Grapevine Heat Stress and Sunburn Management

George Zhuang, UCCE Fresno County
From Vit Tips Sept. 2019

Figure 1. Berry shrivel, raisining, and sunburn during heat wave

Heat waves are common in the San Joaquin Valley (SJV) during the end of the growing season in July and August. In 2017, grape growers in the SJV experienced two-to-three weeks with daily maximum temperature ≥ 110 °F. These conditions lead to severe water and heat stress resulting in yield loss and poor fruit quality for some growers. Berry sugar, organic acid, anthocyanins, and phenolics all can be negatively affected by extreme temperatures. Sugar accumulation can be slowed since leaf photosynthesis is lower when the canopy temperature passes 30 °C. Organic acid degrades faster under higher berry temperature as well as anthocyanins and phenolics. When the heat wave occurs, it usually also causes water stress. High daily temperature coupled with severe water stress will eventually reduce the berry size, reduce fruit quality and make the berry shrivel and raisin (Figure 1). Several vineyard design options (1 - 2) or cultural practices (3 - 6) can be adopted to help minimize the loss of yield and quality.

1.                   Row orientation

2.                   Trellis selection

3.                   Leaf removal

4.                   Irrigation scheduling

5.                   Canopy shading

6.                   Canopy cooling

Row orientation should be optimized to have equal light exposure on both sides of the canopy is southeast to northwest with 45° angle. The traditional row orientation of raisin vineyard in the SJV of east to west is still good. While this row orientation was setup to allow raisins to be dried with southern exposure, it also works to minimize the direct light exposure cluster through canopy shading. North to south row orientation should be avoided for sunburn susceptible varieties, e.g., Muscat of Alexandria and Chardonnay because west-facing side of the trellis intercepts too much sunlight in the hottest part of the day.

Figure 2. Mechanical leafing on morning side of the canopy at bloom

Trellis selection is as important as row orientation. Trellis systems with a sprawling canopy are preferred under a hot climate, allowing the canopy to shade the fruit. Two-wire vertical trellis, or “California Sprawl”, is suitable for the SJV. Single high wire, quad high wire T top and Y trellis systems can all provide the required canopy shading on the clusters to prevent sunburn. Leaf removal is applied to a canopy to provide enough light exposure and air circulation on fruit-zone without exposing the clusters too much direct sunlight. Hand or mechanical leafing (Figure 2) of the morning side of the canopy helps to avoid the afternoon sunlight exposure on fruit zone.

Figure 3. Shade cloth on fruit-zone at afternoon side of the canopy

Proper irrigation management is critical to avoid excessive heat damage/water stress as well as berry sunburn. Severe deficit irrigation should be avoided before the heat wave to make sure vines are not excessively stressed during a heat wave. Soil moisture sensors, pressure chamber measurements, and close observation can help growers to assess soil moisture and vine water status. The grape ET report (https://ucanr. edu/sites/viticulture-fresno /Irrigation_Scheduling/) can help growers to decide the amount of irrigation per week to avoid severe grapevine water stress.

Figure 4. Sun protectant of CaCO3 foliar spray post veraison

Canopy shading including shade cloth (Figure 3) and sun protectant foliar sprays, e.g., Kaolin and CaCO3 (Figure 4), can be used to reflect light away from the canopy and fruit in order to avoid excessive light exposure and sunburn. Cost and timing might be the most important factors when growers decide to use. Generally, the optimum timing to apply canopy shading is after berry set or several days before heat wave. Canopy cooling can also be applied by in-canopy misting, but can pose a risk for disease management. Studies in Australia have found by in-canopy misting it can cool canopy and cluster, and ultimately improve yield and berry composition during heat waves (https://www.wineaustralia.com/research/search/completedprojects/ ua-1502).