MONDAY, JUNE 16, 2025. BY STAN GRANT, VITICULTURIST.
The period of time between veraison and harvest, commonly called ripening period, is an eventful time for grapevines. Berries normally increase in size by a factor of two or more during this time. Additionally, ripening berries soften, their color deepens and they become more palatable as they expand. And if all is working well within the vines, main stems become woody and cold hardy. Such cane lignification is a ripening process.
During the ripening period, several factors contribute to risks of stresses in grapevines. As they ripen, berries become increasingly susceptible to damage. Late in the growing season, leaves can senesce prematurely under stress, with senescence being the sequential deterioration of cells and tissues that concludes with death. Within vines, the ripening process and competition among vine organs can strain the supply of some internal resources, sometimes leading to stress. Finally, environmental conditions during ripening often involve limited soil moisture, high atmospheric temperatures, intense sunlight and high evaporative demand, any of which can cause stress.
Given the risks of grapevine stress during the ripening period, it is not a time for a vineyard manager to ease up. Rather, it is time for careful vineyard monitoring and decisive actions when needed. In this article, we will consider the nature of some common ripening period stresses and how to recognize these stresses as they appear. We will also present some strategies and tactics to minimize them.
Common Ripening Period Stresses
During the ripening period, developing berries draw heavily on available resources for growth and maturation. Carbohydrates and potassium are in particularly high demand within the berries. Due to the strong pull of ripening berries, less competitive vine organs, including leaves and roots, can become somewhat resource-depleted and diminished in activity.
Figure 1. A stressed Petite Sirah grapevine with too few leaves to ripen the crop it bears.
Photo source: Progressive Viticulture©.
This condition, sometimes called crop stress, intensifies as the number of berries relative to the number of exposed leaves increases (Figure 1, above). Restricted shoot growth, incomplete canopy development and delayed fruit maturation are indicators of crop stress.
Deficit irrigation is the practice of applying less than the full amount of irrigation water a vineyard can use with the intent of imposing some degree of water stress on grapevines. When carefully implemented, regulated deficit irrigation (RDI) enhances grape quality and improves fruit production efficiency. For this reason, it has become a standard viticultural practice in winegrape production.
However, if not carefully implemented, deficit irrigation can easily result in severe grapevine water stress. Usually, severe water stress symptoms first appear as randomly occurring yellow patches on leaves near the base of shoots. As the duration of severe stress increases, patches of dead tissues appear among yellowed areas on leaves and symptomatic leaves develop further up shoots. Such leaf damage reduces a vine’s ripening capacity and if accompanied with leaf death and loss, exposes fruit to potentially damaging solar radiation (Figure 2, below).
Figure 2. Severe water stress caused significant leaf death and loss, resulting in a decreased capacity for ripening and excessive fruit exposure.
Photo source: Progressive Viticulture©.
Under deficit irrigation, soil moisture available to grapevine roots is somewhat limited and leaves are slightly dehydrated. Accordingly, leaves transpire at a reduced rate and are warmer than if the vines were well watered. In these circumstances, leaves can easily become stressed and damaged when temperatures are high. Heat stress is initially visible as faded patches on sun-exposed portions of leaves, which feel warm when touched. Later, these patches die (Figure 3A, below).
Compared to other foliar tissues, berries have fewer stomatal openings and a lower capacity for transpirational cooling. As a result, berries are particularly prone to damage when temperatures are high (Figure 3B, below).
Figure 3. (A) A dead patch on a sun-exposed section of leaf is the result of heat stress and (B) poor berry color in this cluster is due to high temperatures.
Photo source: Progressive Viticulture©.
While the nature and degree of heat stress depends on the intensity and duration of heat (Table 1, above), it almost always involves impaired ripening. In California, intense sunlight frequently accompanies high temperatures, causing distinctive leaf and berry damage associated with ultraviolet radiation (Figure 4, below).
Figure 4. (A) Blackened and somewhat bronzy black leaf tissue damage and (B) sunburned berries are due to high temperatures in combination with intense sunlight.
Photo source: Progressive Viticulture©.
Ripening berries have a large requirement for potassium, significantly more than any other mineral nutrient. That being so, the peak demand for potassium in grapevines occurs during the ripening period, coinciding with peak seasonal temperatures, sunlight intensity and vineyard water use (Figure 5, below). At the same time, most California vineyard soils are low in available potassium and under deficit irrigation, limited soil moisture constricts potassium movement towards vine roots. Unsurprisingly, potassium deficiency is a common ripening period stress and it is one that inhibits fruit maturation.
Figure 5. In grapevines, the demand for water and the demand for potassium both peak during the ripening period.
Data sources: Lodi historical average reference ET from Goldhammer and Snyder, 1989 and Chenin blanc grapevine potassium content from Conradie, 1981.
