Utilizing Epidemiological Investigations to Optimize Management of Grape Black Rot

The severity of a plant disease epidemic is determined by the interaction of a susceptible host, a virulent pathogen, and the environment. However, these factors do not remain optimal for disease development over the course of an entire season. In addition to the vagaries of weather, the susceptibility of important host tissues and the availability of pathogen inoculum may be transitory. Furthermore, various cultural practices, including those that provide sanitation, may affect the severity of the outcome, as may variable fungicide programs. Human intervention must be timely and based on a comprehensive understanding of the epidemiology of the disease if we are to implement efficient and effective management strategies. Black rot of grape is caused by the ascomycete Guignardia bidwellii (Ellis) Viala & Ravaz (anamorph: Phyllosticta ampelicida (Englem.) van der Aa), which produces both ascospores and conidia in infected fruit that have mummified and overwintered in the vineyard. Ascospores are the principal form of primary inoculum, although conidia from overwintered mummies and cane lesions (2,12) also may contribute to primary infections. Conidia are formed within pycnidia in the resulting lesions and constitute the secondary inoculum. Both types of spores are released during rain events, reportedly throughout the spring and summer (3,4), and require free water for germination and infection (13). Although black rot does little damage to the overall health of the vine (10), the disease can lead to substantial fruit damage in humid production regions if not properly managed. Even when losses in yield are low, black rot still must be managed to prevent the build-up of inoculum for the following year. Cultural practices also may impact the quantity and quality of overwintering inoculum. For instance, mechanical hedging is a nonselective dormant pruning practice that can retain overwintering inoculum within the trellis, which otherwise would be selectively removed during hand pruning. Although some eastern vineyardists are using this technique to save on labor costs, preliminary evidence (1) suggested that without supplementary sanitation measures, it might exacerbate black rot, although this was not studied in the context of disease management programs. Application of fungicides is the primary tactic used to control black rot in most commercial vineyards. In the United States, the most effective materials for black rot management are the steroldemethylation inhibitor (DMI) fungicides, myclobutanil and tebuconazole. The strobilurin fungicides also provide very good control, and because some members of this class have the advantage of being active against Plasmopara viticola (downy mildew) in addition to Uncinula necator (powdery mildew) and G. bidwellii, they commonly are used during the bloom and early postbloom periods when control of all three diseases is critical. Prior to the start of this work, recommendations typically suggested fungicides be applied every 10 to 14 days from approximately 10 cm of shoot growth until the berries reach 8° Brix (percent soluble solids) (11), i.e., from approximately middle May until middle August in the Finger Lakes region of New York. This approach may require as many as seven or eight fungicide applications per season, and is based upon the largely unproven assumptions that (i) control of prebloom foliar infections is necessary to limit secondary inoculum to levels that are controllable on the fruit; and (ii) fruit remain susceptible to infection for up to 10 weeks after bloom. Although reduced spray programs based upon curative sprays following a forecasted infection event (13) have been tested successfully (3,6), technical constraints on rapidly applying postinfection sprays have limited the commercial acceptance of such strategies. Therefore, the objective of this research was to determine the critical period to fungicidally manage black rot and the factors that influence it, such as age-related resistance of grape berries, sanitation, inoculum dynamics, and weather. An additional objective was to examine and compare the postinfection and antisporulant properties of two commercial fungicides commonly used to control black rot, i.e., the DMI fungicide, myclobutanil, and the strobilurin fungicide, azoxystrobin.