Bitter rot of grape

Bitter rot is a destructive fungal disease caused by the pathogen Greeneria uvicola. It is particularly known for infecting grapevines, though similar fungal complexes affect a wide range of fruit-bearing crops including apple trees, pears, and cranberries. The disease is characterized by rapid decay of the fruit as it approaches maturity.

The fungus is highly aggressive in humid environments. It typically invades the fruit through microscopic skin breaks caused by insect feeding, hail damage, or mechanical trauma. Once the pathogen enters, it colonizes the tissue, leading to the rapid breakdown of berries or fruit flesh, which is often accompanied by an unpleasant bitter taste that renders the harvest inedible.

Symptoms of the infection include the development of light-colored, water-soaked spots that eventually darken. A key diagnostic feature is the appearance of small black pycnidia (fruiting bodies) erupting through the epidermis of the affected fruit. As the infection progresses, the entire cluster or fruit can shrivel and turn into hard, mummified remains.

Environmental conditions such as frequent rainfall and high humidity are critical for the spread of Greeneria uvicola. Spores are dispersed primarily by rain splash from infected overwintering structures, such as dried berries and old rachises (cluster stems) left on the trellis or on the ground. Management of these sources is vital to breaking the infection cycle.

Effective control strategies require a multi-faceted approach. Key practices include:

• Stringent sanitation by removing and destroying all mummified berries.
• Maintaining open vine canopies to promote air circulation and rapid drying of foliage.
• Applying fungicide sprays at critical stages of fruit development.
• Controlling insect pests to reduce entry points for the fungal spores.