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Ministry of Agriculture, Food and Rural Affairs

Magnesium

Magnesium (Mg) is an essential part of chlorophyll and aids in the formation of sugars, oils and fats. Magnesium is a part of the chlorophyll molecule. Magnesium deficient trees have older leaves that are pale in colour. Leaf analysis used in conjunction with a soil test is the best way to evaluate magnesium requirements.

Identification

  • Magnesium is mobile within the plant and therefore deficiency usually appears first on the older (basal) leaves as it translocates to the growing tips and leaves
  • Fruit matures early, is highly coloured and drops prematurely
  • Fruit bud formation may be limited
  • Interveinal chlorosis in leaf centres with an inverted green V around the midrib
  • Leaves develop marginal chlorosis and necrosis
  • Severe deficiencies will cause the leaf margins to curl and turn brown
  • Necrotic spots develop later and eventually the leaves drop prematurely
  • Symptoms show most clearly on spur leaves

Often Confused With 
Leaf scorch – damage to succulent young growth following high winds at high temperatures

Excessive boron - Brown spotting along the midrib on the underside of leaves; Cankers on young twigs and petioles.

Scouting Notes
Symptoms occur on older leaves and are most pronounced in mid- to late summer, and are more severe in wet years.

Management Notes
High applications of potassium or ammonium can induce magnesium deficiency symptoms on soils that originally contained enough magnesium so avoid excess nutrient applications.

Foliar sprays may be used but deficiency should be confirmed with tissue analyses before use. Apply foliar magnesium sulphate (Epsom salts) or chelates beginning immediately after bloom. Depending on severity of deficiency, there may be need for multiple foliar applications.

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