Kiwifruit Nutrition diagnosis of nutritional disorders
Excess manganese severely reduces the growth of kiwifruit grown in solution culture (Photo 9a). In the field, manganese toxicity causes the growth of the current season’s canes to terminate prematurely and the internode length of these canes to be greatly reduced.
Manganese toxicity can be distinguished readily from other nutritional disorders by the appearance of a regular pattern of small black spots which concentrate along the main veins on the older leaves (Photos 9b and 9c). Later, these necrotic spots may spread to the younger leaves (Photo 9d).
The affected leaves usually remain dark green or have a steely blue-grey tinge about them, but in some cases a greenish-yellow chlorosis may accompany the black spots on the older leaves (Photo 9e). As the toxicity becomes more pronounced much larger areas of necrotic tissue develop on the affected leaves; these patches are beige in colour (Photos 9d and 9e).
Many of the affected leaves are shed shortly after appearance of these large necrotic patches.
In addition to the direct effected which manganese toxicity has on kiwifruit, a high concentration of manganese in the root zone can induce iron deficiency in the plant. This secondary symptom of manganese toxicity is characterised by an interveinal chlorosis on the younger leaves (Photo 9f; see also the section on iron deficiency).
The availability of manganese to plants is controlled largely by factors other than the amount of manganese in the soil. Reducing conditions in the soil as a result of waterlogging for example, and a low pH (below 5.5) in the topsoil of specific soil types increase substantially the amount of manganese taken up by plants. Consequently, manganese toxicity is nearly always associated with acid soils and/or poorly drained soils.
While few cases of manganese toxicity have been observed so far in kiwifruit in New Zealand, toxicity symptoms have been found to be associated with concentrations of manganese in fully expanded leaves in excess of 1200 µg/g dry matter. Usually manganese concentrations in leaves of healthy plants range from 50 to 150 µg/g dry matter. In the case of manganese induced iron deficiency, extremely high concentrations of manganese (in excess of 6000 µg/g dry matter) have been found in the leaves.
Manganese toxicity can be corrected by application of lime which increases soil pH and reduces the solubility of manganese, and by improvement of drainage in the orchard.
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| 9a | 9b | 9c |
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| 9d | 9e | 9f |
