Disorders producing symptoms mainly on the older leaves

Deficiencies

The first sign of potassium deficiency in the field is poor growth at bud break. On severely affected vines the leaves are small and pale yellow-green with a slight marginal chlorosis on the older leaves.

As the deficiency becomes more pronounced, there is an upward curling of the margins of the older leaves which is particularly noticeable during the warmer period of the day (Photo 1a). This symptom may disappear the following day giving the plant an appearance which has often been erroneously attributed to a lack of water (Photo 1b).

Later the margins of the affected leaves remain permanently curled (Photo 1c), and the tissue between the minor veins is often ridged upwards. Also the light green chlorosis, which developed initially at the leaf margin, spreads between the veins towards the midrib leaving a zone of green tissue close to the major veins and at the base of the leaf (Photos 1a and 1d).

However, the boundary between chlorotic and healthy tissue is much more diffuse than it is with deficiencies of other elements such as magnesium and manganese. Much of the chlorotic tissue quickly becomes necrotic, turning from light to dark brown as it dies giving the leaf a scorched appearance (Photos 1d, 1e).

As the leaf scorch becomes more extensive, affected tissue becomes brittle and there is a tendency for it to break away at the leaf margin giving the leaf a tattered appearance (Photo 1e). Severe potassium deficiency can cause premature defoliation of the vine, although the fruit will remain firmly attached.

Potassium concentrations in fully expanded leaves of healthy plants sampled in the field at mid season (late February or early March) are usually above 1.8 per cent of the dry matter. Results from the solution culture studies, and analysis of leaf samples taken in the field indicate that symptoms of potassium deficiency do not usually appear until the concentration of potassium in youngest fully expanded leaves falls below 1.5 per cent dry matter.

Severe potassium deficiency of kiwifruit in New Zealand was first reported in 1983 in vines grown at Kumeu near Auckland⁴⁸. Since the observation was made, an extensive survey of the major kiwifruit growing areas of New Zealand has shown potassium deficiency to be much more widespread than is generally appreciated^38,43. Vines of all ages show signs of the deficiency but those just coming into production (4 to 6 years old) seem to be the worst affected. There also appears to be a relationship between potassium deficiency and the incidence of blossom rot (a bacterial infection of the flowers caused by Pseudomonas viridiflava⁵⁵). For example, we have found that incidence of blossom rot was much greater (36 per cent) on potassium deficient vines than on vines of higher potassium status (16 per cent).

Potassium deficiency severely reduces fruit yield, both fruit numbers and fruit size being affected^{38, 43}. Inadequate applications of potassium fertilisers to compensate for potassium required in new cane growth, the large quantity removed in fruit, the competition from grasses and clover in the orchard for potassium, and the small losses from the soil by leaching, probably account for the high incidence of this disorder in the field⁴³.

Potassium deficiency of kiwifruit can be corrected by applying potassic fertilisers. In New Zealand three types of potassic fertilisers are commonly used. They are potassium chloride (muriate of potash, 50 per cent w/w K), potassium sulphate (sulphate of potash, 40 per cent w/w K), and potassium nitrate (37 per cent w/w K). Potassium chloride is obtained from natural ores or brines, while potassium sulphate is manufactured from potassium chloride by a chemical process in which either magnesium sulphate or sulphuric acid is used as the source of sulphate¹⁹. Potassium nitrate is also manufactured from potassium chloride, with nitric acid being used as the source of nitrate¹⁹.

Per unit of potassium the sulphate and the nitrate forms are very much more expensive than the chloride form. In addition to the cost advantage, potassium chloride is the preferred form because of the high chloride requirements of kiwifruit for growth. Kiwifruit will not respond to potassium if they are deficient in chloride (see section on chlorine deficiency).

The quantity of potassium required to correct a deficiency will vary according to the severity of the disorder, the age of the vines, fruit yield, and soil type. The estimated annual loss of potassium in fruit alone from an average yielding orchard in the Bay of Plenty (25 t/ha) is over 80 kg/ha (Table 2). However, losses can be even greater than this figure as some orchards have produced up to 54 t/ha of fruit in one year⁵. These losses of potassium have to be replaced each year from soil reserves or from fertiliser. Thus, for example, between 250 and 300 kg/ha of potassium (500-600 kg/ha of potassium chloride) would be required to correct a moderate to severe deficiency in orchards expected to produce 25 t/ha of fruit⁴³.

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