Magnesium deficiency in orange trees (top photo) and in lemon trees (middle photo) results in a distinct yellow pattern in the older leaves. The yellowing begins near the edge and towards the apex, leaving a triangle of green at the base of the leaf

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Magnesium deficiency affects the older leaves first—note the young leaves are showing no pattern. A heavy crop can accentuate the deficiency in late summer, leading to a heavy fall of the yellow leaves in autumn. The tree can be left severely defoliated and weakened.

Zinc deficiency in citrus

Zinc deficiency, described as ‘little leaf’, ‘mottle leaf’ and ‘rosetting’, is one of the most damaging and widespread nutritional disorders of citrus. Leaf analyses conducted by NSW Agriculture have shown that over 60% of citrus orchards are low in zinc. The deficiency is most acute in alkaline soils. It also affects citrus growing on acid coastal soils.

Even in its earliest stages, zinc deficiency lowers yield, reduces tree vigor and makes fruit small and poor in quality. Leaf symptoms include small, narrow leaves (little leaf) and whitish-yellow areas between the veins (mottle leaf). Leaves also crowd along short stems (rosetting), and smaller twigs die back. Symptoms are often more pronounced on the northern (sunny) side of the tree.

Zinc deficiency is usually most severe in the spring growth (middle photo). Note that leaves from the previous summer flush are not affected. However, when deficiency is severe and is not treated, both the summer flush leaves and the spring flush leaves are affected (bottom photo).

Iron deficiency in citrus

The plant needs iron to produce chlorophyll. Iron deficiency causes a distinct pattern in the leaf caused by a loss of chlorophyll;  only the main veins stay green. Iron does not move easily within the plant, so young leaves are the worst affected, showing the symptoms, while older leaves may remain green.

Iron deficiency is sometimes called ‘lime-induced chlorosis’, as it is worse in calcareous (calcium rich) soils with a high pH. This makes iron unavailable. The problem is accentuated where free lime concretions are found near the surface of the soil. A high water table (which causes a low soil-oxygen supply to the roots) or a low soil temperature can further aggravate the problem.

Effective control measures include careful irrigation (avoid overwatering) and the selection of tolerant rootstocks. Do not plant Poncirus trifoliata rootstocks in problem soils, as this variety is more sensitive to iron deficiency than sweet orange, rough lemon or Troyer citrange. Cleopatra mandarin is one of the most tolerant rootstocks if there are no drainage problems. Lemons are more prone to chlorosis than oranges and grapefruit.

Foliar sprays of iron sulfate or iron chelate are ineffective. If a few trees are affected, try iron chelate applied to the soil under the trees; however, this treatment is uneconomic for a large block—management is the only option here.