Disorders producing symptoms mainly on the younger leaves

Deficiencies

Iron deficiency can be distinguished from other nutritional disorders by the development of a characteristic interveinal chlorosis of the younger leaves grading from yellow through to snow white (Photo 14a). The older leaves often remain green and healthy.

For mildly affected plants, the chlorosis is confined to the leaf margins, leaving a large zone of green tissue at the base of the leaf near the point of attachment of the petiole (Photo 14b). In severe cases the whole leaf becomes chlorotic except for the veins which remain green (Photo 14c). Eventually, even the veins may lose their green colour (Photo 14d). Under these conditions, growth is greatly reduced (Photos 14a and 14e).

Iron concentrations in fully expanded leaves of healthy plants sampled in the field at mid season usually range from 80 to 100 ug/g dry matter. Results from the solution culture experiments indicate that symptoms of iron deficiency do not usually appear until the concentration of iron in youngest fully expanded leaves falls below 60 µg/g dry matter.

While plant analysis may give some indication of the iron status of the plant, considerable caution is required when interpreting these results.

Chlorotic leaves from iron deficient kiwifruit vines may contain as great or greater concentrations of iron than leaves from healthy plants. Studies with other plant species have shown that iron can be readily inactivated within the plant forming compounds which are no longer physiologically effective³³. Thus, older chlorotic tissue may continue to accumulate iron without the deficiency symptoms being alleviated.

A simple test can be used in conjunction with leaf analysis to confirm a visual diagnosis of iron deficiency of kiwifruit. This test involves spraying or painting affected leaves with a solution containing 0.5 per cent (w/v) ferrous ammonium sulphate [(NH₄)₂SO₄.FeSO₄.6H₂0]. Within 10 days of application, patches of healthy green tissue should begin to appear on the treated leaves (Photo 14f).

Up to the present time iron deficiency of any consequence in kiwifruit in New Zealand has only occurred on calcareous soils near Hastings where the natural pH of the soils is over 7.0. Deficiency under these conditions is due to immobilisation rather than inherent deficiency of iron in the soil. Hence applications of compounds which will acidify the soil such as finely ground elemental sulphur, aluminium sulphate, or ammonium sulphate, will increase the concentration of iron previously ‘unavailable’ to the plant¹¹.

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14a	14b	14c
14d	14e	14f