Michigan Leaf Removal and Cluster Thinning Experiment

From taking the U.C. Davis online course, I was familiar with the advantages of leaf removal. I was, however, unaware of the timing and the removal of certain leaves as well as the advantages of cluster thinning. The article that I am reading now¹, was certainly a timely one since we project that our Chenin Blanc grapes are possibly at least two weeks behind in sugar accumulation.
In reading the article, I felt that I needed to go back to basics to get a better understanding on what leaf removal and cluster thinning can do for grapes.
My husband was also looking into cluster thinning and found an online article and pointed me to Crop Thinning: Cluster Thinning or Cluster Removal, written by Patty Skinkis.

• Leaf removal is an adjustment of total vine leaf area and affects the relationship between photosynthetically active leaf area and yield
• Improves the cluster microclimate
• Allows additional sunlight penetration
• Leads to higher cluster temperature
• Cluster/crop thinning is the removal of flower and/or grape clusters on the grapevine
• Affects vine balance (fruit to vegetative growth)
• Necessary for adequate ripening in cool climates
• Improves fruit, when applied with early leaf removal

In the March, 2017 issue of the American Journal of Enology and Viticulture, the Frioni et al. article, Leaf Removal and Cluster Thinning Efficiencies Are Highly Modulated by Environmental Conditions in Cool Climate Viticulture compared experiments done on Cabernet Franc in Michigan in 2011 and 2012, two years with different weather conditions. The following table summarizes the experimental conclusions contained in the article.¹

Treat-ment	Description	Results
C	No thinning, no leaf removal; ~85 clusters	Control
LR	no thinning, leaf removal; ~85 clusters	Cluster morphology and berry size were not changed, when applied at veraison Accelerates anthocyanin biosynthesis Can lead to reduced fruit set and altered cluster morphology, only when applied before or around bloom
TH	thinning, no leaf removal; cluster thinning was applied when clusters reached 50% berry color change, leaving ~50 clusters	Led to a yield reduction consistent with the lower number of clusters per vine at harvest Cluster morphology and berry size were not changed when applied at veraison Particularly effective right after veraison Improved homogeneity of sugars
TH + LR	thinning, leaf removal; cluster thinning was applied when clusters reached 50% berry color change, leaving ~50 clusters	In cooler years, TH + LR was the only treatment that promoted higher amounts of color compounds Accelerates anthocyanin biosynthesis Only method to improve fruit composition at harvest

From the experiments that were conducted, the authors speculated that it is possible that cluster thinning and leaf removal enhance anthocyanin accumulation and have a potential additive effect in cooler seasons. The only method to improve fruit composition at harvest was the dual treatment of leaf removal and cluster thinning. Hmmm.....wondering if cluster thinning might be in our future for our Chenin Blanc, currently with Brix coming in at around 15.
References:
1. Tommaso Frioni, Shijian Zhuang, Alberto Palliotti, Paolo Sivilotti, Rachele Falchi, Paolo Sabbatini, "Leaf Removal and Cluster Thinning Efficiencies Are Highly Modulated by Environmental Conditions in Cool Climate Viticulture", Am J Enol Vitic., March 2017.
2. Patty Skinkis, Crop Thinning: Cluster Thinning or Cluster Removal, July 26, 2012.