During ripening, common symptoms of potassium deficiency develop on mid shoot leaves on the afternoon sun side of canopies. On these leaves, tissues between main (primary and secondary) veins fade from green to yellow, frequently beginning on the leaf edges and extending inward, and yellowed areas often include small specks of dead tissues.
Importantly, ripening period stresses can exacerbate one another. Large crops can intensify stress-induced leaf senescence and berry shrinkage due to severe water stress. Water stress increases leaf temperatures and their susceptibility to heat related stresses, and the likelihood of potassium deficiency. Heat stress accelerates leaf senescence caused by severe water stress. Intense sunlight increases the negative impacts of severe water stress, heat stress and potassium deficiency, including decreased photosynthesis. Finally, potassium deficiency increases the susceptibility of leaves to heat damage.
Ripening Period Stresses – the Nature of Vine and Vineyard Damage
The leaf tissue yellowing (chlorosis) and death that accompanies most ripening period stresses begins at the molecular level as tissues dehydrate and photosynthesis slows (Figure 6, below). Protein molecules denature leading to decreased metabolism. Chlorophyll molecules photooxidize and degrade, liberating excess energy, electrons and oxygen as light capture continues while photosynthesis declines. The surplus electrons attach to oxygen to form active oxygen molecules (free radicals). In the presence of free radicals, chloroplast and cell membranes break down and release reactive oxygen, leading to toxicity in tissues. In sum, these are the effects of uncontrolled oxidation.
Figure 6. Young leaves showing yellowing (chlorosis) and dead areas due to a combination of oxidative stresses during the ripening period.
Photo source: Progressive Viticulture©.
In addition to visible damage, these stress effects have operational and economic consequences. As mentioned above, stresses delay harvest, as well as diminish berry quality and reduce fruit yields. Furthermore, they can deplete carbohydrate reserves in woody vine tissues and inhibit root systems, decreasing a vine’s capacity to support future shoot growth and fruit production. These events decrease returns on farming inputs.
Ripening Period Stresses – Vineyard Management Strategies for Minimizing Them
Optimizing ripening period conditions in vineyards to minimize stress begins with actions taken after harvest in the previous growing season. These actions include postharvest applications of inputs to relieve stress, support leaf activity and promote root growth. This is also the appropriate time to initiate steps to ensure large, uniform and well-aeriated root zones.
Early in the current growing season, encourage a strong flush of root growth to maximize water and mineral nutrient uptake and hormone production. Supportive actions include low-rate applications of fertilizers designed to support root growth. Such fertilizers typically include a small percentage of ammonium-nitrogen, a large percentage of phosphorus and certain organic compounds (e.g., humic acid, fulvic acid). At the same time, foster prompt development of complete canopies (14 to 20 leaves per shoot) and protect them from pests, diseases and stresses (Figure 7, below).
Figure 7. A completely developed canopy composed of mature, healthy leaves. Sustaining such a canopy is a primary vineyard management goal during the ripening period.
Photo source: Progressive Viticulture©.
Before fruit set, thin shoots for a moderate density and when sufficiently long, position shoots and remove leaves as needed to form a thin veneer of leaves with gaps over fruit zones. These actions will facilitate air movement through the canopy while limiting berry exposure to direct sunlight.
Following fruit set, thin clumps of clusters (Figure 8, below). Doing so limits contact and heat transfer between adjacent clusters. Moreover, it maximizes air flow around clusters, which is the principal means of berry cooling. Give thought to maintaining mowed cover crop residue rather than cultivating tractor rows to lessen heat reflected into canopies.
Figure 8. Air movement is restricted within clumps of clusters and heat is readily transferred between them.
Photo source: Progressive Viticulture©.
Prior to veraison, ensure adequate vine tissue calcium, which is essential for heat tolerance. During veraison, collect and analyze leaf blades for nutritional status at the onset of ripening. Based the analysis results, apply fertilizers as needed to assure vines contain adequate phosphorus, potassium, iron, manganese, copper and boron required for ripening and the production of antioxidants.
All through the ripening period, use regulated deficit irrigation schedules that maintain vines on the wetter side of moderate water stress (midday leaf water potential between -10 to -12 bars). Apply low-rate potassium fertigations to lessen the risk of deficiency and to promote normal photosynthesis. If you plan to apply potassium only once during ripening, do so when the fruit is at about 18 degrees Brix. Continue to protect foliage from pests and diseases.
Regularly monitor vineyards for onset of any stresses. Thermal images can be helpful in this effort, watching for canopy temperatures near 95 degrees Fahrenheit. Also, monitor soil moisture for excess depletion in the root zone. Perhaps most important, watch weather reports for forecasted maximum temperatures approaching 100 degrees Fahrenheit and the need to temporarily suspend regulated deficit irrigation schedules.
