Bitter rot gives the berries a bitter taste that is detectable in wine. After flowering, the fungus infects the berry stem and remains latent until the berry is mature. Then the fungus rapidly invades the berry and sporulates in concentric circles, darkening and roughening the surface. Within a couple of days, the berries soften and easily detach. Berries that do not fall off shrivel up, similar to black rot-infected berries. The optimum temperature for infection is 82 to 86ºF (28 to 30ºC), but infection can occur at temperatures as low as 54ºF (12ºC). Fruit injury by insects, birds or cracking can cause bitter rot to spread rapidly throughout the cluster. The fungus invades wounds and overwinters in plant debris and bark of 1-year-old canes.
Bitter rot is common in southeastern growing regions.
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