(continued from last month) Stages in the thinning program - 2.1. Blossom thinning
2.1. Blossom thinning
Regardless of whether thinning is undertaken by hand or with the aid of chemical thinning agents, the thinning process should start early in the flowering period.
NAA
While NAA can thin most cultivars between FB and 21 dAFB, the earlier it is applied the better the response in fruit size.
NAA promotes vegetative growth. This can be advantageous in green apples such as Golden Delicious or Granny Smith, but can be a distinct disadvantage in red apples where extra vegetative growth shades the fruit, inhibiting red colour production.
Interactions: NAA interacts with other plant bioregulators containing the gibberellins GA4+7. Hence it is not compatible with formulations such as Cytolin® when applied at the normal recommended rate. However, if the rate of NAA is reduced to 3-4 ppm then a Cytolin/NAA program works well.
Disadvantages: High concentrations and/or late applications can depress fruit size as well as producing pygmy fruit and reducing seed numbers which can affect fruit quality. The efficacy of NAA is affected by temperature and humidity and rewetting with light rain or dew often causing over-thinning. As NAA is most effective under humid conditions, it can cause fruit russeting.
2.2. Post-bloom thinning
6-benzyladenine (BA)
There are several products registered in Australia with the cytokinin 6-benzyladenine as the active ingredient (CyLex & Maxcel—Valent BioSciences;
BAPSol—Gro-Chem NZ; Exilis—Fine Agrochemicals Limited).
BA is a post-bloom thinner and works most effectively following treatment at flowering with one of the blossom thinners ATS, ethephon, or NAA.
Application conditions: BA is temperature dependent, the warmer the temperature the more effective it will be.
Ideally it should be applied on a rising temperature curve, with maximum daily temperatures in excess of 15C.
Look for the best available weather during the recommended application period.
Advantages: If used correctly, BA is a consistent thinner. It is not persistent or toxic, and is OK for IPM programs. BA increases fruit size independently of the thinning effect and may also increase fruit firmness.
Disadvantages: Temperature dependent—needs warm to hot conditions.
Carbaryl/Thiram
Carbaryl is regarded as a mild thinner and usually only removes the slower growing fruit within bunches.
In Australia, carbaryl is used as a post-bloom thinner, normally in a tank mix with thiram.
Carbaryl can also be used on trees where use of a primary thinner is not warranted, either because the trees are young or because of sparse blossom buds.
Application conditions: Carbaryl is temperature dependent, requiring warm dry conditions for effective thinning. The warmer the temperature the greater the thinning effect. Cool or damp conditions following application can result in fruit russet.
Disadvantages: Carbaryl is toxic to bees and beneficial species important in IPM programs. It is also toxic to mammals, and has been found in waterways.
Carbaryl can leave residues on fruit, and its use is banned on fruit exported to some countries.
Dormancy breakers
As dormancy breakers often compress the flowering period, making thinning chemicals more effective, caution is advised with thinning chemicals when dormancy breakers such as Dormex® or Waiken™ have been applied.
In this situation careful fruitlet counts should be undertaken after the application of primary thinners to determine whether further thinning is necessary.
If using ATS, only one application may be required to reduce the crop load to a suitable level.
Netted orchards
Fruit set in trees under hail netting tends to be lower than uncovered trees.
Due to the lower light levels, chemical thinning of netted trees often induces greater fruitlet drop, hence care should be taken to avoid over thinning.
While low blossom trees under nets may not require the application of chemical thinners, trees with medium to heavy blossoms will still benefit from chemical thinning to minimise or eliminate the need for follow-up hand thinning.
Benefits of chemical thinning
- continued next month -
For more information, see Tree Fruit Sept 2014
