Gibberellic acid (GA) is a naturally occurring hormone or growth-regulating chemical that is found to varying degrees in all parts of plants.
GA stimulates both cell division and elongation and has been used to manipulate flowering and fruit development in selected horticultural crops for many years.
Reason for use
There are four key reasons for using GA on citrus:
- reducing the severity and incidence of albedo breakdown
- reducing the severity and incidence of watermark (mostly on Imperial mandarin)
- reducing the severity and incidence of oleocellosis
- improving rind quality.
Albedo breakdown
Albedo breakdown (sometimes known as creasing) is the separation of the mesocarp or albedo (the layer of white internal rind) from the exocarp or flavedo (the external rind) resulting in the rind developing creases and sometimes cracks.
It is recognised by narrow sunken grooves in the rind. In severe cases the grooves intersect making the fruit appear lumpy and soft (Figure 1).
This is a serious condition and may cause fruit to split open under pressure when packed. GA and up to three calcium (Ca) sprays should be used to obtain maximum control.
Balanced nutrition and irrigation are also crucial. High phosphorus (P) levels are associated with thinner rinds and therefore are more prone to developing albedo. High levels of nitrogen (N) and potassium (K) are associated with thicker rinds.
In districts prone to the disorder it may be necessary to accept thicker rinds in order to obtain economic yields. Moisture stress (not enough or too much) significantly increases the incidence and severity of albedo.
Watermark
Watermark on Imperial mandarins (known as ‘waterburn’ in eastern Australia) occurs during wet weather when the bottom of the fruit is wet for a prolonged period (Figure 2).
The colder and wetter the area the more severe the problem. Up to 60% crop losses were recorded from one property alone in the Harvey region when GA was not used.
GA is a registered treatment that is widely used with great success interstate. Prior to 2000 there was considerable reluctance to use GA particularly on Imperial mandarins in Western Australia. This reluctance was generally based on misinformation and stories distorted with the passage of time. Local concerns related to delays in colour development, reduced yield in subsequent years and tear stain spray damage.
Demonstrations were conducted on orchards in Bindoon, Chittering and Harvey in 2001 and 2002 where GA was applied at label rates and the correct time. The results showed that the incidence of watermark was reduced by up to 80% without any negative effects (Figure 3).
Before and after surveys provided a useful measure of industry adoption.
In 2000, less than 10% of growers surveyed were using GA to reduce watermark. In 2003 after the demonstrations and extensive effort, more than 80% of the same group were using GA as a strategy to reduce watermark in their Imperials.
Oleocellosis
Oleocellosis (sometimes known as ‘oil spotting or burn’) occurs following mechanical injury to the fruit (Figure 4).
It is caused by the rupturing of oil glands releasing toxic oil during harvest and/or transport from the field to the packing house. This oil kills the nearby cells of the flavedo and can also cause spotting on the surface of adjacent fruit.
It is more common when fruit is turgid and the weather is cold and/or wet at harvest.
Oleocellosis predisposes fruit to storage rots, chilling injury and burning from ozone (when used to control diseases that develop during storage).
The oil from the ruptured cells inhibits degreening and discolours the rind. Careful harvesting, transport and use of GA helps reduce the problem.
Rind quality
GA maintains rind quality by strengthening the rind which helps keep the texture looking younger. Rind ageing is delayed in late harvested varieties by preventing collapse due to weakening of the cell walls and subsequent dehydration of mature fruit.
Some climatic and storage conditions can accelerate the ageing process, such as:
- heavy rains plus high humidity, followed by very cold conditions
- dehydration of the fruit in the orchard during hot conditions
- holding fruit too long between harvest and packing (low humidity storage in the packing shed)
- warm and dry conditions when trees are under water stress — this leads to dehydration of the stem end of the fruit.
Other benefits
Other benefits of using GA include:
- better rind firmness at harvest
- fruit appears less prone to postharvest decay (this may be because of greater resistance to injury due to the firmer rind)
- fruit treated with GA seems to better withstand adverse weather (hot, wet or windy), rough handling and staining
- lower incidence of both rind puffiness in mandarins and splitting in varieties with thin rind.
Note: GA has no effect on internal fruit quality or maturity but may delay external colour development.
When to apply
Apply gibberellic acid if:
- your orchard has a history of albedo breakdown and it is going to be a heavy crop year
- your orchard has a history of Watermark on Imperial mandarins
- your area has cold and wet weather during harvest and oleocellosis may be a problem
- you want to delay rind ageing on late harvested varieties.
Note: if spraying to reduce albedo breakdown, oleocellosis and watermark on Imperial mandarins, the earlier you spray the less effect the GA will have on delaying colour change. For example, an early spray at fruit diameter 30 to 50mm (early January), may delay colour change by up to five days instead of two to three weeks if sprayed in early February.
The delay in colour change depends on timing, variety, rate used, location and prevailing seasonal conditions. The earlier you spray the more effective the GA will be.
Timing and rates
Rates may vary between products.
Read the label. The label will provide you with a very good timing guide, however, there is no substitute for on-farm experience and therefore you may wish to conduct your own trials to fine tune the timing of sprays so that they work best for your varieties, under your conditions. Also remember that because conditions are different every year some flexibility is required when deciding on timing.
Check pH before spraying
For albedo (creasing) breakdown the pH must be 4 to 4.5. For all other applications, for example, to delay rind ageing, it should be neutral, pH 7. If necessary use a buffer to achieve this pH.
The easiest buffers to use are those that change colour when the correct pH is reached. Add a spreader/wetting agent at label rates. An adjuvant can be used instead of a buffer, but may contain a spreader that is not at the recommended concentration for the GA product. Note that using too much spreader/wetting agent can burn your fruit.
Use a high volume sprayer that has been calibrated. Check nozzles for wear and make sure they are the correct size. Complete coverage is essential. The spray must penetrate into the canopy, which is why it is critical to correctly prune your trees. Trees must be sprayed to the point of run-off.
Things to consider
To maximise the effectiveness of GA:
- Read the product label and comply with all directions.
- Trees should be healthy and in good condition.
- Irrigation and fertiliser management is important. Trees must not be over- or under-irrigated.
- Canopy management should be effective and up to date. Trees should be pruned well so that the spray can penetrate to all parts of the canopy. During years with a heavy crop load, fruit thinning is also advisable.
- Temperature at application is important. The weather must be cool and the fruit dry when applying GA. Avoid mornings when there is heavy dew or heatwave conditions. The optimal spraying temperature range is 15 to 30°C. The best time to spray is early morning or late evening (wait for fruit to cool down). GA works best during slow drying conditions. If it dries too fast it won’t work. Wait if rain is forecast within the next 6 hours. Cold, wet conditions can cause spray damage, symptoms are a tear-shaped stain/burn marks.
- Time sprays correctly to achieve the desired result.
- Do not apply copper or oil sprays within four weeks of a GA application. Copper and oil blanket the fruit and therefore restrict the uptake of GA. If possible apply GA before your oil and copper sprays. Stop drop sprays can be applied as normal, but do not mix with GA.
- Use the correct rates and check the pH of the water used for spraying.
- GA is best used on its own. Avoid mixing it with other products.