The foliar nutrient ATS has a very useful side effect as a blossom thinner on apples and other crops. If applied at flowering time, ATS works by desiccating and, therefore, damaging the stigmas and styles of apple flowers, so preventing them from setting fruits.
When using ATS as a blossom thinner on apples it is important to consider:
- Spray timings
- The ideal weather conditions
- Spray concentrations and volumes
- Use of adjuvants
- Variations associated with specific varieties
Spray timings
Trials conducted at East Malling and in Denmark have shown that flowers which are at the balloon stage through to those that have been open for 2 days are the most sensitive to the sprays.
- Although the petals of flowers at the pink bud stage are damaged by ATS sprays, the flowers still remain capable of setting fruits.
- Flowers that have been open for more than two days and have been pollinated by bees will often still set fruits, despite being damaged by the ATS sprays.
- This is because the fruit receptacles and ovaries are not damaged by the sprays and once pollen tubes have penetrated the ovary they will not be influenced by the sprays.
- Nevertheless, in certain climatic conditions, such as very slow drying, the skin finish of these fruits may be slightly damaged.
In seasons when flowering is concentrated over just a few days, then a single treatment with ATS will often be sufficient to thin the trees effectively.
- This is because a high proportion of the total number of flowers will be at a vulnerable stage on the day of the spray treatment (70% full bloom).
In years when the blossoming period is extended two sprays may be required; the first when 25% of the blossoms have opened and a second when most of the spur flowers have opened.
- A single spray in such seasons is rarely sufficient to effect adequate thinning.
With varieties that are prone to set large numbers of fruits on one-year-old wood (axillary blossoms) growers often endeavour to selectively prevent this fruit set using sprays of ATS applied after full bloom.
- Axillary blossoms, which often give rise to smaller than average fruits at harvest, flower several days after spur blossoms.
- To thin them ATS sprays must be delayed until early petal fall on spur blossoms.
- Care must be taken with this late treatment, however, as trials in Denmark have shown that in their climatic conditions (high humidity and slow drying) sprays applied to thin axillary blossoms may also damage and cause abscission of fruitlets already set on the spur blossoms.
- Although this is contrary to the results noted at East Malling, it does indicate that there is a potential problem with this strategy in some climatic conditions.
Trials at East Malling (Webster and Spencer, 1999; Webster and Spencer, 2000), on the varieties Royal Gala and Queen Cox, showed that, although single sprays with ATS usually thinned very effectively in the first year of treatment, repeating the same sprays on the same trees in the subsequent season could produce disappointing results.
- The reason for this was the very abundant blossom numbers induced in the second year by the treatments in the first year.
- Although the treatments reduced set per 100 flower buds similarly in both seasons, the thinning was insufficient in the second season to compensate for the big increase in flower numbers.
- Where this is likely to be a problem, growers should consider increasing spray volumes, or ATS concentrations or even applying a second spray during the flowering period.
The ideal weather conditions
Research conducted in controlled environment chambers in Holland has shown that as temperatures increase the degree of thinning achieved with ATS sprays also increases.
- So, when temperatures are 15oC or higher thinning should be very efficient. In contrast, when temperatures are very cool <10oC the sprays are likely to prove much less effective.
- Temperatures are influential, not just at the time of spraying but for several hours after the treatment is applied.
Thinning is also influenced by the humidity at the time of spraying. Slow drying conditions (high humidities) improve thinning slightly, but may also cause phytotoxicity on the spur leaves.
- Spraying high concentrations of ATS in slow drying conditions is not recommended on account of the potential problems of phytotoxicity to the spur leaves.
- Spur leaves are essential to the early growth of the persisting fruitlets.
Research in Canada has indicated that when sprays of ATS are applied at low volumes the spray concentration needs to be increased as much as 6 to 12 times (Sanders and Looney, 1993).
- Such concentrations can, however be very phytotoxic (Irving et al., 1989) and further work is needed before similar recommendations can be made for UK conditions.
- However, in contrast to the above findings, recent work conducted in Poland indicates that ATS thins best if applied at higher concentrations and low spray volumes.
- Differences in climatic conditions between Canada and Poland may help explain these anomalies.