatment against Black rot.

Even the best hygiene practices will, however, inevitably leave some inoculum in the vineyard, which makes the use of fungicides necessary. Since spore production peaks just before the flowering period, the best time to use them is from the days immediately before to two weeks after the start of flowering.