Part Two is available HERE.
This article is based on a presentation made on January 8, 2018 at the Mid Valley Agricultural Services Annual Grape Grower Meeting, and the information was updated for today’s blog post.
Further Reading
Basile, B, Marsal, J, Mata, M, Vallverdu, X, Bellvert, J, Girona, J. Phenological sensitivity of Cabernet Sauvignon to water stress: vine physiology and berry composition. Am. J. Enol. Vitic. 62, 452-461. 2011.
Bernardo, S, Dinis, L-T, Machado, N, Moutinho-Pereira, J. Grapevine abiotic stress assessment and search for sustainable adaption strategies for Mediterranean-like climates. A review. Agron. Sustain. Dev. 38: 66. 2018.
Bertamini, M, Zulini, L, Muthuchelian, K, Nedunchezhian, N. Effect of water deficit on photosynthetic and other physiological responses in grapevine (Vitis vinifera L. cv. Riesling) plants. Photosynthetica. 44, 151-154. 2006.
Bolton and Grant. Ripening period potassium deficiency. Lodi Winegrape Commission Coffee Shop Blog. September 17, 2018.
Conradie, WJ. Seasonal uptake of nutrients by Chenin blanc in sand culture: II. Phosphorus, potassium, calcium, and magnesium. S. Afr. J. Enol. Vitic. 2, 7-13. 1981. Copp, C, Levin, A. Managing grapevines during a heat spike. Oregon State University Extension Service. May 2022.
Edwards, EJ, Smithson, L, Graham, DC, Clingeleffer, PR. Grapevine canopy response to high-temperature event during deficit irrigation. Aust. J. Grape Wine Res. 17, 153-161. 2011.
Gambetta, GA, Herrera, JC, Dayer, S, Feng, Q, Hochberg, U, Castellarin, SD. Grapevine drought stress physiology: towards an integrative definition of drought tolerance. Journal of Experimental Botany. 71, 4658-4676. 2020.
Garde-Cerdan, T, Porto, J, Lopez, R, Santamaria, P. Effect of foliar applications of proline, phenylalanine, urea, and commercial nitrogen fertilizers on Stilbene concentrations in Tempranillo musts and wines. Am. J. Enol. Vitic. 66, 542-547. 2015.
Goldhammer, DA, Snyder, RL. Irrigation scheduling: a guide for efficient on-farm water management. University of California, Division of Agriculture and Natural Resources Publication. 1989.
Grant, S. Five-step irrigation schedule: promoting fruit quality and vine health. Practical Winery and Vineyard. 21 (1): 46-52 and 75. May/June 2000.
Grant, S. Balanced soil fertility management in wine grape vineyards. Practical Winery and Vineyard. 24 (1): 7-24. May/June 2002.
Grant, S. Fertilizer efficiency for wine grape vineyards. Practical Winery and Vineyard. 28 (1): 35-41. March/April 2006.
Grant, S. Regulated deficit irrigation, parts I and II. Lodi Winegrape Commission Coffee Shop Blog. July 18 and August 04, 2014.
Grant, S. Shoot thinning for multiple benefits. Lodi Winegrape Commission Coffee Shop Blog. May 02, 2017.
Grant, S. Managing vineyard mineral nutrient efficiency beyond the 4 R’s. Wines and Vines. pp. 77-78. April 2016.
Grant, S. Post harvest vineyard management. Lodi Winegrape Commission Coffee Shop Blog. October 15, 2019.
Grant, S. Prebloom vineyard management. Lodi Winegrape Commission Coffee Shop Blog. March 02, 2020.
Grant, S. Remote sensing and aerial images in vineyard management. Lodi Winegrape Commission Coffee Shop Blog. March 30, 2020.
Grant, S. Post bloom vineyard management. Lodi Winegrape Commission Coffee Shop Blog. May 11, 2020.
Grant, S. Ripening period vineyard management. Lodi Winegrape Commission Coffee Shop Blog. July 07, 2020.
Grant, S. Systematic vineyard monitoring for effective vineyard management. Lodi Winegrape Commission Coffee Shop Blog. May 3, 2021.
Grant, S. The seasonality of vineyard mineral nutrient management. Lodi Winegrape Commission Coffee Shop Blog. May 24, 2021.
Grant, S. The ultimate goal of vineyard soil management: optimized root zone function. Lodi Winegrape Commission Coffee Shop Blog. December 20, 2021.
Grant, S. There is more to vineyard floors. Lodi Winegrape Commission Coffee Shop Blog. March 28, 2022.
Grant, S. Canopy management revisited – part two. Lodi Winegrape Commission Coffee Shop Blog. January 1, 2024.
