Some of the manipulation practices are listed below. More information is provided in each heading in the pages that follow.
- Flower suppression
- Chemical thinning
- Hand thinning
- Harvest timing
In the south west of Western Australia floral initiation of citrus usually occurs between June and July however identifying the difference between vegetative and floral shoots cannot be determined until early spring.
Many factors can influence flowering of citrus such as:
- past physiological history of the tree
- water stress
Gibberellic acid (GA) inhibits flower initiation in citrus, and has been used in other citrus growing countries for this purpose for a number of years. Ralex® is presently the only registered gibberellic acid compound for flower suppression in Australia where it is registered for use in both navel oranges and mandarins.
Leafless inflorescences produce a lot of white blossom but few leaves and produce smaller and less desirable fruit than leafy inflorecences which produce fewer flowers but fruit of better size. GA applications during flower initiation have been shown to reduce the number of leafless inflorescences and increase the more desirable leafy inflorescences which produce larger fruit.
The objective of flower suppression is to manipulate the forming bud into a vegetative state before any determination of floral parts has occurred.
Applications of GA should be made as directed on the label and be trialed on small areas to fine tune the technique on your own property.
Pruning citrus is an important tool which can be used to reduce crop load. Pruning gives the orchardist control over how much of the tree is removed so the amount of crop removed can be tailored to suit seasonal requirements. If a heavy crop is predicted pruning unwanted branches will have many benefits such as:
- increasing fruit size (because of reduction in yield)
- improving light and spray penetration
- reduction in the amount of wind damage
- confining trees to their allotted space allowing easier management and harvest
- increasing the evenness of colour throughout the canopy.
Examine blocks prior to pruning to determine the desired outcome as different blocks will have different pruning requirements.
Pruning should be performed after harvest but prior to flowering where possible. This may not be possible for late harvested varieties which will need to be done as soon after harvest as possible if no loss of current crop is desired. The emphasis should be placed on removing weak or dead branches, crossover limbs, water shoots, or undesirable limbs. See Citrus pruning for further information.
One of the most important benefits of pruning is increased light distribution into the canopy which increases the fruit set inside the canopy where fruit is better protected from sunburn and frost. If pruning is done correctly trees are encouraged to produce consistent crops of good quality fruit.
Hedging during flowering is an effective way of removing excess flowers however it is non-selective so care should be taken as to how much canopy is removed.
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.
At the end of natural fruit drop in December the number of fruitlets remaining on the tree will provide a good indication of the final crop load. At this point it is important to do a crop load estimate to allow you to make decisions about any further thinning that may be required.
Hand thinning is the final management tool available to manipulate crop load and increase fruit size before harvest. Once the cell expansion stage is reached (Jan - May) the cells inside the fruit stop dividing or multiplying. This means the fruits final size may be predetermined as early as January providing the trees are not stressed.
Hand thinning should be done as early as possible after fruit drop to achieve the greatest size benefit. Hand thinning varieties that continually produce large crop loads with fruit clusters (e.g. some mandarin varieties) will result in increased fruit size and more marketable fruit. The competition for nutrients between fruits, particularly those in clusters, is reduced by thinning resulting in an increase in the supply of carbohydrates to the remaining fruit.
The option of hand thinning gives growers the ability to manipulate crop load and remove poor quality fruit from the tree in one operation.
The objective of hand thinning is to:
- remove damaged fruit first
- remove small fruit
- thin clusters to one or two fruit
- thin to prevent fruit touching
- remove fruit hanging close to the ground
- leave terminal fruit on Imperials
For varieties sensitive to sunburn and sun bleach, leave extra fruit on the tree at the first thinning to allow for the removal of sunburnt fruit after the hazardous period. This is especially important where many fruit are exposed and will depend on the amount of protection offered by the tree canopy.
Hand thinning is limited by factors such as the high cost of labour and the limited extra growth achievable in years with moderate crop loads. It remains an option if crop load is excessive and no other method of crop manipulation has been implemented.
Harvest timing can significantly impact on flowering and fruit set in the following season. Where fruit of a variety is left to hang on the tree late (to obtain a better market window or improve fruit eating quality) a reduction of flowering the following season can often occur. This is particularly true of many mandarin varieties.
Harvest management can also increase the current seasons fruit size. By carrying out multiple picks the largest and most coloured fruit can be removed on the first pick allowing smaller and less mature fruit more time to size. GA sprays are often used to control rind ageing but will have the effect of prolonging the harvest season. If applied too early GA will cause fruit to retain green colour and may make degreening more difficult.
Nutrition management in a citrus orchard is a critical practice for growers. To achieve optimal fruit size, trees must get the right balance of nutrients at the correct growth stages. Major and minor nutrients all play important roles and interact with each other to determine the overall health of trees. Some obviously have more influence on fruit size than others, but at the same time they may also play an important role in relation to fruit quality. For example calcium and its role in reducing albedo breakdown.
All good fertiliser and nutrition programs require effective monitoring of nutrients to ensure the correct balance is maintained. One of the most important tools to monitor nutrient changes within the plant is a regular leaf analysis program and good record keeping. It is important to maintain optimum nutrition at each growth stage to promote better fruit size and tree health.
Irrigation is one of the most important factors in producing a good yield of quality citrus and plays an important role in determining fruit size. The correct scheduling of irrigation in your orchard is an important tool in achieving desired fruit sizes.
Information in this webpage has been adapted from the "Fruit Size Management Guide Part 1" which is a publication distributed by the Australian Citrus Growers (2003). The guide was compiled by Ken Bevington, Sandra Hardy & Peter Melville (NSW DPI), Kym Thiel (Citrus Growers of SA), Garry Fullelove (QLD DAF) and Peter Morrish (Murray Valley Citrus Board).