Yellowing leaf caused by chlorosis
What is Chlorosis?
Chlorosis is a common term used to describe symptoms of uniform yellowing of leaves. It may be caused by any number of stresses and although it can affect many plant families is most commonly seen in acid loving plants. These plants are generally known as 'ericaceous' but this is not a strictly accurate classification. Plants such as such as Rhododendrons, camellias Pieris and Liquidambar are all typical example of the family Ericaceae which will show the classic symptoms of Chlorosis, however there are many plants outside of this family which will also suffer to a lesser or greater extent. So just what is chlorosis?

In alkaline soils, plants from the Ericaceae family have difficulties in taking up iron and magnesium from the substrate through the roots. This is typified by the characteristic interveinal yellowing - known as chlorosis - as both iron and manganese are vital for the formation of chlorophyll pigments within the leaves.

Yellowing rhododendron leaf caused by chlorosis
What is Chlorosis?
This characteristic patterning is a direct response to these specific nutrient deficiencies and occurs because the chlorophyll pigment found in the vascular bundles – the leaf veins – will remain unaffected for longer periods than chlorophyll pigment found in the cells between the vascular bundles.

Also, because of the low mobility of iron within the plant and relatively higher concentrations within older leaves due to the formation of iron binding proteins, leaf discolouration is far more prevalent in the new, juvenile leaves found near to the growing points. In extreme cases newly formed leaves can grow through almost pure white in colour.

Chlorosis can be treated by spraying the leaves with soluble iron foliar feeds every 2 -4 weeks or by lowering the soil pH. This is achieved by applying chelates, ferrous sulphate, aluminium sulphate, or sulphur to the soil surface and allowing them to dissolve into the soil by watering and rainfall. At the very least, use soluble, acidic plant fertilisers such as Miracid or Sequestration as a weekly liquid feed. Be aware that it will take weeks and not days for the effects to show through.

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