Climate and pests dictate the cultivars that can be sustainably grown within a region, and management practices are used to achieve production goals within those cultivars. Two goals of vineyard and winery enterprises are to produce economical crop yields and consumer-preferred wines. Cultural practices used to achieve these goals vary by growing region. In humid, subtropical growing regions, such as in the southeastern United States, excessive grapevine canopy growth results in shaded leaves and fruit zones (Giese et al., 2015; Hatch et al., 2011; Hickey et al., 2016). The humidity of the southeastern United States macroclimate is intensified within a shaded fruit zone microclimate. Management strategies are implemented to increase grape cluster exposure by thinning dense canopies that can otherwise exacerbate rot incidence and severity (English et al., 1989; Hed et al., 2009; Hickey et al., 2018b; Wolf et al., 1986). Fruit zone leaf removal is used to decrease rot incidence (Hed et al., 2015; Smith and Centinari, 2019), increase spray penetration (Hed and Centinari, 2018), and promote the development of desirable (Bubola et al., 2017) and reduce the presence of undesirable (Ryona et al., 2008) wine sensory impact compounds.
Fruit zone leaf removal is conventionally implemented after fruit set and before bunch closure (Poni et al., 2006). Removing leaves from only the morning-sun canopy side (e.g., the east side of north/south-oriented rows) has become standard practice in the eastern United States, where the current recommendation is to retain an average of one to two fruit zone leaf layers (Reynolds and Wolf, 2008). In humid regions, more late-season bunch rots are observed in fruit zones with one to two leaf layers relative to fruit zones devoid of leaves (Bubola et al., 2017; Hed et al., 2015), even in ‘Cabernet Sauvignon’ (Hickey and Wolf, 2018), which is generally tolerant to late-season bunch rots compared with ‘Chardonnay’, ‘Vignoles’, and other white-berried winegrape cultivars. However, questions persist regarding optimal timing and degree of fruit zone leaf removal across cultivars and climatically unique growing regions. Optimal leaf removal method is dictated by the radiation and temperatures experienced within a region (Spayd et al., 2002; Tarara et al., 2008).
Leaf removal affects grape soluble solids, TA, and pH (Palliotti et al., 2012), which are important for wine alcohol, acidity, mouthfeel, and microbial stability. As berries are subjected to radiant heat with increased fruit exposure, TA generally decreases as a function of malic acid respiration (Lakso and Kliewer, 1975; Jackson and Lombard, 1993). In some regions, lower acidity may be desirable for the production of less astringent wines (Reynolds et al., 2006). Although best fruit zone management practice differs across climatically distinct regions and cultivars (Hickey et al., 2018a; Hickey and Wolf, 2019; Spayd et al., 2002; Tarara et al., 2008), removal of some leaves surrounding clusters can positively affect wine quality potential by increasing or decreasing several metabolites (Crupi et al., 2010; Hunter et al., 1991; Jackson and Lombard, 1993; Lee et al., 2005; Ryona et al., 2008).
Fruit set, berry size, and cluster compactness can be reduced when leaves are removed before bloom (Poni et al., 2006). The reduced fruit set has also been associated with an increase in skin thickness, skin-to-pulp ratio, and phenolics (Diago et al., 2012; Poni et al., 2006, 2009). Reduced fruit set results in a reduction in berry number per cluster, ultimately resulting in looser clusters. Although a decrease in cluster compactness can improve rot management (Hed et al., 2009; Sabbatini and Howell, 2010), prebloom leaf removal may not always result in superior rot management relative to post–fruit set leaf removal (Hickey et al., 2018b; Liggieri et al., 2018). Further, removal of excessive fruit zone foliage before bloom substantially reduces crop yield (Diago et al., 2012; Hickey and Wolf, 2018; Poni et al., 2006, 2009).
Post–fruit set leaf removal has numerous benefits. Like prebloom leaf removal, post–fruit set leaf removal increases airflow and pesticide spray penetration, leading to decreased rot (English et al., 1989; Hickey and Wolf, 2018; Wolf et al., 1986). The resulting fruit exposure can decrease TA, increase soluble solids, and balance pH compared with no leaf removal (Bavaresco et al., 2008; Bubola et al., 2017; Reynolds et al., 2007). Unlike prebloom leaf removal, which can drastically reduce crop yield (Sabbatini and Howell, 2010), post–fruit set leaf removal maintains crop yield (Hickey and Wolf, 2018; VanderWeide et al., 2018). Post–fruit set leaf removal may offer greater economic sustainability relative to prebloom leaf removal.
Best leaf removal practice should be based on previous findings, optimized for vineyard production goals, and refined for specific cultivars. Further investigation of best fruit zone management practice is required in regions where no formal leaf removal studies have been conducted, such as in the U.S. state of Georgia. The present study evaluated the effect of different leaf removal regimes on crop yield, rot incidence, rot severity, and primary fruit composition of ‘Chardonnay’ grown in north Georgia, a humid, subtropical region. We hypothesized that prebloom leaf removal would reduce crop yield and that leaf removal to the greatest extent would reduce bunch rot and juice TA.
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