Disorders producing symptoms mainly on the younger leaves

Deficiencies

The appearance of small irregular patches of yellow tissue near the centre of the younger leaves is the first sign of boron deficiency (Photo 16a). These patches enlarge and coalesce to form an extensive area of yellow tissue on both sides of the midrib (Photos 16b and 16c). A zone of healthy green tissue is usually retained at the margins of the affected leaves. At the same time the immature terminal leaves thicken, becoming mis-shapen and twisted (Photos 16d and 16e). When the deficiency is severe, stem elongation is restricted due to a lack of extension of the internodes, giving the plant a stunted appearance (Photo 16f).

Boron concentrations in fully expanded leaves of healthy plants sampled in the field mid season usually range from 40 to 50 µg/g dry matter. Results from the solution culture studies and of analysis of leaf samples taken in the field indicate that the symptoms of boron deficiency do not usually appear until the concentration of boron in youngest fully expanded leaves falls below 20 µg/g dry matter.

Boron deficiency most often occurs on light textured sandy soils and soils which are low in organic matter. Overliming can also induce boron deficiency by reducing the solubility of boron compounds in the soil³⁰.

Although there are a number of soils in New Zealand on which kiwifruit are grown that are naturally low in boron, such as the yellow-brown pumice soils of the North Island and the yellow-brown earth soils near Nelson in the South Island⁵³, boron deficiency of kiwifruit is not common in this country. So far this disorder has been observed in only one isolated case in an orchard near Wanganui (Photo 15c).

One possible reason for the low incidence of boron deficiency in the field is the comparatively small annual removal of boron from cropping orchards (Table 2).

In many situations, sufficient boron is added to the soil from sources such as airborne sea spray (Table 3), irrigation water (Table 4), and from impurities in fertiliser such as superphosphate (Table 2), and some sources of potassium chloride (Table 9)to compensate for new growth and the amount removed in fruit.

Table 9: Quantity of boron as an impurity in sources of potassium chloride (muriate of potash) commonly used in New Zealand.

However, in genuine cases of boron deficiency it can be readily corrected by the application of boron fertilisers such as borax (11 per cent w/w B). While foliar sprays of boron (100 g boron/100 l water) compounds like boric acid (18 per cent w/w B) and Solubor® (21 per cent w/w B) are often beneficial, soil applications remain more effective in the longer term³⁰.

As kiwifruit appear to be very sensitive to excess boron (see section on boron toxicity) extreme care should be taken when applying fertilisers since there is no reliable information yet for kiwifruit on the quantities of boron required to safely correct a deficiency.

Click any image to view an enlargement
16a	16b	16c
16d	16e	16f