Chemical thinning with NAA and NAAm (NAD)
NAA, or its amide NAD (NAAm), has long been used in continental Europe for thinning apples.
- Although approved for use in the UK up until the 1970s, these auxins failed to prove consistently effective when used on Cox’s Orange Pippin and they were withdrawn from use.
NAA
Thinning results using NAA are very variable from season to season, variety to variety and site to site.
- In recent years, it has proved consistently effective only in Polish trials.
- NAA is either sprayed a little later than NAAm, (up to 14 days after petal fall), or during flowering.
- Concentrations of up to 20 ppm are used and many different wetters and surfactants have been tested in attempts (largely unsuccessful) to improve the consistency of response.
- However, recent work in Germany has provided some explanation of the variability of response and made suggestions for future use of more appropriate wetters (Schönherr et al., 2000).
- Recent trials in Norway have shown NAA to be ineffective in thinning the early ripening variety Summered, when applied at the 10 mm fruitlet diameter stage
NAAm (NAD)
Trials on several varieties grown in central Germany (Link, 1998) showed that the NAAm thinned Cox in some situations but did not result in any increase in monetary value when the results of trials over an eight year period were analysed.
- In contrast, hand thinning either by removing whole blossom clusters at the time of flowering or of fruitlets after completion of ‘June Drop’, usually produced more economically worthwhile results.
Effects of thinning treatments on the yields and grade-outs of Cox grown in Hohenheim, Germany (mean of years 1982-1990)
| Unthinned | NAD (NAAm) | Hand thinning of bloom clusters | Hand thinning of fruitlets post June drop | |
|---|---|---|---|---|
| Total yield/tree (kg) | 23.2 | 21.5 | 21.4 | 20.6 |
| Yield of large/medium sized fruits/tree (kg) | 12.0 | 15.2 | 17.3 | 16.2 |
| Monetary value of fruit/tree/year (£) | 7.20 | 7.10 | 8.40 | 7.90 |
- Recent trials in Germany have shown good thinning of the variety Delbard Estivale using a combination of NAAm and Ethrel applied at the late bloom stage.
- NAAm in liquid formulations is normally applied at a concentration of 40 ppm in high volume sprays of 1500 l/ha just after petal fall.
- However, concentrations of up to 100 ppm have been used in some situations. It is normally sprayed shortly after flowering.
In the absence of carbaryl or other potential fruitlet thinning chemicals NAAm is likely to remain a major thinner in many orchards in mainland Europe.
- NAA and NAAm are still approved for use as fruitlet thinners on apples in many European countries. However, they are not approved on apples.
- The thinning effects achieved are inconsistent and further work is needed on ensuring that the pH of the water used is optimal and that the best surfactants are used.
Chemical thinning with Ethephon (Ethrel C)
Ethephon (Ethrel C) applied at the 12 mm fruitlet diameter stage has been shown to thin many varieties of apple.
- Unfortunately, the results are extremely variable from season to season and between varieties.
- A major influence on the efficacy of Ethrel sprays in thinning is the temperatures during and shortly after treatment.
- Unless the day temperatures are 15oC or above, the response to the sprays is likely to be poor.
- Another factor influencing the variability of response of Ethrel thinning sprays is the pH of the water used.
- Alkaline spray solutions are needed to maximise the release of ethylene by the product.
Knight (1981) showed that sprays at concentrations of 150 ppm were effective in thinning Discovery but that the growth rate of the persisting fruitlets was often reduced by the treatment.
- Similar effects on fruit growth rates have also been noted in Dutch trials.
Although still used in several European countries as a thinner, Ethrel is extremely variable under UK climatic conditions and in some situations may cause slight russeting and poor storage potential in fruits.
- The results achieved with Ethrel are often variable. Thinning efficacy is improved by temperatures higher than 15oC and by use of alkaline water.
- Ethephon (Ethrel) is still approved as a fruitlet thinning chemical for apple in several European countries, but is not approved for this use in the UK.