Phytosanitary Protection Against Summer Hailstorms

Hail in vineyard



Hailstorms can be disastrous for a grape harvest. Just look at the size of the hailstones that destroyed homes and businesses, halted traffic, and totaled vehicles on a highway in Fidenza, Northern Italy, on July 26th. If this summer is any indication of what’s to come, then viticulturists need to take steps to prepare for extreme weather down the road.

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Severe weather appears to be the new normal in Italy. While the Glass Fire scorched over 67,000 acres of wine country in California in 2020, hailstorms, thunderstorms, and powerful winds rained down on Italian crops, including wine grapes. In 2018, growers and vintners reported some yields suffered losses of up to 70% after spring and summer hailstorms. Viticulturists estimate that Italy sustained over 500 million EUR in damages to agricultural crops that year. Among the affected wine regions of Italy include some parts of Piedmont, Emilia-Romagna, Lombardy, and Veneto. As a result, some farmers have turned to the secrets of their ancestors… installing anti-hail wires to better resist future hailstorms. Other farmers in Italy, like those on the slopes of Mount Etna in Sicily, have turned to precision viticulture to continue to ensure high-quality wines despite changes in the climate.

In France this summer, a hailstorm hit the Vouvray region of Indre-et-Loire with as much as 50 mm of rain and hail in just three minutes. The storm arrived on the heels of a severe springtime frost that created yield losses for that year’s vintage. The same month, in Spain, as many as 2,000 hectares of Ribera del Duero were affected by a hailstorm, representing 10% of the region, with Pesquera de Duero, a village noted for its balanced and fruity red wine, receiving the brunt of it. Winemaker Vega Sicilia, with its prized Alión wine, saw damage to its 32-hectare vineyard from a storm that left a carpet of hail as much as 10 cm deep in certain areas.

Summer harvests in Italy, France, and Spain are unlike most. Just as winegrowers see the finish line after surviving cold Mediterranean winters, spring frosts, and scorching heatwaves, the promise of veraison—when white grapes replace chlorophylls with carotenoids and red grapes blush with pigmentation from anthocyanin— is beset with violent hailstorms that appear out of nowhere to destroy thousands of hectares in minutes. For centuries, a typical mild summer season rang the proverbial bell to mark another milestone. And yet, as of late, when heatwaves push farmers to apply sunscreen to grapes and increase water usage, there seems to be no break in the warm atmosphere that brings on strong thunderstorm clouds loaded with ice pellets that plummet from the sky to strip leaves from vines and damage the stem and berries.

Hailstorms are not new, but winegrowers fear hail damage because unlike other forms of severe weather, it strikes suddenly and without warning, and causes irreparable damage to crops. When cumulonimbus clouds appear in summer storms, these clouds rise high into the colder parts of the atmosphere and the water vapor inside them turns to ice crystals. A stronger updraft gives hailstones plenty of time to get bigger as it allows the hailstones to go around several cycles inside a cloud. The size of an individual hailstone is directly linked to a thunderstorm’s strength. The more powerful a storm, the longer the hail can remain and grow inside the cumulonimbus cloud. When the weight of these ice crystals becomes too much for the cloud to hold, they fall to earth. Small hailstones often melt on their trip to the ground, but a hailstone larger than 1-inch in diameter is considered “severe” and part of a severe thunderstorm. Most thunderstorms feature at least six different sizes of hailstones.

Meanwhile, the heavy pellets do little to help drought conditions, but instead, cause landslides. This summer, Italy experienced record heavy rains in addition to the hailstorms that caused landslides and soil erosion. One of the positive outcomes of precision viticulture practices has been analyzing data to bring about results, like reducing tillage of the soil to minimize soil erosion.

Strong Plants Are Hardier Against Physical Damage

Nothing will test the strength of a plant like a hailstorm. The underlying health of the vine is a factor in how much damage a hailstorm will cause to the plant. As climate change worsens, dangerous weather events are becoming more frequent and severe. If extreme weather is the new normal for viticulturists, phytosanitary measures are increasingly important prior to the months in which extreme weather is already known to strike. June is particularly conducive to hailstorms. Phytosanitary measures are methods used to control the health and vigor of a plant. Strong plants are hardier against physical damage. Hail can cause significant damage to leaves, stems, shoots, flower clusters, and fruit berries. Leaves, flowers, or berries can be damaged or knocked to the ground, as well as bruised, torn, or holed. Trunks, cordons, and shoots can be broken or sustain cracks.

Before summer hailstorms hit, be sure to fertilize your soil properly. Along with proper watering and care, fertilizing can give your plants some extra strength. Nitrogen-rich grapevine fertilizers (such as urea, ammonium nitrate, and ammonium sulfate) should be applied after the vine has blossomed or when grapes are about ¼ inch (0.5 cm) in size. Balancing the three types of pesticide—herbicide, fungicide, and insecticide—can also help keep plant diseases from weakening the plants.

With extreme weather comes a rise in insurance claims. Violent hailstorms in southwest France1 caused so much damage across hundreds of vineyards that the winemakers almost filed bankruptcy had it not been for the ability to file agricultural claims. Producers in Cognac and Bordeaux—France’s second-largest wine-producing region, estimate 10,000 hectares out of a total 70,000 hectares were affected. Precision viticulture tools, like TerraviewOS’ all-in-one software platform, provide alerts and recommendations to warn crop farmers of weather conditions that may be conducive to hailstorms.

If every hailstorm has a silver lining, it must be that the pellets of frozen rain will test the vigor of the vine crops. A wound is considered any break in the outer protective bark of the tree that exposes the xylem. One of the worst consequences of mechanical injury to the trunk, vine, and root system is the wound creates an entry point for pests, like infecting spores and insects. At the same time, the wood cells are reacting to these new invaders and a chemical barrier full of phenolic compounds closes the wound caused by hail to contain any infection. In most cases, the plant does not generate new wood cells to replace the lost cells but rather confines the wound with the new callus tissue. The rate at which the vine closes its wounds is correlated with vine vigor. If a vine has many long shoots as well as larger leaves and yields a large crop, it generally has a lot of vine vigor and will close its mechanical injuries faster than a yield with less vigor.

Good practices, such as fertilization, smart irrigation, pest control, and crop load management, encourage vigor and help speed the wound closure process as well as prevent decay. After a storm, growers need to check for callus formation of the vines, lateral shoot formation if defoliation occurred, scarring or berry loss if the damage occurred during the early stages of fruit set, and fruit rot during or after veraison.

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