Wines & Vines

May 2016 Packaging Issue

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82 WINES&VINES May 2016 GRAPEGROWING WINE EAST Benefits and Costs of Early Leaf Removal Researchers study ways to reduce bunch rot while maintaining yields By Bryan Hed and Michela Centinari T he development of bunch rot disease is very dependent on cli- mate, especially when there is frequent rainfall and high hu- midity during the ripening pe- riod—a common scenario in Pennsylvania and other parts of the eastern United States. In addition, ripening generally coincides with the onset of hurricane season, which can often deliver abundant rainfall and cloud cover throughout the eastern seaboard region at that time. Unfortunately, there's not much we can do about the weather, but there are ways to improve rot control in spite of that. Fungicides are an important part of bunch rot control programs. The ubiquitous fungus Botrytis cinerea is often the predominant cause of late- season bunch rots, and fruit rot control programs rely heavily on Botrytis spp.-specific fungicide applications made at bloom, pre-bunch closure, véraison and pre-harvest (Wilcox 2012). How- ever, pesticides are not always enough to main- tain commercial levels of control, and Botrytis spp.-specific fungicides generally carry a high risk of the development of resistance, making heavy reliance on them unsustainable. Furthermore, other organisms besides Botrytis are involved in the bunch rot complex (such as sour rot bacteria and other fungi) that are not well controlled by any pesticide options. Finally, growers are increasingly con- scientious about public interest in reducing chemical inputs in agriculture. Therefore, bunch rot control strategy in the eastern United States must integrate non-chemical methods to be most consistently effective. Conditions leading to bunch rot and possible solutions Research has shown that the compactness of grape clusters plays a major role in bunch rot susceptibility (see photo at lower left). Indeed, the more compact a grape cluster, the more likely rot is to develop in that cluster (Vail and Marois 1991, Hed et al. 2009), and once initi- ated, the more likely it is to spread quickly and severely throughout the cluster (Zitter and Wilcox 2004). Berry cuticle, known as an im- portant barrier to pathogens, was found to be thicker among berries grown in looser clusters with less berry-to-berry contact (Percival et al. 1993). Bloom "trash," a mix of spent caps, flowers and stamens that can act as a substrate for bunch rot pathogens after bloom (Nor- thover 1987), is less likely to be retained inside loose clusters, and when retained, is less likely to exacerbate bunch rot disease in loose clus- ters than compact clusters (Hed et al. 2009). Loose clusters may also improve pesticide spray penetration onto interior surfaces of clusters (Hed et al. 2011), which may improve pesticide efficacy—not only for control of Botrytis, but other diseases as well. The development of bunch rots is strongly influenced by the fruit-zone microclimate, which can be modified to reduce fruit suscepti- bility. For example, leaf removal around clusters improves the microclimate in the fruit zone by increasing air circulation and cluster sunlight exposure, thereby reducing fruit wetness peri- ods (English et al. 1989), and improving pesti- cide penetration into the fruit zone (see photos on page 85). The currently recommended tim- ing of this widely adopted practice in cool-cli- mate regions is generally from after fruit set to "pea-sized berries" stage in order to minimize potential reduction in yield and bud develop- ment at defoliated nodes (Candolfi-Vasconcelos and Koblet 1990, Wolf et al. 1986). Post-fruit set leaf removal does not address the pitfalls of compact cluster architecture. However, when fruit zone leaf removal is ap- plied earlier in the season—just before or at the beginning of bloom (pre-bloom; trace bloom)— most photosynthetically exporting leaves are removed, starving inflorescences of carbohy- drates during bloom and reducing the number of flowers that set fruit (Coombe 1959, May et al. 1969, Candolfi-Vasconcelos and Koblet 1990). This in turn reduces the number of ber- ries per cluster, cluster compactness and predis- position of clusters to bunch rots (Poni et al. 2006, Sabbatini and Howell 2010), combining the benefits of an open fruit zone with less disease-susceptible clusters (Hed et al. 2015). In the past 10 years, pre-bloom leaf re- moval has gained popularity in some Euro- pean countries as a practice for reducing crop level, improving grape composition and loos- ening clusters of high-yielding varieties such as Barbera, Sangiovese, Trebbiano and Tem- pranillo (Poni et al. 2006, Intrieri et al. 2008, Tardaguila et al. 2010). Recent studies con- ducted in Italy and Spain suggested that pre- b l o o m l e a f r e m o v a l co u l d b e a v ia b l e KEY POINTS Researchers in Pennsylvania studied multiple ways to increase cluster looseness in order to reduce bunch rot. Pre-bloom leaf removal was more consistent in reducing both the compactness of clusters and the potential for bunch rot. While leaf removal at trace bloom can be done me- chanically, it reduces the number of berries per cluster, and therefore reduces yield to some extent. Yield reductions depend on the percentage of leaf area removed, the variety and weather conditions. Cluster architecture plays a major role in bunch rot susceptibility. The small, overcrowded Vignoles cluster (left) is extremely susceptible, whereas the open, loose architecture of Chambourcin (right) generally renders it free of bunch rot in most years.

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