Black Rot of Grapes: Symptoms and Control

What is black rot of grapes?

Black rot is a fungal disease of grapes caused by Guignardia bidwellii (also known as Phyllosticta ampelicida). The disease can compromise both grape yield and wine quality. Under warm and humid conditions, infections can spread rapidly, and severe outbreaks may result in crop losses of up to 100% if left unmanaged. The disease is particularly damaging because fruit infections continue to develop throughout the season, transforming healthy clusters into shriveled, unmarketable berries and severely reducing harvestable yield.

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Black rot damage symptoms

Black rot symptoms can appear on all green, expanding tissues of the grapevine. Recognizing symptoms early in the growing season is critical for timely management interventions.

Leaves

Leaf infections are typically the first visible indicators of black rot, appearing roughly 10 to 14 days post infection.

Initial stages: Symptoms show up as small, circular, tan or cream-colored spots that typically measure 2 to 10 mm in diameter.
Margins: As lesions develop, they become surrounded by a distinct dark reddish-brown margin.
Pycnidia formation: Within the older central parts of these lesions, tiny black fruiting bodies (pycnidia) become visible, often arranged in a black ring or cluster.

Shoots, leaf stalks and tendrils

Infected shoots, leaf stalks, and tendrils may develop the following symptoms:

Elongated cankers: Sunken, dark brown to black lesions form on affected tissues.
Discoloration and girdling: Lesions may completely encircle leaf stalks or stems, restricting water and nutrient transport. This can weaken plant tissues, cause shoot dieback, and increase the risk of stem breakage.

Berries and fruit infections

While leaf spots serve as an early warning sign, fruit symptoms typically result from infections that occur between flowering (anthesis) and bunch closure.

Early symptoms: Small, localized, light-brown spots appear on developing green berries.
Concentric expansion: Lesions enlarge rapidly, spreading concentrically until the entire berry is affected within a few days.
Mummification: Infected berries turn dark brown to black, shrivel, and become hard, wrinkled "mummies" covered with spore-producing pycnidia. These mummified berries often remain attached to the grape cluster.
Late-season infections: Berries infected later in the season may not fully mummify. Instead, they develop fruit rot on the vine, reducing fruit quality and marketability.

The black, dried mummified berries left hanging in the trellis or dropped are the primary overwintering structures for the pathogen.

Life cycle of Guignardia bidwellii

The black rot fungus survival and dispersal are tightly linked to spring and summer weather patterns.

Overwintering

The fungus survives the winter in infected plant debris, including mummified berries, infected canes and tendrils. These sources of inoculum may remain on the vineyard floor or attached to the trellis system.

Primary infection (Spring)

Spring rainfall triggers the release of overwintering sexual spores (ascospores). These spores are dispersed by wind and splashing water and infect young, actively growing tissues. Primary spore release typically peaks around the flowering stage of the vine.

Secondary infection (Summer)

Once infections become established, the fungus produces asexual spores (conidia) within newly formed lesions. These spores initiate additional infection cycles throughout the growing season.

Disease development

Disease development is favored by warm temperatures, frequent rainfall, and prolonged periods of leaf wetness. Under these conditions, infections can increase rapidly and spread throughout the vineyard.

Management of Black rot

Successful black rot management relies on prevention and early intervention. An integrated approach that combines sanitation, canopy management, regular monitoring, and biological control measures can significantly reduce disease pressure throughout the growing season.

Cultural practices and sanitation

Effective sanitation helps reduce the amount of overwintering inoculum available to initiate infections in spring.

Inoculum removal: Prune out infected canes and remove all mummified berries and clusters during the dormant season.
Pruning strategy: Avoid minimal-pruning systems and excessive mechanical hedging in vineyards with a history of black rot, as dense canopies can favor disease development.
Canopy management: Optimize airflow and sunlight penetration by selecting appropriate training systems, controlling weeds, and managing canopy density. A more open canopy dries faster after rainfall, reducing infection risk.

Monitoring

Regular scouting is essential for early detection and timely intervention. Monitor vineyards closely during high-risk periods, particularly from pre-bloom through bunch closure. Pay special attention to young leaves, shoots, tendrils, and developing fruit for the first signs of infection.