Appears in
Oxford Companion to Wine

By Jancis Robinson

Published 2006

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magnesium, mineral element essential for healthy vine growth. It is an essential component of chlorophyll, and so chlorosis is a common symptom of magnesium deficiency. The most conspicuous symptom is discoloration between the main veins of the leaf, which becomes particularly noticeable around veraison. This zone is yellow for white varieties, and red for dark fruit varieties. This deficiency can be severe in some situations, reducing yields, and slowing fruit ripening. Magnesium deficiency may be associated with bunchstem necrosis, in which the bunch stems and berries shrivel before ripening. Maturity is affected and wine quality suffers.