Jellies

In plants, more or less serious disorders can appear when temperatures drop below a certain limit, they sometimes become fatal for the vine. Thus, frosts can occur in winter, early in autumn, but also late in spring. Negative temperatures mean that part of the water in the tissues will escape from the cells and form crystals in the inter-membrane space, causing their destruction. The lamina then becomes twitched and blistered on the upper surface, while the lower surface is tense and sometimes torn.

The different types of jellies

• The early frosts of autumn trigger prematurely the dormancy of the vines. They cause the leaves to drop, and the plants not having enough accumulated reserves present a bad growth of the wood.
• The winter frosts are a limiting factor for growing vines. Thus, depending on the region, it is advisable to cultivate grape varieties that are more or less resistant to cold. Note that if the lowering of the temperature is gradual, the negative values ​​are very well supported by the vine which gradually goes into vegetative rest. However, too long a period of intense cold can cause the formation of frostbite , radial slits penetrating more or less deeply into the trunk at the base and at the top of the vines. The most feared situations are mild winters allowing premature recovery of the vines which are starting to bud. It only takes a sudden return of cold to cause significant damage. We then observe the destruction of the buds which sag easily. Frozen tissue shows a dark to brown tint and eventually necroses. Many buds may not develop, in which case secondary and tertiary buds will appear. When lifting the bark of the frozen branches, a blackish to brown coloration is observed in the cambium and the bast which have been destroyed. The section of the arms and branches shows a red coloration of the bast. Frequently the graft weld bead is affected while the roots are occasionally damaged, the smallest being the most sensitive.
• The spring frosts occur when the vines resumed activity. The symptoms are variable and depend on the vegetative stage. When budding, thebudsandtwigsmeasuring a few cm turn brown and dry out (figures 1 to 12). If the frost is intense, there may be total destruction of the vegetation which turns brown after the thaw and takes on the appearance of a cooked salad, before drying out (Figures 3, 4, 7 and 10). The inflorescences also turn brown and dry out (Figures 4 and 5). If the frost is weak, only the high leaves and the apex are affected (Figure 1). Remember that 2 types of spring frosts are known:
  • the white frosts which are characterized by the formation of a thin "skin" of ice crystals. It manifests itself in cold, clear weather with nighttime temperatures between -3 ° C to -5 ° C. These frosts mainly affect the lower slopes and wetlands. The intensity of the damage varies from a few affected shoots to all of the young vegetation;
  • the black frosts occur during a massive influx of cold polar air, overnight temperatures are very low of -5 ° C to -10 ° C. These can completely destroy young shoots and cause significant crop losses.

Factors more or less influencing the effects of frosts

• Favorably: the presence of weeds on the ground, plowed ground, the absence of hedges downstream from a plot of vines located on a slope;
• Unfavorably: the practice of late pruning which shifts bud break, stripping away the buds from the ground, pruning methods that delay bud break, the presence of a hedge upstream, the choice of plots not very conducive to frost (avoiding as far as possible the shallows, the basins (figures 11 and 12), and the proximity of a wood).

Actions to be taken

In the presence of damage to vines, it is often too late to intervene. If the shoots are completely frozen, there is nothing more to do. Secondary buds will gradually develop over a few days. If the frost is partial and the inflorescence is intact, the development of new twigs may take place. If the inflorescences are destroyed, the vine should be pruned (Figure 13). Note that some protection methods can be considered:

• the sprinkling of the vine uses the principle of the energy given up by the water when this one passes from the liquid state to the solid state. It is necessary to maintain a constant quantity of water in the process of freezing so that the temperature of the plant does not drop below zero. It is a water-consuming method and it is not practicable in all vineyards.
• the establishment of turns anti-ge l , a "gel hunter" (Frostbuster), a straw cap on vines,
• installation of heating more or less artisanal (heaters oil and paraffin candles) ... are also used to prevent frost.
• Finally, a product was authorized fairly recently to reduce the incidence of frost. It would cause "the accumulation in the leaves of protective molecules lowering their freezing point and strengthening their resistance" ( e-phy ).