Final Report: This is a completed project for the Washington Grape & Wine Research Program
Title: Influence of cultivar, environment and management on grape yield components and quality (Objective 3: Determine weight loss associated with long hang time)
Project Duration: 2014-2016
Primary Investigator(s): Markus Keller
Organization: Washington State University
Address: IAREC, 24106 N. Bunn Road, Prosser, WA 99350
Telephone: 509-786-9263
Email: mkeller@wsu.edu
Project Summary:
It is becoming increasingly common for wineries to request that wine grapes not be harvested until they reach 26-30 Brix. This is well above the 23-25 Brix that most grapes are able to accumulate through sugar import from the leaves. The higher sugar concentration of such fruit may come at the cost of weight loss through water loss from the fruit. This not only constitutes a potential loss of income for growers, but the long ‘hang time’ required to achieve such high sugar levels also puts growers at risk from inclement weather (e.g. rainfall, early fall frosts) and disease pressure. Consequently, growers would like to quantify the extent of the potential weight loss for their cultivars. This project therefore studied berry water loss in a range of wine grape cultivars, using grapes from our new research vineyard. Overall, we found that sugar levels above 23-25 °Brix often come at the cost of weight loss from the berries. However, maximum sugar levels and the extent of weight loss varied widely among cultivars. The average daily weight loss of berries at advanced stages of maturity ranged more than 10-fold, from 2.5 mg to 28.3 mg, depending on the cultivar. Similarly, the extent of weight loss for each unit (in °Brix) of late-season gain in sugar varied from insignificant to almost 0.5 g per berry. Much of the weight loss occurred by water evaporation from the berry surface (berry transpiration), which in turn varied more than 30-fold depending on air temperature and humidity. Hot, dry air greatly increased such water loss. In some cultivars, weight loss was accelerated by berry splitting (cracking). Although the present results must be viewed as preliminary, this study has provided important insight into the relationship between hang time and berry weight loss. Growers and wineries may use this information to negotiate a pricing structure that takes desired Brix levels and harvest dates into account, and acknowledges that advanced grape maturity constitutes a potential loss of crop yield and imposes additional risks on the grower.
Project Major Accomplishments:
The objective of this project was to determine the extent of weight loss associated with long hang time. The new WSU wine grape research vineyard planted in 2010 was used for this study. The vineyard is drip-irrigated and spur-pruned, with cultural practices applied as uniformly as possible across the entire block. Grape samples were collected repeatedly during ripening from 25 cultivars (2014) or 27 cultivars (2015) to determine the variation in berry weight, total soluble solids (TSS), organic acids, and transpiration, using established methods (Keller and Shrestha, 2014; Keller et al., 2015; Zhang and Keller, 2015). In 2015, but not in 2014, the vines were thinned at fruit set to a single cluster per shoot.
Significant results and conclusions: The two growing seasons differed markedly with respect to crop yield. The 2014 average yield across all cultivars was 8.2 tons/acre, while cluster thinning reduced the average yield for 2015 to 3.6 tons/acre. In 2014, only four of the 25 sampled cultivars showed a significant late-season decrease in berry weight: Muscat blanc (-20%), Riesling (-15%), Viognier (-21%), and Zinfandel (-20%). None of these cultivars showed visual symptoms of berry shrinkage. Furthermore, only four cultivars reached average TSS levels >25 °Brix: Barbera (25.3 °Brix), Grenache (26.3 °Brix), Nebbiolo (25.4 °Brix), and Viognier (29.7 °Brix). Viognier was also the cultivar with the lowest yield (2.4 tons/acre). The high crop levels in the remaining cultivars increased the need for a longer growing season to ripen the large amount of fruit. In addition, the later portion of the season saw an increase in days with low vapor pressure deficit (VPD), cloudy weather, and repeated rainfall, especially after 9/26/2014. The wet weather, combined with high crop loads, decreased the rate of sugar accumulation, while berry weights fluctuated or continued to increase. In 10 cultivars, the final berry weight at the end of the 79 day postveraison observation period was also the maximum berry weight. Moreover, Grenache showed a high incidence of berry cracking (splitting), and Zinfandel suffered from severe bunch-stem necrosis. However, the heavy crop combined with the somewhat unfavorable weather conditions in 2014 permitted only very limited insight into the issue of late-season berry weight loss.
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