Dr. Patty Skinkis, Viticulture Extension Specialist & Associate Professor

Excessive plant water stress can cause damage to grapevines, particularly young vines. The recent hot, dry seasons of 2014 and 2015 and the west coast drought drew awareness to water stress and its potential detrimental impacts. However, water stress can occur in any season, and an understanding of how to monitor and manage it is important to the production of healthy vineyards in any region.

This article briefly describes ways to determine whether vines are under water stress. This is the first step used in determining when to begin irrigation in response to stress. For sites that are not irrigated, the methods may be used to determine whether management tactics should be employed to conserve soil moisture or reduce soil water competition. The three most common methods to determine water stress include 1) visual symptoms, 2) leaf or stem water potential and 3) stomatal conductance.

Visual symptoms may be used to determine whether a plant is under stress. It requires training and understanding of the vine’s lifecycle, as symptoms may also be caused by other factors. When visually assessing canopies, it is important to look at shoot tips, tendrils, leaves, clusters, and overall growth. When vines are under water stress, tendrils become limp, shoot tips begin to flop downward, leaves begin to bend (petiole juncture at leaf blade begins to form a shepherds hook) and berries may begin to shrivel. Under prolonged water stress shoot tips stop growing and abscise, tendrils dry up and fall off, and basal leaves may turn yellow and abscise. It is important to consider that tendrils and shoot tips may dry up and fall off as a result of the natural cessation of growth late season, in preparation for dormancy and may not indicate water stress. Visual symptoms are important to document and are strengthened by quantitative measures. Keep in mind that water stress can lead to nutrient deficiencies  which may lead to other visual symptoms.

Leaf water potential is the most common measure conducted by commercial vineyards to determine whether vines are under water stress. A pressure chamber is used to measure the amount of pressure required to push water out of the cut end of the petiole and reflects the amount of water potential (or tension) of the water column in the leaf. It is a measure of negative pressure (- bars), although the gauge on most pressure chambers does not indicate a negative number. Typically, irrigation is initiated when leaf water potential readings reach -12 bars, which is considered moderate stress. Stem water potential, also uses a leaf to measure plant water stress, but it requires additional steps in the process, including covering a leaf with a special reflective bag for at least one hour prior to measurement. Covering the leaf limits transpiration, equalizing the water potential in the leaf close to what is experienced in the shoot (or stem). Stem water potential is usually 1-2 bars less negative as shoots are under less tension than leaves. For example, if leaf water potential readings are at -12 bars, the stem water potential on the same plant may be at -10 bars, and the leaf reading would suggest that you need to start irrigating while the stem reading does not. More information about monitoring plant water stress can be found here.

pressurebomb1

Above: The pressure chamber. The leaf is placed inside a sealed chamber and pressurized gas is added to the chamber slowly. As the pressure increases water will be forced out of the xylem and will be visible at the cut end of the stem. Note the magnifying glass for easier viewing.

pressurebomb3

Above: Stem water potential, also uses a leaf to measure plant water stress, requires additional steps in the process, including covering a leaf with a special reflective bag for at least one hour prior to measurement. Covering the leaf limits transpiration, equalizing the water potential in the leaf close to what is experienced in the shoot (or stem).

pressuremomb2

Above: When measuring water or leaf potential, make sure to cut end of the petiole for an accurate measurement.

pressurebomb4

Above: A leaf attached to a petiole is placed inside a sealed chamber and pressurised gas is added to the chamber slowly.

Some find the pressure chamber to be cumbersome due to its bulky size and have been interested in using a portable leaf porometer. A leaf porometer measures the rate of water that moves out of the stomata, known as stomatal conductance. When a vine is under water stress, stomata close and stomatal conductance of water is reduced. A vine that is not under any water stress can have stomatal conductance of >250 mmol H20/m2/s while a vine under moderate stress has <150 mmol H20/m2/s. There is a porometer on the market that is affordable for commercial vineyard use, and the cost is comparable to a pressure chamber. It may be small, but it requires training, attention to detail in collecting the data, and requires a calibration step before using the meter. This calibration needs to be done each day, before sampling begins, and again hen environmental conditions change (approximately every hour).

