by Pierre Helwi (June 2017)
If you ask a winegrower in Texas about his biggest fear, the answer depending on region may well be hailstorms during the season.
Damage from hail can range from random spots on leaf blades and scares on shoots to total defoliation, broken branches and complete crop loss. Usually, if damage occurs early in the season, vines have the ability to recover by reshooting from secondary buds and wounds can “heal” properly. However the degree of recovery depends on vine health and vigor, timing of the damage and its intensity.
In which situations should the vineyard be retrained after hail damage?
If damage happens early in the season and the potential number of healthy clusters is significant and economic (more than 60%), simply leave the inflorescences and wait for the canopy to regrow and reshoot. In this case, many new shoots on the vine will grow, producing a second crop which will most likely not ripen to an acceptable level by the time the initial crop has achieved target Brix levels. Consequence harvest decisions may be more difficult.
If damage is more than 60%, growers may consider knocking broken shoots to the basal buds and allowing new ones to develop from secondary buds. In fact, shoot loss will stimulate the initiation of fruitful buds from secondary and latent buds, with minimal effect on bud fruitfulness or crop in the following season. This practice would allow the development of healthy canes with a good quality wood for the next season.
Secondary bud fruitfulness is lower than primary buds and it is variety dependent. Varieties such as Cabernet-Sauvignon and Syrah have relatively fruitful secondary buds compared to Riesling or Chardonnay. Clusters on secondary shoots will ripe later in the growing season.
In a later season hail damage scenario (after flowering), retraining vines is not recommended. Shoot loss may stimulate some new canopy growth from dormant buds, however fruit ripeness will be greatly delayed, and the quality is likely to be affected.
In young vines, the scarring on a shoot that will eventually become the trunk can interfere with sap flow and may provide sites for entry of trunk diseases. In this case, cutting back wounded shoots and retraining a new shoot as a trunk should be considered.
Vines (A) three days and (B) one month after hailstorm. Secondary shoots emerged from secondary and latent buds after an intense hailstorm early in the season (April 16, 2017).
What about the pest management program?
In wet and warm season, scarred berries will be prone to bunch rots and Botrytis cinerea, and sour rot organisms in addition to different insects and pests. An adequate fungal disease program will protect sound berries from infection but cannot prevent the development of rot on damaged berries (attention required to the pre-harvest interval for all fungicides). An opened canopy allowing the circulation of air at the fruit zone will also help to maintain a lower disease pressure.
After hail storm, the application of a broad-spectrum fungicide may help avoid opportunistic fungi, including Botrytis cinerea. A botrytis-specific fungicide may be helpful as well. Elevate, Vangard, Scala or the highest label rate of Pristine would be suitable choices. The development of Crown gall and wood diseases such as Eutypa dieback and Botryosphaeria can also be an issue for wounded shoots and canes.
After a hailstorm, it is important to inspect damage to plants as soon as possible. If damage is extensive and early in the season (between budbreak and bloom), re-training of vine parts may be necessary and pruning may need to be adjusted to obtain healthy wood and sufficient buds for next season. If the damage occurred pre-veraison, the injured berries may scar over and continue developing, or they may shrivel without rotting. Crop thinning might be appropriate for a homogenous ripening pattern and for young and weak vines.