Backyard Grapes Identify & treat grapevine problems
Disorder low severity

Magnesium deficiency

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

Magnesium deficiency: interveinal yellowing with palmate vein banding on an older grapevine leaf Yara UK
Magnesium deficiency: interveinal yellowing and marginal browning on an older grapevine leaf Yara UK
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)

Description

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.

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 options

Cultural & prevention

Start here — habits that stop problems before they spread.

  • 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.

Organic / low-toxicity

Gentler sprays and biologicals for active outbreaks.

  • Epsom salts (magnesium sulphate) as a foliar spray for a fast in-season correction.
  • Compost and dolomite for lasting soil-level supply.

Sources

This entry is based on the references below. Read them for full detail and for spray timings and rates valid where you live.

  1. Red Leaves in the Vineyard: Biotic and Abiotic Causes (FS1260) — Rutgers NJAES Cooperative Extension
  2. Red Leaves in the Vineyard - Diagnosis and Management — University of Maryland Extension
  3. North Carolina Winegrape Grower's Guide, Chapter 9: Vine Nutrition — NC State Extension