Black rot of grapes

Guignardia bidwellii (anamorph Phyllosticta ampelicida) is an ascomycete fungus belonging to the order Botryosphaeriales and is the causal agent of black rot, one of the most destructive diseases affecting grapevines globally. This pathogen specifically targets the tissues of the vine, leading to significant yield losses if left unmanaged.

The fungus impacts all aerial parts of the grapevine, including leaves, shoots, petioles, and berries. On leaves, symptoms appear as reddish-brown circular spots. The most damaging effect is observed on berries, which shrivel, turn black, and develop small, pimple-like structures called pycnidia, eventually hardening into mummified fruits that remain attached to the cluster.

The lifecycle of Guignardia bidwellii involves both sexual and asexual reproduction. It overwinters as mycelium or pycnidia in mummified berries and infected canes that persist on the vine or fall to the vineyard floor. In spring, under warm and wet conditions, spores are forcibly ejected or washed out, serving as the primary inoculum for new infections on young tissues.

Environmental factors are critical for the spread of this fungus. High relative humidity, persistent rainfall, and temperatures ranging from 20°C to 28°C are optimal for spore germination and rapid infection. The pathogen thrives in canopy environments with poor air circulation, where foliage stays wet for prolonged periods after rainfall, facilitating disease establishment.

The impact of black rot is severe, causing not only the total loss of individual berries but also a reduction in overall vineyard productivity and vine vigor. Management requires a multi-faceted approach centered on cultural practices that minimize moisture retention and chemical strategies utilizing fungicides during critical growth stages, specifically from bud break through veraison.

• Strict removal of mummified fruits and dead wood during winter pruning.
• Canopy management to ensure adequate sunlight penetration and airflow.
• Regular fungicide applications, focusing on the period from early spring to late summer.
• Sanitation by disposing of or burying infected plant debris.
• Monitoring weather patterns to time preventive sprays before predicted rain events.