By Michela Centinari
It is August already, which, for many grape growers in Pennsylvania, means veraison and the beginning of fruit ripening. It seems a good time to comment on the seasonal weather and how it can affect the vines.
Figure 1. Map of drought intensity for Pennsylvania released on August 4, 2016
In July, above average temperatures were recorded in Pennsylvania [1], and drought conditions varied from ‘none’ to ‘severe drought’ across the state (Figure 1). The regions most affected by drought are North Central, Northwest, and some areas of Northeast PA [1].
In Figures 2 and 3 I reported the cumulative growing degree days (GDDs) (April to July) and precipitation (March to July) recorded at the two Penn State research and extension stations located in the South Central (Biglerville, Adams County) and Northwest (North East, Erie County) part of the state (http://newa.cornell.edu/). I also included the 2014 and 2015 data so you can compare the heat accumulation (GDDs), precipitation patterns and amount this year with those of the two previous seasons.
When looking at figures 1, 2, and 3, please keep in mind that local weather conditions vary greatly, shower and thunderstorm activity was hit or miss across the state. It is indeed recommended that growers install a weather station at their site to carefully monitor weather conditions and assist with disease control programs.
Figure 2. Cumulative growing degree days (GDDs) recorded from April to July 2014, 2015 and 2016 at the two Penn State research and extension stations.
Compared to 2014 and 2015, this growing season started with lower heat accumulation in some areas of Pennsylvania, such as the Northwest (Figure 2A) and South Central (figure 2B) regions. Higher than average temperatures recorded in July however, pushed GDDs close to or above those of the same period last year. For example, in Erie County, cumulative GDDs were, by the end of July, above those accumulated in 2015 or 2104. In South Central PA GDDs are reaching the 2015 values and they are above those accumulated in 2014 for the same period (April-July).
The hot temperatures recorded in July can accelerate fruit ripening [2]. For example, in Central Pennsylvania, Noiret (Vitis hybrid), which is not one of our earliest varieties, started to turn color last week (i.e., the first week of August), approximately 10 days earlier than last year.
While drought conditions have not been recorded in the Southeast and most of the Southwest regions, it has been dryer than average in the rest of Pennsylvania. For example, in North East (Erie County, Northwest) cumulative precipitation from March to July (13.6²) was 40% and 36% lower as compared to last year (22.6²) and two years ago (21.12²). In Biglerville (Adams County, South Central) cumulative precipitation from March to July (12.7²) was 33% and 31% lower as compared to last year (19.2²) and two years ago (18.4²).
Figure 3. Cumulative precipitation recorded from March to July 2014, 2015 and 2016 at the two the two Penn State research and extension stations.
Drought doesn’t always equal water stress
In- and across-season precipitation patterns in the eastern US are unpredictable. In our humid climate, precipitation and the soil water reservoir are usually sufficient to meet (or exceed) vine water requirements through ripening. Even if a drought period occurs, its duration and severity are not usually sufficient to warrant concern about moderate or severe vine water stress. Growers do however need to be aware that non-irrigated grapevines in temperate climates can occasionally face water stress during drought periods in the growing season [3; 4].
Hot temperatures, like those recorded in July, increase evapotranspiration and how much water the vine needs. This could facilitate the occurrence of vine water stress in areas that have been experiencing persistent lack of rain. The risk of water stress, indeed, not only depends on the amount of soil water available (supply), but also on how fast this water is used by the vines (demand) [5].
Along with seasonal rainfall and winter soil moisture other factors affecting the amount of water available (water supply) to the vines are:
- Soil water holding capacity which is determined by the soil textural properties: heavier soils (loam and clays) hold more water than light sands or gravels. For example, a unit volume of sandy-loam soil can hold about 50% as much water as a clay soil [5].
- Soil depth: deep soil can hold a greater volume of moisture than shallow soil [6] allowing grapevines, in the absence of restrictive layers, to develop a more extensive and deeper root system which can access deep resources of water during drought periods.
- Grapevine root system size and rooting depth: In addition to soil characteristics, also the age of the vine will influence root system size and rooting depth. Young vines have restricted root systems and rooting volume for several years, thus they are more sensitive to water stress than mature vines with well-established root systems [5].
- Presence of competitive plants, as green and actively growing cover crops and weeds in the middle-row and in-row areas.
