Disorders producing symptoms mainly on the older leaves

Deficiencies

For many plant species, zinc is usually considered to be an element with very limited mobility in the phloem⁴⁷. Any interruption in the external supply of zinc will cause deficiency symptoms to appear on young actively growing parts of the plant.

For kiwifruit, there is some doubt as to the degree of phloem mobility of zinc: It has been concluded from observations of kiwifruit in California⁵⁰ that zinc was relatively immobile in the plant, the deficiency resulting in the "little leaf" symptoms on the young growth. This symptom is common to many other plant species.

However, solution culture studies at Ruakura and observations made in the field have consistently shown that symptoms of zinc deficiency are confined to the older leaves, particularly those on canes near the junction of the leaders with the trunk of the vine; while the young leaves remain green and healthy even on severely deficient plants. Moreover, there is no reduction in the size of the younger leaves. These observations suggest that zinc could be relatively mobile in the phloem of the kiwifruit.

Symptoms of zinc deficiency of kiwifruit include a bright yellow interveinal chlorosis on the older leaves with the veins remaining dark green (Photos 4a, 4b, 4c and 4d) - the contrast between the dark green veins and the yellow chlorosis is very striking (Photo 4c).

In the field there is a tendency for the chlorosis to be more intense along the leaf margins (Photo 4d) than it is on plants grown in solution culture in the glasshouse (Photos 4a and 4b). There was no indication of any necrosis developing on deficient leaves from either plants in the glasshouse (Photo 4a) or in the field (Photo 4e).

Severe zinc deficiency can also markedly reduce lateral root development (Photo 4f). Leaf symptoms do not usually appear in the fields until mid season.

Zinc concentrations in fully expanded leaves of healthy plants sampled in the field at mid season usually range from 15-28 ug/g dry matter. Results from the solution culture experiments and from leaf samples taken in the field indicate that symptoms of zinc deficiency do not usually appear until the concentration of zinc in the youngest fully expanded leaves falls below 12 ug/g dry matter.

Zinc deficiency of kiwifruit has been observed in orchards on the Waimea Plains near Nelson, at Motueka, Wanganui, and near Hastings. Low levels of zinc also occur in soils at Kerikeri⁸, but there has been evidence of a deficiency in kiwifruit grown on these soils.

The relatively low incidence of this disorder in the field is probably a reflection of the natural reserves of zinc in New Zealand soils and also of the widespread use of superphosphate. Superphosphate manufactured in this country has a relatively high zinc content (200-400 ug/g) as an impurity¹⁴. If the recommended rate of phosphate for mature cropping vines is applied as superphosphate (9 per cent P), sufficient quantities of zinc to compensate for the removal in fruit are also applied (Table 2).

Table 2: Estimated annual loss of macro- and micronutrients in fruit from a mature kiwifruit orchard. Fertiliser recommendations are those of Sale^31.

Excessively high rates of phosphate fertilisers should be avoided, however, as numerous studies with other plant species have shown that phosphorus can reduce the ‘available’ zinc in the soil and zinc concentrations in the plant to deficient levels³³. Foliar sprays (1 kg zinc sulphate 100 l water) or soil applications of zinc sulphate (23 per cent w/w Zn) at a rate of 20 kg/ha early in the season before bud break may be used as corrective treatments for zinc deficiency.

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4a	4b	4c
4d	4e	4f