Disorders producing symptoms mainly on the older leaves

Toxicities

Early symptoms of boron toxicity include a yellow-green interveinal chlorosis developing first on the older leaves and spreading progressively to the younger leaves (Photos 7a and 7b). It is also usual to find the affected leaves cupped either upwards (Photo 7b) or downwards (Photo 7c).

As the toxicity becomes more pronounced, the interveinal chlorosis quickly gives way to small patches of brown necrotic tissue which develop between the minor veins and extend to the midrib (Photos 7c and 7d). Necrosis of the leaf margins is also common (Photo 7d).

Eventually, necrotic patches link up forming a continuous zone of dead tissue between the major veins. As this necrotic tissue weathers, it changes from brown to a silvery-grey colour (Photo 7e). By this stage the necrotic tissue has become very brittle and may break away giving a ragged appearance to the leaf (Photo 7f).

As with other boron sensitive plants, the margin between boron sufficiency and toxicity is very narrow for kiwifruit. Concentrations of boron in the leaves only slightly above the required level can cause serious injury to the plant.

Analyses of kiwifruit leaves sampled in October through to December quite commonly show low boron concentrations of 20-30 µg/g dry matter. However, this should not be interpreted as a deficiency but rather as a part of a natural pattern.

Further analyses will show that from December through to the end of the growing season, boron concentrations usually double. Consequently, if boron is applied to correct what may appear to be an early season deficiency, there is a distinct danger that toxicity will result.

In New Zealand boron toxicity in kiwifruit has been observed only following heavy applications of boron fertiliser to the soil (in excess of 2 kg/ha of boron), in foliar sprays, or where bore water naturally high in boron (>0.8 mg/l) has been used for irrigation.

Table 4: Quantity of boron applied in irrigation water (g/ha/week)

The residual effect of boron in soils is determined largely by the soil type and form of boron applied. Boron leaches readily from sandy soils but is more strongly retained by soils high in silt and clay. Soil acidity has been shown to accentuate the toxic effects of boron in plants²¹. Although the effects on kiwifruit are not yet known, for some plants boron toxicity may be reduced by adding lime and /or organic matter to the soil³³.

While boron concentrations in youngest fully expanded leaves of healthy plants sampled in the field at mid season are usually about 50 µg/g dry matter, plants showing symptoms of severe toxicity have boron concentrations in excess of 100 µg/g dry matter.

Excess boron severely reduces fruit yield, both fruit numbers and the weight of individual fruit being reduced. The storage quality of the fruit is also affected, with the fruit ripening prematurely in cool storage³⁸.

Until further information is available on the effects of rainfall and soil type on boron accumulation in the root zone, boron concentrations in irrigation water should not exceed 0.5 mg/l.

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7a	7b	7c
7d	7e	7f