Heavy crop loads can be reduced by the use of chemical thinning compounds which may reduce a citrus crop by around 20 to 30%. This method of thinning should only be considered for trees carrying a heavy crop load. See also NSW DPI primefact 788, chemical thinning.
A number of chemicals including Ethephon (various trade names), Triclopyr (trade name TOPS®) and DICHLORPROP-P 2-ETHYLHEXYL ESTER (trade name CorasilTM) are currently registered in Australia to thin or increase the size of oranges and mandarins. Growers should check labels or the APVMA website for rates. Varying amounts of fruit can be removed by the application of these chemicals depending on fruit load, application rate and environmental conditions.
Growers are encouraged to trial the application of these chemicals on a small number of trees and fine-tune their use before committing to large scale application. This may involve trialing over more than one season. Unsatisfactory results can be achieved as a result of poor timing or over application of chemical. However, if used carefully and under the right environmental conditions, chemical thinning can be a useful management tool to increase fruit size by reducing crop load.
Things to consider:
- monitor fruit load using frame counts before deciding to chemical thin. Only chemical thin if trees are carrying a heavy crop load.
- only apply to healthy trees.
- ensure trees are receiving adequate irrigation.
- make applications around the end of the natural fruit drop cycle in accordance with product label directions.
- use a water application rate of 3500L/ha.
- do not apply in cool weather (below 18°C), in the afternoon, or if rain is likely within two days after spraying.
- avoid slow drying conditions.
- ensure spray equipment is calibrated accurately.
- fruit drop should occur 7-14 days after application.
Chemical thinning will be indiscriminate, taking off small, large, marked and unmarked fruit. If there is marked fruit or a mixed second crop (poor first flowering set) then hand thinning targeted at these unwanted fruit may be the better option.
Post application temperature also has an influence on the effectiveness of chemical thinning so weather forecasts should be checked prior to application.
Warning - the over application of thinning chemicals can excessively reduce fruit number and also has the potential to cause excessive leaf drop.