Wine grape yields not affected by early leaf removal

Leaves on this vine were removed prebloom. (Courtesy of Washington State University)

In eastern Washington State, leaf removal in wine grapes is typically done between fruit set and bunch closure to open up the canopy, reduce disease pressure, and improve fruit quality.

But is that the best time?

Washington State University researchers studied fruit zone leaf removal in eastern Washington the last two years, looking at the effects on vine development and fruit quality in white wine grapes.

This year, they are expanding the research to red varieties and will study how leaf removal influences wine quality.

“Initially, we looked at white varieties and leaf removal from the aspect of disease management,” Dr. Michelle Moyer, WSU extension viticulturist, told Good Fruit Grower. “White varieties give us a lot of disease problems because they are the most susceptible to fungal diseases like powdery mildew and Botrytis bunch rot.”

Moyer and WSU graduate student Brittany Komm wanted to learn if growers could remove leaves earlier than fruit set—before or during bloom—to improve disease management without impacting vine development or yield.

They also wanted to learn if earlier leaf removal would reduce sunburn severity by acclimating fruit to high levels of solar radiation earlier in the season.

In the past, studies that evaluated fruit zone leaf removal showed significant reductions in fruit set and yield when leaves were removed early, around bloom time, said Moyer.

“However, these studies were conducted in cooler climates than that of eastern Washington. We wanted to find out if the warmer, more arid climate of eastern Washington would help reduce ­possible negative side effects on fruit set and yield when leaf removal was done early.”

Moyer and Komm set up trials in commercial vineyard blocks of Sauvignon Blanc and Riesling planted in 2007.

The blocks, located north of Prosser, have identical pest management programs, are both spur pruned and drip irrigated, and have canopies trained to a modified vertical shoot position trellis.

During the growing seasons of 2012 and 2013, WSU tested four leaf removal treatments: control (no leaf removal); prebloom; bloom; and four weeks postbloom. All leaves were removed by hand. Vines were treated the same both years.

Leaf removal was defined as removing all basal leaves and lateral shoots from the base up to and opposite of the second cluster on all fruiting shoots.

Conclusions

Moyer highlighted the following results:

Vine development
• Fruit-zone leaf removal did not alter the vines’ ­nutrient status.
• Timing of leaf removal did not significantly impact fruit set or berry size at harvest.
• Exposing buds through leaf removal before and during bloom helped increase bud surface ­temperatures during this key developmental period.
• Leaf removal did not negatively impact cold ­hardiness, based on preliminary results.

Fruit quality
• Prebloom and bloom leaf removal significantly improved spray penetration. Bloom period is a critical time for disease management; maximum spray penetration into clusters can help prevent powdery mildew and bunch rot.
• No significant differences were observed in fruit quality at harvest (Brix, titratable acidity, and pH) between the different leaf removal timings.
• The prebloom and bloom leaf removal in Riesling in 2013 accumulated sugar faster than the other treatments and may have led to the severe bunch rot in the prebloom and bloom treatments.
• In Sauvignon Blanc, all leaf removal treatments in both years of the study resulted in higher skin tannins and phenolics than the control.
• Early leaf removal may prove to be a practical addition to current vineyard ­management practices in eastern Washington.

Unique discovery

Moyer was most surprised that removing leaves early in the season near bloom did not result in looser clusters and smaller yields, something researchers have observed in Spain, Italy, and elsewhere.

“We didn’t see that,” said Moyer. “We didn’t see a change in yield or a change in cluster appearance, even going into the third year of study. That’s a unique discovery relative to the European studies, but similar to other studies conducted in the Pacific Northwest.”

She said a possible explanation could be that vines in eastern Washington are able to recover quickly from leaf removal because of abundant sunlight and long days.

“Young growing tissue is photosynthetically active, and we have so much more productive ­sunlight hours than in Europe and other places.”

Red varieties

Moyer is now applying some of the leaf removal concepts to red wine grape cultivars.

Similar studies with Pinot Noir have been conducted by Oregon State University’s Dr. Patty Skinkis. OSU researchers are finding positive results from early leaf removal in reds, including reduced disease incidence and improved color and tannins.

In 2014, Moyer will begin testing different timings of fruit zone leaf removal in commercial vineyards of red varieties.

Both hand and mechanical leaf removal will be compared to no leaf removal. Mechanical leaf removal will be implemented at later timings (not prebloom or during bloom) to avoid damaging flowers and developing clusters. Fruit from the trial will be harvested and made into wine by WSU extension enologist Dr. James Harbertson.

“We’ll be looking to see if early leaf removal influences berry color or tannin structure in finished red wines,” she said. “Will it impact wine quality? And does late-season, mechanical leaf removal affect wine quality? These are some of the questions we want to answer.” •