By Kathryn Carter (OMAFRA) and John Warbick (OMAFRA)
Response of Grape Vines to Dry Weather
Grapevines are more tolerant of dry seasons than other fruit crops, and in some instances low levels of water deficit can increase fruit quality. However, excessive water stress can have a negative impact on yields, vine health and wine quality.
Excess drought stress on vines reduces root formation, vine size and the capacity of the vine to produce a full crop. Drought stress reduces the berry size, resulting in a reduction in yields. Dourght reduces photosynthesis resulting in delayed ripening, impacting the development of sugars and phenolic compounds within the fruit. In wine grapes, this can create an off flavouring (green flavouring) in white wines (Dr. Andy Reynolds, CCOVI, Brock University). Wines created from the drought stressed vines can also age poorly and have a short shelf life.
Drought also reduces fruit bud formation having a negative impact on next years crop. Drought stressed vines are more prone to nutrient deficiencies, and cold injury.
Mild water stress can be used by growers to help regulate the growth of vigorous vines and manage berry growth.
Factors affecting the amount of water available to the vine
The amount of water available to the vine is affected by the following:
Soil water holding capacity: Heavier soils such as loam and clay can hold more water than sands.
Soil Depth: Deep soil holds more moisture than shallow soils.
Grapevine root system: Young vines have smaller root systems and are more susceptible to water stress. The majority of roots on mature vines can be found throughout the top metre of soil.
Competitive plants: Green and actively growing cover crops and weeds in the middle row and between vines can compete with vines for water.
Varietal differences in susceptibility to water stress
Grapes vary in their susceptibility to drought stress. Table grapes are the most sensitive to drought due to the negative impacts on berry size. Juice grapes and some hybrids tend to be more vulnerable than vinifera because of the need to maximize yields. Red wine grapes are able to handle water stress better than white wine grapes. Large vines (Concords) need more water than smaller vines (Reisling). Divided canopies use more water than single canopies (Martinson and Lakso, 2016).
Impact of Water Stress at different developmental stages
The timing and severity of the drought stress can also have a significant affect on crop response.
Pre-bloom- Soil moisture levels are usually adequate in the spring, and temperatures are cooler meaning water stress is unusual at this time.
Bloom to fruit set-Mild stress should gradually develop after bloom so that good fruit set can occur, but the growth of the berries and shoots are slowed somewhat.
Fruit set to veraison- After fruit set, the canopy is filling in, and at this time mild water stress can help to manage vegetative growth, improve fruit set, reduce berry growth and increase fruit colouring and aroma compounds. However, excess drought stress at this time can stunt the vines and leave them with non-functional leaves, affecting ripening later in the season. High levels of water stress at fruit set can cause reduced fruit set or berry drop sulting in reduced yield by up to 50%. Extreme water stress at this time can result in small berries, berry shrivel, and delayed or prevented fruit maturation. From fruit set to veraison, severe water stress can reduce yields by up to 40%, mainly due to a reduction in berry size. Drought stress at this time can also reduce bud fruitfulness, impacting next years crop.
Veraison to harvest-In the first few weeks after veraison, berry growth and sugar are less impacted by drought. With moderate stress after veraison fruit will develop normally, but fewer resources will be available for shoot and root maturation. Flavours in the fruit tend to develop in the last few weeks before harvest, and extreme water stress close to harvest can limit flavor development, fruit sizing and cause premature fruit drop. In addition leaves may exhibit symptoms of severe nutrient deficiency (potassium or nitrogen). Excess water stress after veraison may prevent canes from acclimating for the winter.
Young vines-Drought stress is particularly harmful to young vines (0 to 5 years old) which have smaller root systems. Lack of moisture can negatively impact the growth and development of young vines. If you notice signs of water stress in young vines and don’t have an irrigation system available, temporary irrigation systems could be used to apply water (flex tank and hose). As vines mature they become more tolerant to drought stress. Remember to irrigate new “replants” in existing vineyards, as sometimes they can be forgotten.
Figure 1: Young vines with a drip irrigation system
Symptoms of Water Stress in Grapes
There are several observable signs on water stressed vines that can indicate a stressed plant. Leaves are pale green or bleached in appearance and leaf margins may appear scorched. Leave may wilt, feel hot to the touch, and begin to senesce resulting in premature leaf drop. Leaf blades may orient to the sun and the angle between the leaf blade and petiole will decrease to less than 45 °. Tendrils may be dry, flaccid, or wilting and may dry up and fall off. The shoots will stop growing and turn a greyish-green colour, and leaf formation is slow or non-existent. Drought stressed vines produce berries that are smaller in size, shrivelled and have looser clusters. Drought may reduce or cease soluble solids accumulation resulting in a delay in fruit maturation. Drought may reduce the uptake of nutrients resulting in nutrient deficiency symptoms (ie potassium deficiency and N deficiency).
Although grape vines are incredibly tolerant to lack of moisture, extreme drought stress can have significant negative impacts on grape vine growth, health and wine quality. For options to mitigate drought stress in vineyards, stay tuned for the next ON fruit blog posting.
Fiola, 2020. Drought stress, vine performance and grape quality.
Martinson and Lakso, 2016. How grapevines respond to stress.
Pool and Lakso, 2000.Recognizing and Responding To Drought Stress In Maturing Grapevines