Viticulture – Understanding and Managing Water Stress Session

Charles Obiero | Markus Keller*

Extreme Drought Causes Persistent Impacts on Vine Growth and Productivity After Renewed Irrigation

Charles Obiero and Markus Keller*

*Washington State University, 24106 N. Bunn Road, Prosser, WA, 99350, mkeller@wsu.edu

As weather extremes intensify, grapegrowers in the western United States and other dry regions face increasingly difficult decisions during prolonged droughts, including whether to replant vines after repeated stress or reduce crop load in hopes of supporting vine recovery. In 2025, we assessed shoot growth, yield, and fruit composition in field-grown Cabernet Sauvignon and Riesling vines that had been exposed to three consecutive years (2022 to 2024) of drought stress in the arid Yakima Valley of Washington. Stress was imposed annually by suspending irrigation from either fruit set or veraison (no irrigation until postharvest), combined with 0, 50, or 100% fruit removal. Relative to the regulated deficit irrigation control, early-season shoot length in 2025 declined by one-third in Cabernet Sauvignon and by one-quarter in Riesling following drought initiated at fruit set in the preceding 3 yr. Fruit removal during the drought years enhanced subsequent shoot growth only in Riesling. In Cabernet Sauvignon, up to 10% cordon dieback occurred in vines previously stressed from fruit set, and fruit removal did not reduce this damage. Relative to the control, yield during the recovery year declined by 50 to 60% in vines previously stressed from fruit set, and by 34 to 38% in those stressed from veraison, when no fruit had been removed. In Riesling, but not in Cabernet Sauvignon, 100% fruit removal with drought starting at fruit set in previous years alleviated yield losses during the recovery year. Unlike with drought from fruit set, 50% fruit removal with drought starting at veraison alleviated subsequent yield losses in both cultivars. Fruit composition remained largely unaltered by the previous 3-yr drought stress and fruit removal. These results demonstrate that extreme drought imposes persistent constraints on vine growth and productivity, and recovery is slow and incomplete, necessitating multiyear management strategies, such as severe pruning, to renew cordons.

Funding Support: the USDA Northwest Center for Small Fruits Research, USDA National Institute of Food and Agriculture, the Washington State Grape and Wine Research Program, and Chateau Ste. Michelle Distinguished Professorship

Liesl Heinemann | Alli Zamora | Miguel Pedroza | Suraj Kar*

Rootstocks Reduce Viticultural and Enological Impacts of Severe Early-Season Drought in Sauvignon blanc

Liesl Heinemann, Alli Zamora, Miguel Pedroza, and Suraj Kar*

*California Polytechnic State University, Wine and Viticulture Department, California Polytechnic State University, San Luis Obispo, CA, 93407, surajkar@calpoly.edu

Viticulture has historically depended on relatively stable climate patterns to support consistent grape and wine quality. However, ongoing shifts in weather patterns threaten many established winegrowing regions, including California’s Central Coast. More frequent and severe drought is posing a significant challenge to the region’s grape and wine industry. Drought negatively affects grapevine growth and vigor, reduces vine water status and photosynthesis, and lowers yield, berry composition, and wine quality. While grapevines generally tolerate water stress postveraison, early-season stress can severely limit canopy and root development, reduce berry size, and decrease bud fruitfulness, resulting in long-term declines in vine productivity. As severe drought events become more frequent, identifying drought-tolerant plant material is essential to maintain vineyard health and sustain wine quality in California’s minimally irrigated regions. Traditionally bred for resistance to Phylloxera and nematodes, grapevine rootstocks can serve as a long-term solution to drought, given that vineyards are typically managed for 25 to 30 yr and deliver sustained economic benefits. To evaluate this potential, we conducted a field trial at a commercially managed block in Cal Poly’s research vineyard. In this randomized field trial, we tested the effects of three irrigation regimes: 80% ET_c over the course of the season (sustained deficit irrigation), 0% ET_c prebloom followed by 80% ET_c postbloom (prebloom deficit), and 0% ET_c preveraison followed by 80% ET_c postveraison (preveraison deficit) on viticultural and enological properties of Sauvignon blanc grapevines grafted onto six rootstocks (101-14, 1103P, 110R, 1616C, 420A, and 5C). Rootstocks significantly influenced bloom date, vine physiology, yield, and berry composition under early-season water-deficit conditions. Additionally, rootstocks influenced the volatile and phenolic composition of resulting wines. These findings highlight the role of rootstocks in mitigating seasonal drought stress and provide valuable insights for long-term rootstock selection tailored to regional growing conditions and winemaking objectives.

Funding Support: This research was supported by the California State University Agricultural Research Initiative. Additional funding for student involvement during the summer was provided by Joe and Florence Silva and the CSU Summer Undergraduate Research Program.

Shijian Zhuang* | Karl Lund | Qun Sun | Matthew Fidelibus | Philippe Rolshausen

Rootstock Selection for Drought and Boron Tolerance in the San Joaquin Valley of California

Shijian Zhuang,* Karl Lund, Qun Sun, Matthew Fidelibus, and Philippe Rolshausen

*University of California, Riverside, 550 E Shaw Ave, Suite 210-B, Fresno, CA, 93710, gzhua003@ucr.edu

Rootstock selection is critical when vineyard replanting decisions are made, particularly under increasing boron (B) toxicity and water limitation in the San Joaquin Valley of California. Two scions (Barbera and Colombard) grafted onto five rootstocks were planted in 2021 to evaluate B tolerance and drought response. Freedom, RS-3, 140 Ruggeri, and 1103 Paulsen were compared with own-rooted vines under elevated irrigation water B (>1 ppm) and deficit irrigation conditions imposed in 2024 and 2025. The experiment was arranged as a two-way factorial split-block design with five rootstocks ´ two irrigation levels, replicated four times, with three vines per experimental unit. Two deficit irrigation treatments (40% and 80% ET_c) were applied from fruit set to harvest in both seasons. Own-rooted vines exhibited the lowest B uptake in bloom leaf blades, while Freedom consistently had the second-lowest B uptake for both cultivars. Water deficit increased veraison leaf blade B concentrations in both Barbera and Colombard. No significant differences in leaf gas exchange or midday leaf water potential were found among rootstocks; however, both parameters were significantly reduced under water deficit. Yield per vine was reduced by water deficit due to decreased berry size, while rootstock did not significantly affect yield. Among the rootstocks evaluated, Freedom showed the greatest yield potential while maintaining relatively low B uptake and produced fruit with higher juice pH and lower titratable acidity at harvest.

Funding Support: California Grape Rootstock Commission

9:10 am – 9:30 am	Extreme Drought Causes Persistent Impacts on Vine Growth and Productivity After Renewed Irrigation Charles Obiero, Washington State University, Prosser
9:30 am – 9:50 am	Rootstocks Reduce Viticultural and Enological Impacts of Severe Early-Season Drought in Sauvignon blanc Liesl Heinemann, California Polytechnic State University, San Luis Obispo
9:50 am – 10:10 am	Rootstock Selection for Drought and Boron Tolerance in the San Joaquin Valley of California Shijian Zhuang, University of California, Riverside

Viticulture – Understanding and Managing Water Stress Session

Research Reports

Moderator:

Speakers:

Share This Page