2.4.1.c Black rot

What are the symptoms of Black Rot on leaves?

Black Rot (Guignardia bidwellii) causes small regular spots (a few millimetres in diameter), reddish-brown on both sides with a dark brown border. Three to four days later, small black pustules, called pycnidia, appear on the upper surface and concentrically over the spot.
Developing leaves are more vulnerable than mature leaves that become resistant. The characteristic symptoms are referred to as "shotgun". Symptoms can also be seen on berries as illustrated below.

What are the symptoms of black rot on bunches?

Inflorescences are rarely attacked. The most susceptible period is from fruit set to bunch closure. Diseased berries become a tawny colour.
Shortly afterwards, the berries begin to shrivel and eventually dry out after three to four days. They then turn brown and mummify, becoming covered in numerous pycnidia, or perithecia, at the end of the season, giving them a rough appearance. The severity of the attack ranges from a few affected berries to completely dried out bunches.

What is the biology of the fungus?

Winter inoculum is stored on mummified berries in the form of perithecia. Very early in the spring, especially if the winter was rainy, these perithecia mature, releasing ascospores with the rain, which cause the initial contamination. Symptoms first appear after an incubation period of 10 to 25 days, depending on the temperature.
The pycnidia cause secondary contamination via pycniospores dispersed by the rain on all the organs within splash range (shotgun blast). The dispersal has a small radius, so the fungus has a relatively slow expansion compared to mildew and powdery mildew, and has localized outbreaks. Temperatures between 23 and 25°C and prolonged damp conditions favour the development of the fungus.

Extracts from Vigne Vin Occitanie.

What practices favour Black Rot? What are the preventive measures?

• Management of treatments or delays in treatments: treating only when visual symptoms appear may be too late when the plot's history indicates frequent outbreaks in previous years. Prophylactic solutions: Carry out the treatment programme on a preventive basis: at the post-budding stage by the synergy of sulphur and copper.

• Plot maintenance: leaving mummified grains on plots preserves the winter inoculum and risks contaminating the vine again the following year. Prophylactic solutions: remove dried (mummified) grains from the base of the vines during pruning, pull up abandoned vines, and remove any affected shoots from the plot.

What treatments are available to combat this fungus?

The resurgence of Black Rot attacks in recent years has led to a reconsideration of this disease in treatment strategies. These are usually based on the plot's vulnerability and level of contamination the previous year. In organic agriculture, as in conventional winegrowing, there is no specific treatment against Black Rot, only the synergy and the secondary effects of sulphur and copper (against powdery mildew and downy mildew) offer any protection against this fungus.

At what stage of the vines' development should treatment be started?

Black rot treatment depends on the vines' stage of development and degree of contamination the previous year. At the post-budding stage, it is a preventive treatment, at the flowering to bunch closure stage and then until véraison when symptoms exist, it becomes curative.

For normal situations, i.e. when there were no symptoms the previous year, and when weather conditions are unfavourable, it is advisable to treat when the first symptoms appear, i.e. spots on the leaves. On the other hand, if the vines were contaminated the previous year, it is more prudent to start the treatment earlier, when the perithecia are mature, at the leaf unfolding stage for example, if the weather conditions are rainy.

For plots at risk or when the symptoms are still visible, it is advisable to continue the treatment until the véraison stage.