Achieving adequate levels of phosphorus in Cox and Bramley apples at harvest reduces their susceptibility to low temperature breakdown (LTB) during storage. This is particularly important where climatic conditions during development increase susceptibility to LTB. Fruits that are low in phosphorus also tend to be softer although this may not be a causal association.
Crop load affects the concentration of phosphorus in fruits. Heavier cropping reduces the amount of phosphorus per apple but also restricts the amount of dry matter available to the fruit. It is likely that the shortage of dry matter for cell wall formation is linked to poorer texture rather than the associated depression in phosphorus levels.
Flower and fruit thinning increased firmness and phosphorus concentration in Cox apples. Minimum recommended levels of phosphorus in Cox and Bramley apples are 11 and 9 mg 100g-1 (see Section 3). Minimum recommended levels of phosphorus in the leaves of Cox to achieve good storage quality is 0.24% (dry weight) which is at the high end of the range (0.20‑0.25%) recommended for satisfactory yield.
In surveys carried out in the 1980’s 90% of commercial Cox orchards achieved target levels for phosphorus in fruit compared with only 68% of Bramley orchards. In the pursuit of even higher yields the proportion of orchards that achieve satisfactory levels of phosphorus is likely to decline. The problem will be aggravated where soil management is based on the use of herbicides and by increased rates of nitrogen fertiliser.
Phosphorus problems in established orchards are unlikely to be rectified by the application of phosphate fertiliser, as phosphorus is poorly mobile in the soil. Foliar sprays containing phosphorus are recommended for both Cox and Bramley, particularly where there is a history of breakdown during storage and fruit analysis indicates that low fruit phosphorus may be a contributing factor.
Sprays containing phosphorus applied to Cox trees in the period mid-June to September markedly reduced susceptibility to LTB and senescent breakdown. Similar effects of phosphorus sprays were found in Bramley, although only those sprays applied during the period mid-June to mid-July gave effective control of breakdown.
The effect of time of application of phosphorus sprays on the development of LTB in Cox has not been examined critically but it would be advisable for growers to make applications in the period mid-June to mid-July. This advice is supported by the correlation between low phosphorus in fruitlets sampled in July and increased LTB in stored fruit.
There has been no evaluation of the effects of phosphorus sprays on the storage quality of other cultivars of apple important in UK production. Although the concentration of phosphorus in Crispin (Mutsu), Spartan, Red Pippin (Fiesta), Jonagold and Gala apples are much lower than in Cox there is little evidence of LTB problems in commercial storage. Phosphorus content of Gala sampled two weeks prior to harvest averages 10.2 mg/100 g FW, while Braeburn averages 11.9 mg/100 g FW. In both cases these cultivars are stored at much lower temperatures than Cox without signs of low-temperature breakdown. Attempts to raise the concentration in these cultivars appears unwarranted.