Disorders producing symptoms mainly on the older leaves

Deficiencies

Early symptoms of magnesium deficiency include a pale yellow-green interveinal chlorosis of the older leaves on the current season’s extension canes. The chlorosis usually develops at the leaf margin and spreads inwards between the veins towards the midrib, often leaving a relatively wide zone of healthy tissue each side of the main vein (Photos 2a and 2b).

In some cases, the margin of the leaf remains green and the chlorosis and subsequent necrosis start some distance in from the margin (Photos 2d and 2e). On these leaves the necrotic tissue usually forms a regular pattern of isolated patches roughly parallel to the leaf margin giving a distinctive "horse shoe" appearance (Photos 2d and 2e).

A large zone of green tissue is also retained at the base of the leaf close to the point of attachment of the petiole, even on severely deficient plants (Photos 2c, 2d and 2e).

Initially, there is no necrotic tissue associated with the chlorosis but as the deficiency becomes more pronounced the chlorotic tissue turns bright yellow and a marginal or interveinal necrosis may develop (Photos 2c and 2d). The necrosis may also extend a short distance between the veins towards the midrib.

Off all the visible symptoms, those associated with magnesium deficiency are the ones most likely to be confused with other nutritional disorders. However, there are important points of difference which help distinguish magnesium from these disorders, of which boron toxicity and manganese deficiency are the most important.

In the case of boron toxicity, the interveinal chlorosis quickly gives way to a necrosis which extends from the leaf margin to the midrib, whereas with magnesium deficiency the necrotic tissue is confined to the margins or a discrete zone parallel to the margins of the leaf.

The boundary between healthy and chlorotic tissue is also more pronounced with magnesium deficiency than it is with boron toxicity. An additional distinguishing feature is that the symptoms of magnesium deficiency do not spread to the younger leaves, even on severely deficient plants, as they do with boron toxicity.

Manganese deficiency, on the other hand can be distinguished from magnesium deficiency in that the symptoms of deficiency appear first on the recently mature leaves and not on the oldest leaves as they do with magnesium deficiency. Also, with manganese deficiency the entire leaf becomes chlorotic, leaving only a small zone of healthy tissue on each side of the main veins; with magnesium deficiency large areas of tissue remain green, particularly at the base of the leaf.

Furthermore, with magnesium deficiency large areas of leaf become necrotic. This does not occur with manganese deficiency. A further feature distinguishing magnesium deficiency from the other two disorders is that the symptoms do not usually appear in the field until mid to late season.

Magnesium concentrations in fully expanded leaves of healthy plants sampled in the field mid season usually exceed 0.38 per cent of the dry matter. Analysis of leaves from the solution culture studies and from the field indicate that symptoms of magnesium deficiency do not usually appear until the concentration of magnesium in youngest fully expanded leaves falls below 0.10 per cent dry matter.

Magnesium deficiency of kiwifruit is not uncommon in New Zealand. Deficiencies can be contributed mainly to very low levels of ‘available’ magnesium in the soil¹². Much less common is where magnesium deficiency is induced by an excess of other cations in the soil, usually potassium but sometimes calcium. These cations impede the uptake of magnesium by the plant even though relatively high levels of ‘available’ magnesium may be present in the soil.

Magnesium deficiency can be corrected by applying magnesium fertilisers to the soil. Application rates of at least 200 kg/ha of magnesium are required to overcome a deficiency¹². Possible fertiliser sources include Kieserite (15 per cent w/w Mg) and Epsom salts (10 per cent w/w Mg) both of which are highly soluble materials and probably best used for correcting existing deficiencies; the slower release materials such as calcined magnesite (50 per cent w/w Mg), Dolomite (11 per cent w/w Mg) and magnesium oxide (60 per cent w/w Mg) are more suitable for building up or maintaining soil reserves ²⁴.

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2a	2b	2c
2d		2e