Black Rot

BLACK ROT
Description
Symptoms
Disease Cycle
Control

Description:
Black rot is the most common and destructive disease of bunch grapes in the eastern United States. Most varieties of vinifera, French/American hybrids and American bunch grapes are susceptible, while varieties of muscadine range in disease susceptibility from resistant to very susceptible (Table 1). Crop loss due to black rot can range from 5 to 80% (Fig. 1), depending on weather conditions, level of inoculum, and susceptibility of the cultivar.

Symptoms:
Black rot disease affects leaves, shoots, tendrils and fruit of grapevines. Leaf spots on both bunch grapes and muscadines are characteristic tan, circular lesions with small black fruiting structures (pycnidia) scattered within (Fig.2). Infections on young shoots, tendrils, and petioles first appear as small dark lesions that later develop into elongated, often sunken lesions. Elongated black cankers may develop on shoots, and can eventually girdle them, causing a shoot blight. Fruit symptoms vary between muscadine and bunch grapes. On muscadine fruit, infections appear as small, black, superficial scabby lesions, which may coalesce to form large superficial lesions that exhibit a crusty or scabby appearance (Fig. 3). Fruit do not mummify, but pycnidia develop within the lesions. On bunch grapes, lesions are initially small and scabby but as they expand they become sunken (Fig 4). As the entire fruit becomes colonized, it turns light brown in color, and begins to shrivel (Fig. 5). It eventually turns dark brown and numerous black pycnidia develop over the surface. Eventually, the fruit becomes dry and shriveled, turning into hard, blue-black mummies (Fig.1 and Fig. 6).

Disease Cycle:
Black rot on bunch grapes is caused by the fungus, Guignardia bidwellii. On muscadine grapes, a distinct race of the fungus, G. bidwellii f. euvitis causes black rot. The disease cycle of both fungi is similar. Guignardia bidewellii overwinters on stem cankers, on clusters left hanging on the vine, and on mummified fruit on the soil (bunch grapes only). During spring rains ascospores and/or conidia are ejected and carried by rain and wind to leaves, blossoms and young fruit. Lesions may develop on all young, green tissues when temperatures and duration of leaf wetness are favorable for infection. Infection may occur after 6 hours of wetness at 81F, but at 50 F 24 hours of wetness is required, and 12 hours at 90 F. Very little infection occurs above 90 F. Pycnidia, produced within the lesions, continue to release conidia during wet weather throughout the season. Fruit are most susceptible to infection by the fungus from mid-bloom to about 6 weeks after bloom, and become resistant to infection at maturity.

Control:
Cultural control - An important consideration in black rot control is the amount of overwintered inoculum left on the trellis and ground from the previous season. Mummified fruit and infected canes are the major source of primary inoculum for early season infections, and should be removed from the vine and vineyard floor before spring arrives; mummies may be disked into the soil. Good canopy management practices are essential for control of black rot. Shoot thinning, leaf removal, pruning, cluster thinning, and shoot positioning are all cultural practices that open the vine canopy to air and light, reducing the amount of moisture trapped within the canopy, and allowing better penetration and spray coverage of biological or chemical fungicides.

Chemical control - Refer to the Winegrape Spray Program in the North Carolina Agricultural Chemicals Manual for current recommendations for black rot control on bunch grapes http://ipm.ncsu.edu/agchem/chptr7/706.PDF or muscadine grapes http://ipm.ncsu.edu/agchem/chptr7/707.PDF . Fungicide applications for black rot control are most critical in the prebloom and first two postbloom sprays. In vineyards where black rot is a problem, it may be necessary to initiate fungicide treatment 2 weeks earlier. Control of primary infection of fruit and leaves early in the season will minimize the need for additional fungicide protection later in the summer. Fungicide sprays should be timed to assure coverage during periods conducive to disease development. Wetting and temperature requirements necessary for infection to occur have been defined for black rot on bunch grapes (Spott's Chart Table 1).