If fruitlets are to expand and develop into high quality large fruits at the time of harvest it is essential to maintain adequate supplies of water and nutrients to the tree throughout the growing season.
Irrigation
The influence of different amounts and timings of water supply to the growth and final size and quality of apple fruits has been studied in many regions of the world.
- Unfortunately, the results are often variable, and are probably influenced greatly by local climatic conditions and soil types.
- Reduced supply of water to trees growing in areas experiencing hot dry summers, results, as expected in a higher proportion of smaller fruits at harvest time.
- As might be expected, application of increasing amounts of irrigation result in faster growth rates for fruitlets and larger fruit size at harvest.
- However, several reports show irrigation of fruits also reduces fruit firmness and lowers the soluble solids content, in some but not all years (Bonany and Camps, 1998).
In most UK orchards and in most seasons it is not a problem in UK climatic conditions to maintain adequate supplies of water and nutrients to the tree throughout the growing season.
- However, where trees are grown on shallow or sandy/gravel soils supplementary irrigation will be a necessity in most seasons.
- This is best provided using a trickle irrigation system, which can supply water regularly and in low controlled amounts.
Water needs are estimated using calculations of water deficits based on evapotranspiration or are measured using various other systems (neutron probes, tensiometers etc.).
- Research conducted on Braeburn trees has shown that witholding necessary irrigation water early in the season (up to 104 days after full bloom) reduced shoot growth and mean final fruit weight, although rates of photosynthesis did not appear to be affected.
- In contrast, witholding water later in the season (i.e. 104 to 194 days after full bloom) had no influence on mean fruit size (Kilili et al., 1996a).
- The same authors (Kilili et al., 1996b) noted that witholding water late in the season increased total soluble solids, soluble sugars (fructose, sucrose and sorbitol), flesh firmness and red skin colour intensity.
- This late witholding of water, therefore, may have merits in certain situations. However, care must be exercised when using these Regulated Deficit Irrigation (RDI) systems of management.
Research in the USA (Ebel et al., 1993) has shown that whilst RDI systems produced apples with higher soluble solids, the fruit sizes at harvest were reduced.
- If fruit size is not to be reduced deficits should only be imposed late in the growing season.
Recent work in New Zealand showed that irrigating Braeburn trees on MM.106 rootstock improved the concentrations of calcium in the fruits at harvest (Mills et al., 1994).
- Fruits from non-irrigated trees developed sugars slightly earlier than fruits from irrigated trees.
Nutrition
Nutrient deficiency is unlikely to influence fruitlet growth in most conventional orchards, which are fertilised to established recommendations. However, nitrogen and potassium deficiency in organic orchards is likely to be a problem.
- Ensure that the trees are supplied with adequate water and nutrients so as to maintain the cell expansion and growth of apple fruitlets.
- Monitor water requirements regularly, using calculations of evapotranspiration or other equipment marketed to measure soil water levels.
- Check soil and leaf nutrient levels regularly and adjust nutrition accordingly.