FACTORS WHICH MAY INFLUENCE THE INCIDENCE OF BITTER PIT NUTRITION

Nitrogen

Quantity: No clear influence of nitrogen supply in normal quantities on the incidence of bitter pit has been found (37) unless fruit size or shoot growth is increased too much in relation to the amount of set fruit (12, 14, 4, 84). In large fruits there is a lower calcium content of fruit flesh. Vigorous shoot growth and - later in the season - enhanced transpiration lead to competition for Ca supplied by root uptake between leaves and fruit. Occurence of bitter pit decreased when shoot growth was reduced by growth retardants, particularly when additional sprays of Ca chloride were applied (64).

High doses of nitrogen or unbalanced nitrogen supply (34) promote the incidence of bitter pit, especially on soils with low pH (44). Hill (42) found high incidence of bitter pit at a N-level above 2,3 %. Increased N- and K-dressing with neglection of calcium supply leads to an increase of bitter pit (66).

Time of application: In order to get a regular growth of trees a regular nitrogen supply is necessary. According to results of pot experiments (14, 15) varying availability of nitrogen during summer leads to high incidence of bitter pit whereas regular nitrogen supply, either low or high, results in a low bitter pit attack.

In orchards regular supply of nitrogen can be achieved by sprinkler irrigation (65). Late topdressing should be avoided (17, 29, 45), it is most unfavourable if the fertilizer is not leached in by irrigation (67). Thus, nitrogen works unfavourable by stimulating a too strong growth and also by leading to a "shock"-effect (14) after suddenly increased availability.

Kind of nitrogen: In soil ammonium will suppress the uptake of cations, also that of calcium. Fertilizing with calcium nitrate means supply of available calcium, which can increase the calcium level of leaves (70), but no clear results were obtained in view of

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