Both leaf water potential and stomatal conductance measures require the right equipment and training. Both tools should be used on clear, cloudless days within 1 hour before and after solar noon and not under extreme heat events, to give the most accurate readings. We have tested both pressure chambers and porometers in various trials under western Oregon conditions to monitor plant water stress (Skinkis and Schreiner Labs), and both can be good tools to determine when vines are under stress. If you are interested in those findings, contact Patty Skinkis or Paul Schreiner for more information. Whenever using quantitative measure of plant water stress, it is important to make note of visual symptoms, as they may help interpret conditions of the vines.

NOTE:  The reference to porometers and pressure chambers are for local companies that have developed these devices. There may be other companies that provide similar products, and the mention of these two companies does not imply recommendation or endorsement of those products over any other similar product.

Additional Reading

The Pressure Chamber (The Bomb) – UC Davis Fruit & Nut Research Information

Measuring Water Status Using a Pressure Chamber – eXtension.org

Grapevine Management under Drought Conditions

Irrigation Basics for Eastern Washington Vineyards

Dr. Patty Skinkis, Viticulture Extension Specialist & Associate Professor

There has been an increase in the number of reported cases of stunted vine growth and potential trunk disease this spring in the Willamette Valley. The symptoms ranged from delayed bud break to lagging shoot growth compared to healthy blocks, and in some cases led to shoots with distorted and almost tattered-looking leaves. The symptoms looked different than the typical culprits of herbicide drift, frost damage, rust/bud mite feeding, or micronutrient deficiency. Upon closer inspection by sawing into cordons and trunks, significant cankers (dead areas within the vine trunk) were found, and this suggested the potential cause of the limited shoot growth. Although visual symptoms suggested trunk disease, samples were submitted to OSU Plant Clinic to confirm which disease organisms may be causing the damage.

Knowing what trunk disease organisms are present is helpful in understanding next steps for managing the disease. Dr. Melodie Putnam, OSU Plant Clinic Director, summarized the importance of identifying the disease-causing organisms and provides visual examples of trunk disease symptoms in a seminar archived online here. Trunk disease has become more of a “hot topic” in recent years both nationally and internationally. In 2015, Dr. Jose Urbez Torres visited OSU and growers in the north Willamette Valley and southern Oregon to share his expertise and research about trunk diseases in California and British Columbia. His archived seminar is available online here. Research on trunk diseases of grapevines is currently being led by Dr. Kendra Baumgartner, a USDA-ARS plant pathologist from Davis, CA. The work is funded by a federal grant and is aimed at understanding both basic and applied aspects of managing trunk diseases in grapevines and other tree fruit and nut crops. You can learn more about the research here.

Grapevine trunk diseases don’t lead to immediate vine decline. The vine symptoms that are being expressed this spring are likely due to infection years ago, and the vineyards are just now showing the symptoms due to some prior vine stress. The two record breaking yield and heat/drought vintages of 2014 and 2015 may have led to more nutrient and/or water stress that could lead to poor nutrient or carbohydrate storages for early spring growth. The research team on the federal trunk disease grant are working to understand how water stress impacts the disease.


Please see the links below for more information.

The Oregon Wine Research Institute is pleased to announce that Alexander Levin will be joining the OSU Department of Horticulture as Assistant Professor of Viticulture in early fall, 2016. He will be based at the Southern Oregon Research and Extension Center in Central Point.

Dr. Levin completed his Ph.D. degree in horticulture and agronomy at UC Davis under Drs. Mark Matthews and Larry Williams. Prior to pursuing a Ph.D., Alexander completed a bachelor’s degree in psychology at University of Michigan. He gained hands-on experience
in the wine industry while working in Napa Valley, CA as a harvest intern, and at Beringer Vineyards in St. Helena, CA. His primary research interests include investigating genetic differences on drought response between cultivars, evaluating plant water status, leaf conductance and vegetative growth, and the development of deficit irrigation regimes to optimize production goals. He conducted a multi-year field study investigating vine performance of 17 red winegrape cultivars under deficit irrigation.

Alexander’s future research and Extension plans include working with grape growers to identify appropriate management practices, developing suitable irrigation practices for the region, and utilizing field data to optimize grape and wine quality. He also looks forward to collaborating with other researchers, Extension specialists, grape growers, and stakeholders to further develop the Southern Oregon grape and wine industry using effective
management practices.

For more information, please contact: Richard Roseberg, Director- Southern Oregon
Research & Ext. Center. Phone: 541-772-5165, email: richard.roseberg@oregonstate.edu.