# Magnesium deficiency

- URL: https://backyardgrapes.com/diseases/magnesium-deficiency/
- Category: Disorder
- Affects: Leaves
- Severity: low
- Favored by: Sandy, acidic soils; high potassium (which competes with magnesium); leaching after heavy rain.
- Easily confused with: Potassium deficiency, Iron deficiency, Grapevine leafroll

**Summary:** Yellowing between the veins of older leaves while the veins stay green — one of the commonest grape disorders.

**What to look for:**
- Interveinal yellowing on older, lower leaves; the veins themselves stay green (a herringbone look).
- On red varieties the interveinal patches turn reddish rather than yellow.
- The pattern climbs the shoot as the season goes on; margins may dry in severe cases.
- Vigour and yield are usually little affected unless the shortage is severe.

**Treatment — Cultural / prevention:**
- Soil-test and check the potassium–magnesium balance; too much potassium induces magnesium shortage.
- On acidic soils, dolomitic lime raises both magnesium and pH over the longer term.
- Keep organic matter up to steady nutrient supply.

**Treatment — Organic / low-toxicity:**
- Epsom salts (magnesium sulphate) as a foliar spray for a fast in-season correction.
- Compost and dolomite for lasting soil-level supply.

Magnesium deficiency is one of the most frequently seen — and most frequently
photographed-and-panicked-over — grape leaf symptoms. Magnesium sits at the centre of the
chlorophyll molecule, so when it runs short the leaf can't stay green between the veins,
which keep their supply longest; the result is the tell-tale herringbone of green veins on
a yellow (or, on red grapes, reddish) background, always on the older leaves first.

It's usually more cosmetic than damaging, and it often isn't a true shortage of magnesium
in the soil at all but an imbalance — too much potassium crowding magnesium out of the
vine. A foliar Epsom-salts spray greens the leaves up quickly for the season, while
dolomitic lime (on acid soils) and attention to the potassium–magnesium ratio fix the
underlying cause. The main thing to rule out is leafroll virus, which produces similar
colours but reddens the whole blade and doesn't respond to feeding.

**Image credits:**
- Magnesium deficiency: interveinal yellowing with palmate vein banding on an older grapevine leaf — [Yara UK](https://www.yara.co.uk/crop-nutrition/grapes/nutrient-deficiencies-grapevines/magnesium-deficiency-grapevines/)
- Magnesium deficiency: interveinal yellowing and marginal browning on an older grapevine leaf — [Yara UK](https://www.yara.co.uk/crop-nutrition/grapes/nutrient-deficiencies-grapevines/magnesium-deficiency-grapevines/)
- Magnesium deficiency in a grape leaf caused by low soil pH (yellowing between the veins, green wedges along the veins) — [Mississippi Fruit and Nut Blog (Mississippi State University Extension)](https://msfruitextension.wordpress.com/2014/08/11/magnesium-deficiency-in-the-vineyard/)

**Sources:**
- [Red Leaves in the Vineyard: Biotic and Abiotic Causes (FS1260)](https://njaes.rutgers.edu/fs1260/) — Rutgers NJAES Cooperative Extension
- [Red Leaves in the Vineyard - Diagnosis and Management](https://extension.umd.edu/resource/red-leaves-vineyard-diagnosis-and-management) — University of Maryland Extension
- [North Carolina Winegrape Grower's Guide, Chapter 9: Vine Nutrition](https://content.ces.ncsu.edu/north-carolina-winegrape-growers-guide/chapter-9-vine-nutrition) — NC State Extension