Bitter Rot

BITTER ROT
Description
Symptoms
Disease Cycle
Control

Description:
Bitter rot can be a problematic disease of grape in warm, humid regions, causing from 10-30% loss of ripening fruit. All Vitis spp. are susceptible. Diseased fruit develop an unpleasant, bitter taste, which may ultimately affect the quality of wine produced and/or the ability to market the crop.

Symptoms:
The bitter rot pathogen infects all above-ground vegetative parts of the vine. Leaf infections occur as tiny, sunken reddish-brown flecks with yellow halos (Fig.1). Lesions on stems and petioles are round to elliptical, slightly raised, and reddish-brown to black in color. Flecking of the sepals and blighting of the flower buds can also occur. Infected grapes soften and become completely covered with concentric rings of fruiting bodies known as acervuli (Fig.2). Light colored fruit often turn brown (Fig.3), while dark colored fruit appear roughened and sparkly when acervuli develop (Fig.4). Infected fruit may abscise, or may dry into mummies and stay firmly attached. Bitter rot is often confused with black rot, but the black rot pathogen primarily infects immature or green fruit before veraison while the bitter rot fungus infects fruit at maturity.

Disease Cycle:
The fungal pathogen, Greeneria uvicola (syn. Melanconium fuligineum), overwinters on plant debris, canes, and mummified fruit. In the spring, gelatinous masses of spores (conidia) are exuded from acervuli and carried by rain to all green parts of the vine, including the pedicels. The pathogen invades the pedicels and becomes latent, or inactive, until fruit mature. In the weeks leading up to harvest, the pathogen grows from the pedicels into the ripening fruit, causing them to rot and eventually become completely covered with concentric rings of fruiting bodies. Secondary infections can occur when conidia from infected grapes are rain splashed to fruit that has been mechanically wounded by birds, insects, or hail, or is weather-cracked due to heavy rains.

Control:
Cultural - Good weed control and canopy management practices, including pruning, leaf removal, and shoot positioning, will promote air circulation and light penetration to improve drying of leaves and clusters, and will result in a less favorable environment for bitter rot development. Sanitation is also important for management of bitter rot. Diseased canes, mummified fruit and other debris on the vine and ground are over-wintering sites for G. uvicola. Thus, it is essential to prune and carefully dispose infected plant material during the dormant season to reduce the inoculum carried over to the next season. Muscadine cultivars vary in susceptibility (Table 1). Little is known about the susceptibility of cultivars of bunch grapes to bitter rot (Table 2).

Chemical - Refer to the Winegrape Spray Program in the North Carolina Agricultural Chemicals Manual for current recommendations for bitter rot control on bunch grapes (http://ipm.ncsu.edu/agchem/chptr7/706.PDF) or muscadine grapes (http://ipm.ncsu.edu/agchem/chptr7/707.PDF). Successful management of this disease involves protecting fruit with fungicides during periods of warm wet weather, which are favorable for infection. The spray program devised to control black rot will help to manage early season bitter rot activity, but to prevent fruit infections and subsequent rot, late season and preharvest fungicides should be applied.