Vines with too much yield are often maturing too slowly or producing a fruit of suboptimal quality for wine. A typical practice to reduce yield and hasten maturity with high vigor vines consists of shoot thinning. The removal of undesired shoots reduces canopy density and crop load. However, if this is done too drastically, the vine compensates and produces longer shoots with more laterals while yield is reduced. The ratio of yield/wood index decreases as a result. How can a winegrower control vine vigor and excess yield without creating an imbalance between yield and wood production?
By reducing the amount of active leaves, defoliation of the first basal leaves before bloom reduces fruit set and consequently reduces yield. On top of that, defoliation has the benefit of decompacting clusters, reducing disease risk. Interestingly, since defoliation reduces yield and shoot production, the ratio “yield /wood index” does not necessarily decrease as a result. In fact, many authors have reported an improvement of wine composition in response to pre-bloom defoliation. During the 2017 XXth Giesco meeting in Mendoza, a very interesting talk given by Dr. Cesare Intrieri reviewed the consequences of mechanical defoliation on ripening profiles.
Today as bloom, is observed in many regions, we discuss the pros and cons of bloom time defoliation.
When performed before set, defoliation of young and very active leaves affects strongly vine photosynthetic activity. When performed after set but before veraison, defoliation of older leaves does not affect photosynthetic activity as much. Instead, it essentially modifies microclimate conditions during fruit maturation: providing more aeration and more light exposure to the cluster. Thus, according to its timing, defoliation will affect photosynthetic assimilation more or less.
Observations and results
Using Sangiovese, a team of italian researchers reported interesting results. They investigated the impact of canopy management techniques on yield and grape composition over 3 years. Among the different treatments, authors specifically studied the distinction between shoot thinning followed by a) defoliation before bloom ; b) defoliation pre veraison.
Authors observed that shoot thinning with defoliation pre-veraison produced the same yield and same fruit quality as shoot thinning only. However, shoot thinning with defoliation pre-bloom lead to significant differences: a strong yield reduction (by one-third!) and increased sugar concentration, which can be an advantage when sugar accumulation is too slow, as can be observed when vines yield is in excess. Moreover, after the third year, a “memory effect” was observed onto the fourth year since vine capacity was also reduced as a result (ie. less cluster/vine, lower main shoot length).
Authors concluded that, when vines grow under high vigor conditions, shoot thinning combined with pre-bloom defoliation offers “the strongest potential for long-term regulation of vine yield and grape quality”. Wine growers should consider this powerful strategy as a way to modify yield and the fruiting zone microclimate. The benefit from such practices under the conditions of the experiment is a reduction of yield loss due to rot infection associated with an improved grape quality.
It is important to note that yield reduction will not only be limited to one year but the following year as well. Furthermore, pre-bloom defoliation will increase sugar concentration which may be a challenge in the context of warmer temperatures leading to earlier sugar accumulation, as we discussed in a previous blog (here).
Perspectives and practical take home
More recently, other work has been reported to nuance those results and select the practice that best fit your need around bloom time.
- Zenoni and her team have investigated more specifically the general responses to defoliation over contrasted locations with Sangiovese as well as 3 other varietals. Their results suggest that defoliation effects may be weaker than environmental effects. Thus, the benefits of defoliation practice will be conditioned by environmental conditions.
- With cv. Merlot, a research team led by Sivilotti compared defoliation before vs. after bloom. Results confirm that pre-bloom defoliation improve wine color and reduce bell pepper flavor while reducing yield at the same time.
Overall, these results show that it is necessary to connect fruit ripening profiles with defoliation around bloom. Benefit of defoliation on wine quality and vineyard performance will vary according to your local environmental conditions.