What is the cause?
Localized or generalized withering symptoms can appear as the grapes ripen due to various causes:
-
Dehydration is the main cause. This is a natural result of the grapes ceasing to accumulate sugar once they have reached maturity. The berries then wither in an uneven manner on the bunch and the stalk is unaffected. A loss of berry weight and a concentration of sugars can be observed but the quality of the grapes is not affected. Severe water stress can also cause the supply to the berries to be cut off in favour of the plant's survival. In extreme cases, this can lead to a check in ripening and withering of the entire bunch. Qualitative losses are then significant. It is advisable to monitor this evolution and to harvest earlier in order to limit any negative consequences due to severe dehydration.
-
On the other hand, stem dehydration is generally observed at the plot level, and its consequences can be more serious. This drying out is due to a physiological disorder that affects the vines from véraison onwards. It is mainly due to deficits in magnesium, calcium, water and/or excess potassium. The ends of the stems become necrotic and reduce the flow of water, sugars and nutrients in general, which eventually causes the berries to wither. Some grape varieties and rootstocks are more sensitive than others, such as Cabernet Sauvignon and SO4. When this phenomenon occurs in a plot, it may be partly due to fertilizer and input management. Foliar fertilization, mainly with magnesium, at véraison can reduce this phenomenon. This does not require early harvesting.
-
Berries can also wither because of damage inflicted during the growth period. This can be when the trimmers pass between the rows and a herbaceous projection hits part of the stalk. Some berries may be cut, no longer be nourished and therefore wither. It is the same during thinning, the positioning of the remaining bunches within the vegetation can cause twisting and pinching of the stalks. But this only happens occasionally and does not reflect the state of the entire vineyard. This does not oblige the winegrower to harvest earlier.
-
Finally, another phenomenon, more anecdotal but no less serious, called "berry shrivel" or "sugar accumulation disorder" (SAD) may be observed. It is a disorder related to sugar metabolism and nitrogen compounds. This phenomenon is similar to stem dehydration, but the stalk remains green and the consequences are even more serious. A lower yield and a decrease in the quality of the harvest are observed. The grapes wither suddenly, lose their colour, and have lower pH and sugar concentrations that can drop by up to 70% compared to an unaffected berry. This shrivelling and very low sugar accumulation is thought to be related to necrosis of phloem tissue and discolouration of xylem tissue. Earlier harvest dates will not change this phenomenon observed in many parts of California.