Grant, S. Towards an Understanding of Vineyard Potassium. Lodi Winegrape Commission Coffee Shop Blog. August 26, 2024.
Grant, S. In Pursuit of Yield: grapevine capacity, balance, and crop load – Part Two. Lodi Winegrape Commission Coffee Shop Blog. May 28, 2024.
Greer, DH, Rogiers, SY, Steel, CC. Susceptibility of Chardonnay grapes to sunburn. Vitis. 45, 147-148. 2006.
Greer, DH, Weedon, MM. The impact of high temperatures on the Vitis vinifera cv. Semillon grapevine performance and berry ripening. Front. Plant Sci. 4, 491. 2013.
Hannam, KD, Neilsen, GH, Neilsen, D. Midwood, AJ, Millard, P, Zhang, Z, Thornton, B, Steinke, D. Amino acid composition of grape (Vitis vinifera L.) Juice in response to applications of urea to the soil and foliage. Am. J. Enol. Vitic. 67, 47-55. 2016.
Keller, M. Deficit irrigation and mineral nutrition. Proceedings of the Soil Environment and Mineral Nutrition Symposium. Christensen, LP, Smart, DR (ed.). Amer. Soc. Enol. Vitic. pp. 267-283. 2005.
Kramer, PJ. Plant water relations. Academic Press, San Diego. 1983.
Lang, NS, Wample, RL, Smithyman, R, Mills, L. Photosynthesis and chlorophyll fluorescence in blackleaf-affected Concord leaves. Am. J. Enol. Vitic. 49, 367-374. 1996.
Matsui, S, Ryugo, K, Kliewer, WM. Growth inhibition of Thompson Seedless and Napa Gamay berries by heat stress and it’s partial reversibility with application of growth regulators. Am. J. Enol. Vitic. 37, 67-71. 1986.
Keller, M. The science of grapevines. 2nd Ed. Academic Press, Burlington, MA. 2015.
Marschner, H. Mineral nutrition of higher plants. 2nd Ed. Academic Press, London. 1995.
Paulsen, GM. High temperature responses in plants. pp. 365-389. In Boote, KJ, Bennett, JM, Sinclair, TR, Paulsen, GM (ed.). Physiology of determination of crop yield. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America; Madison, WI. 1994.
Poni, S, Intrieri C, Silvestroni, O. Interactions of leaf age, fruiting, and exogenous cytokinins in Sangiovese grapevines under non-irrigated conditions. I. Gas exchange. Am. J. Enol. Vitic. 45, 71-78. 1994.
Prajitna, A, Dami, IE, Steiner, TE, Ferree, DC, Scheerens, JC, Schwartz, SJ. Influence of cluster thinning on phenolic composition, resveratrol, and antioxidant capacity of Chambouricin Wine. Am. J. Enol. Vitic. 58, 346-350. 2007.
Romero, P, Fernandez-Fernandez, JJ, Martinez-Cutillas, A. Physiological thresholds for efficient regulated deficit irrigation management in winegrapes grown under semi-arid conditions. Am. J. Enol. Vitic. 61, 300-312. 2010.
Scholasch, T, Rienth, M. Review of water deficit mediate changes in vine and berry physiology; consequences for the optimization of irrigation strategies. OENO One. 3, 423-444. 2019.
Sepulveda, G, Kliewer, WM. Effect of high temperature on grapevines (Vitis vinifera L.). I. Translocation of 14C-photosynthates. Am. J. Enol. Vitic. 37, 13-19. 1986.
Sepulveda, G, Kliewer, WM. Effect of high temperature on grapevines (Vitis vinifera L.). II. Distribution of soluble sugars. Am. J. Enol. Vitic. 37, 13-19. 1986.
Sepulveda, G, Kliewer, WM. Stomatal response of three grapevine cultivars (Vitis vinifera L.) to high temperatures. Am. J. Enol. Vitic. 37, 44-52. 1986.
Smithyman, RP, Wample, RL, Lang, NS. Water deficit and crop level influences on photosynthetic strain and blackleaf symptom development in Concord grapevines. Am. J. Enol. Vitic. 54, 364-375. 2001.
Snyder, RL, Connell, JH. Ground cover height affects pre-dawn orchard floor temperature. Calif. Agric. 47, 9-12. Jan-Feb 1993.
Have something interesting to say? Consider writing a guest blog article!
To subscribe to the Coffee Shop Blog, send an email to stephanie@lodiwine.com with the subject “blog subscribe.”
To join the Lodi Growers email list, send an email to stephanie@lodiwine.com with the subject “grower email subscribe.”
To receive Lodi Grower news and event promotions by mail, send your contact information to stephanie@lodiwine.com or call 209.367.4727.
For more information on the wines of Lodi, visit the Lodi Winegrape Commission’s consumer website, lodiwine.com.
