Epidemiological Factors Affecting Bitter Rot Infection in Vitis vinifera L. in North Carolina

Abstract

Bitter rot, caused by the fungus Greeneria uvicola (Berk. & Curtis) Punith., is one of the most important fruit rot diseases that threaten the burgeoning winegrape (Vitis vinifera L.) industry in the southeastern United States. Epidemiological studies were conducted to examine the variation in aggressiveness among isolates, period of fruit susceptibility in V. vinifera, relative susceptibility of cultivars to bitter rot, and influence of temperature and duration of wetness on infection. Detached V. vinifera 'Chardonnay' fruit were inoculated with 10 isolates of G. uvicola obtained from fruit of V. vinifera, V. rotundifolia (muscadine grape), and a French-American hybrid. Isolates HCMD1 and HCMD5, obtained from V. vinifera grapes from a Maryland vineyard, were the most aggressive. Severity of disease on fruit inoculated with isolates collected from V. vinifera was significantly higher than with isolates collected from V. rotundifolia. The period of fruit susceptibility was distinguished by inoculating intact clusters of grapes in vineyards in Alamance Co. and Rockingham Co., NC, every 2 weeks from bloom until 2 weeks before harvest. Susceptibility of V. vinifera 'Merlot,' 'Chardonnay,' and 'Cabernet Franc' fruit increased from bloom until véraison in 2003 and from bloom until 2 weeks before véraison in 2004. The relative susceptibility of 38 cultivars and selections, including 23 V. vinifera cultivars and 5 French-American hybrids, was determined by inoculating and incubating detached fruit at 26°C. Fruit of V. vinifera were significantly more susceptible to infection by G. uvicola than French-American hybrids. V. vinifera 'Petite Sirah,' 'JB97-8-0-7,' 'MissBlanc,' 'Roussanne,' 'Mourvédre,' and 'Petit Verdot' were among the most susceptible to the bitter rot pathogen. V. aestivalis 'Cynthiana Norton,' V. vinifera 'Arkansas 1271' and 'Riesling,' and French-American hybrid 'Traminette' and 'Chardonel' were among the most resistant. Growth chamber studies also were conducted to examine the influence of temperature and duration of wetness on infection. Detached fruit of V. vinifera 'Cabernet Sauvignon,' 'Cabernet Franc,' and 'Chardonnay' were inoculated and incubated at 14, 18, 22, 26, or 30°C for 6, 12, 18, or 24 hours. The optimal conditions for infection of fruit by G. uvicola were a temperature of 23.7°C and 9 hours of wetness.