Adopting Best Practices After a Hailstorm Episode in Viticulture

Technical articles - Climate resilience

Viticulture Viticulture Pruning and vine management

Hail damage on vine

Vine plots are regularly confronted with episodes of hail significantly impacting the health of the vines and the season's harvest. While there are means to prevent damage caused by hail, it is also important to know the practices to implement after a hail episode in order to care for the vine and ensure a small harvest.

The vine's resilience capacity

First of all, it is important to recall that the vine has significant resilience capacities thanks to the energy reserves stored in its wood throughout autumn.

The amount of these reserves from photosynthesis depends on various factors:

• Exposure to light
• The amount of leaves relative to fruiting (the load)
• The persistence of foliage after harvest
• Parasitic pressure
• The quality of lignification

If these reserves provide the plant throughout the year with the energy necessary for its growth and flowering, they also ensure the vine's resilience to various stresses it may face. To allow the plant to retain all its resilience capacities, it is important to help it maintain a sufficient energy stock throughout the year.

For this, one must be aware of the sources of energy deficit:

• Reduction of a cane or topping of the vine
• Pruning impacting wood older than 2 years
• Excess load in grape relative to effective leaf area
• Soil fertility problems
• Poor sanitary condition of foliage during the season

Plot diagnosis

The first step after a hail episode is to assess the damage on the vines.

In vine, damages can be various:

• Complete defoliation
• Impacts on old wood
• Potential losses for the current season's harvest
• Difficulties in pruning future shoots and yield reduction due to reserve problems, in case of hail from July onwards
• Destruction of buds

Fertilization

It is not necessary to apply fertilization to the soil or foliar immediately. Indeed, the vine is weakened, its metabolism reduced, and the supplied nitrogen would not be available at the right time; it could even increase breakage and the risks of botrytis.

Strengthening the vine with plants

After a hail episode, it is possible to support the vine in its healing process using plant extracts.

If the pressure of diseases (powdery mildew and downy mildew) is strong, treat quickly with sulfur and copper, ideally using sulfur dust at flowering, otherwise wettable sulfur (5-6 kg/ha), copper (200 g/ha of copper metal) and calcined kaolinite (1 to 2%).

Without disease pressure or under low pressure:

1. Spray as soon as possible a mixture of:
   • Valerian (biodynamic preparation 507) at a dose of 5 to 10 ml/ha in 35L of warm water dynamized for 20 min. Valerian extract is known for its healing action on plants.
   • A few drops of Arnica mother tincture to reduce plant stress.
   • Nettle tea to remineralize the plant and help it heal. You will find the conditions for a successful tea in this article.
   • Clay at a dose of 25 to 30 kg/ha or wettable calcined kaolinite at 5 to 10 kg/ha to sanitize the environment by drying it.
2. In case of significant damage, it is possible to:
   • Dust with 25 kg of Clay / ha and 10 to 15 kg of sulfur dust.

Pruning the vine after hail damage

Arbre & Paysage 32 organized a webinar following the 2020 hail episodes, find the speakers' advice in this video:

Early hailfalls

In case of early hailfalls (before June 15):

• If all the fruit is destroyed and the shoots are cracked and peeled down to their base, pruning is necessary. The objective in this case is to promote the growth of correct shoots that can be used for the next winter pruning and avoid weak, bushy growth on all the vine's injured parts. This re-pruning should be done as soon as possible after the hail and it is recommended to stop it 15 days after the hail to avoid creating a second stress for the plant.

• If the herbaceous shoots are neither cracked nor peeled and intact inflorescences remain, re-pruning is unnecessary, the development of the inner shoots will contribute to vegetation renewal.

If re-pruning is necessary, its objective is to maintain 4 to 5 vigorous shoots per vine:

• For vines trained in Cordon de Royat or goblet, simply cut the shoots with pruning shears half a centimeter from the spur wood. It is also possible on the spurs to cut back the top bud of the spur with pruning shears to start the basal shoot. After the new budburst, monitor the number of new shoots and remove some if necessary.
• For vines pruned in simple or double Guyot, cut back the long wood leaving 2 to 3 buds at their base. Green buds peeled by hail are cut back with pruning shears half a centimeter away. This is especially the case for green shoots peeled on the spurs.

Late hailfalls

In case of late hailfalls (late June, July, August):

• On adult vines, it is advised to leave the vine as is and not consider re-pruning. If possible, remove the affected clusters that may develop diseases (bunch rot or botrytis). This helps relieve the vine by reducing the load.
• On young vines or weak vigor vines, relieve the vine by dropping the clusters.

Following winter pruning

Prune as late as possible, taking several precautions:

• On plantings of the year: since lignification of shoots is not always satisfactory, it is possible to hill up young plantings to protect them from frost.
• On vines aged 2 to 3 years: damage assessment is done by removing bark strips from the young trunk. If lesions reach the wood and necrosis is observed, do not hesitate to cut back the trunk by 2 to 3 buds above the soil to rebuild a new trunk.
• On adult vines: damage to shoots can cause pruning problems. Flower primordia differentiate in buds from June to July, so hail in late July or August will have little effect on next year's fertility. It is advised to keep the short prunings on spurs that are little or not damaged. For Guyot-pruned vines, it can be quite easy to find a long shoot little or not damaged by hail. Switching to Cordon de Royat for a year is also possible.

Examples of re-pruning

Find examples of vine re-pruning following a hail episode in this article.

Treatments

Only plots still having green organs present a high risk regarding diseases (downy mildew, bunch rot, powdery mildew). To protect this remaining vegetation from future contamination, they must be treated quickly after hail. Other "bare" plots require no intervention.

Protection against bunch rot

It is usual to treat a hail-damaged vine to prevent the development of bunch rot (Coniothyrium diplodiella), a parasitic fungus present in the soil. Spores of this fungus can be splashed by hail onto grape wounds. The spores germinate and produce mycelium, which invades the berry and neighboring berries via the peduncle canal. The entire cluster eventually dries out and mummified berries are covered with white pustules. After hail, treatment must be done very quickly (ideally within 12 hours) because beyond 20 hours, treatment is ineffective against the parasitic fungus. It is preferable to start treatment on less affected vines and finish with the most damaged plots.

When intervention within this timeframe is impossible, it is better to wait for new vegetation to develop from the inner shoots. Treatment must be done with a contact fungicide, preferably folpel, or captan. Copper should be avoided, as it has no healing effect and can slow shoot growth. The vintner's best ally remains the weather: warm and dry conditions will stop bunch rot development.

Protection against downy mildew and powdery mildew

Generally, early hail-damaged vines, whether re-pruned or not, require increased monitoring against downy mildew and powdery mildew, until harvest and beyond. To allow good lignification, foliage must remain functional until autumn. Moreover, regrowing vegetation is extremely sensitive to parasites.

Unfortunately, multiple hail episodes can sometimes occur in the same year, up to three times in some areas (as in 2009). In case of exceptionally low and prolonged winter temperatures, some vines may fail to restart. This situation is explained by a very late last summer regrowth of vegetation, exhausting the vine's reserves. The too short vegetation cycle then did not allow reserves to be rebuilt before autumn.

Cette technique s'applique aux cultures suivantes

• Vine

La technique est complémentaire des techniques suivantes

• Protect perennial crops from hail