Water demand is primarily driven by weather conditions (solar radiation, air temperature and humidity). For example, evaporation from an open pan under hot and dry weather (i.e., California) can be around 8-10 inches of water per month, whereas under cool and humid condition, typical of the northeast US can be less than 5 inches [5]. Also the amount of sun-exposed transpiring leaf area and crop load will affect the amount of water used by the vines [5]. For example vines trained to GDC or high-wire cordon tend to have greater sun-exposed leaf area that can capture more sunlight and use more water than those trained to vertical shoot positioning (VSP) [5]. Heavily cropped vine vines also require more water for fruit ripening than vines with a smaller crop [2].
Vine response to water stress varies with the severity of the stress and the timing of the season it develops
Growth processes (i.e., shoot growth, early berry growth) are more sensitive to water deficit than photosynthesis [7]. Therefore, a mild/slight water stress between fruit-set and veraison can favorably diminish vegetative growth and reduce berry growth leading to smaller berries with potentially higher skin to pulp ratio without compromising photosynthesis and carbohydrates/sugars production [7]. Under moderate to severe water stress conditions, however, photosynthetic activity is reduced possibly leading, early in the season, to poor canopy development and function. Later in the season (after veraison) a reduction in photosynthesis can decrease sugar accumulation in the berries with a negative effect on fruit ripening and flavor development. Further, a reduced storage of carbohydrates and other nutrients in perennial organs may occur. Thus, it is crucial to maintain a healthy and functional canopy after veraison to avoid negative effects on fruit or wine quality and cold hardiness. Furthermore, because after veraison, berry growth is quite resistant to water stress, a post-veraison water deficit is not as effective in reducing berry size as a pre-veraison one [5].
Growing up in Italy, I remember the old-world “wine dilution theory” that supported the idea that any irrigation after veraison would lead to an increase in berry size (due to water dilution) and a reduction in wine quality [8]. There was not strong scientific evidence, however, supporting this assumption. It was actually found that water doesn’t move into the berry after veraison due to complete or partial lost in xylem functionality [7] which proved that irrigating the vines after veraison doesn’t actually impact berry size [8]. Thus, nowadays it is recommended to avoid moderate to severe water stress after veraison to ensure vine health and proper ripening and flavor development.
Symptoms of vine water stress
Since vines change in appearance under water stress conditions it is a good practice to walk through the vineyard and look for sign of water stress, starting with young vines. A comprehensive table that summarized visual symptoms of increasing water stress from mild to severe can be found in the “Wine grape production guide for eastern North America” (page 172) and also available in the July issue of Viticulture Notes [2] edited by Tony Wolf, professor of viticulture at Virginia Tech University.
Below I summarized some of the visual indicators of vine water status, from ‘well-watered’ to ‘severe drought’ conditions [6]
Well-watered vines (Figure 4):
- Shoot tips are actively elongating
- Tendrils are turgid and expand well beyond the shoot tip
- Leaves orientation: leaf blades are oriented toward the sun
- Leaf color and temperature: canopy is green and healthy and leaves are cooler than our body temperature
- Berries are turgid
Mild to moderate water-stressed vines:
- Shoot tips are compressed and they are enclosed when the last formed leaves are pushed toward the growing tip (Figure 5A)
- Tendrils are drooping or wilted
- Leaf orientation: leaves are oriented away from the sun
- Leaf color and temperature: leaves (starting from the basal leaves) are grayish-green to light-green and they are warm to touch at mid-day (> 100°F)
- If it occurs around bloom/ fruit-set, berry-set may be reduced
Severe water-stressed vines:
- Shoot growth has stopped and shoot tips are dry or aborted
- Tendrils dried or abscised
- Leaf orientation: leaves may roll and dry
- Leaf color and temperature: leaves (starting from the basal leaves) are yellow with necrotic edges (Figure 5B) and they are very warm (well above 100°F)
- Cluster rachis tip may dry if stress occurs at bloom, fruit-set may be reduced, berries may become flaccid if water stress occurs post-veraison
stress in a young planting must be avoided because it can compromise root system establishment and overall vine growth, delay its capability to carry a crop, and reduce cold hardiness. If you notice signs of water stress in young vines and you don’t have a permanent and functioning irrigation system in place, temporary irrigation systems could be used such as a flex tank and hose. It is a very labor intensive operation but it is crucial to ensure the long-term success of your investment. If you notice any sign of severe water stress on your mature vines and you are not able to irrigate them you may want to consider shoot and crop-thinning (especially in heavily cropped vines) to reduce vine demand for water, as well as avoid growth of weeds which can compete with vines for water supply [9].