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 (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.