This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

Fruit size was the most important production issue facing citrus growers in domestic markets and particularly in export markets. Market returns were directly linked to fruit size with most markets preferring fruit in the larger size classes. Crop load manipulation could be used to overcome smaller fruit size and alleviate the problems of alternate bearing. Ralex®, a gibberellic acid (GA3) based formulation (a product of Valent BioSciences) offered a means of crop load manipulation via flower suppression. Bud responses to different Ralex® rates and timings for flower suppression were studied for two seasons. Experiments were carried out on Washington navel, Navelina, Bellamy (Washington) navel and Barnfield navel trees. Ralex® was applied to individual trees in an “on-flowering” year 2003 at four different rates (0 ml, 100 ml, 150 ml, or 200 ml/100 L) and at five different timings ranging from early May to late July during first and second sensitivity peaks. The sensitivity peaks refered to the ability of developing buds to be influenced by treatments.

The results indicated that higher rates of Ralex® caused flower reduction by targeting the leafless inflorescence. The best rate identified was 150 ml/100 L when applied around mid June. This rate produced a higher percentage of fruit in large (77-87 mm) size class at harvest compared to the control treatment. Ralex® treatments were also applied to a new set of individual trees in an “off-flowering” year at four different Ralex® rates (0 ml, 100 ml, 150 ml, or 200 ml/100 L) and at three different timings ranging from late May to late June targeting the buds during first sensitivity peak. Results indicated a strong reduction in flowering, although there were not any fruit size benefits at harvest when Ralex® was applied in an “off-flowering” year.

There were no biennial bearing effects found after Ralex® use after two years of study. Ralex® effects on fruit skin colour were also assessed in a different set of trials in early, mid and late maturing cultivars. At the time of this report's publication there was no colour delay indicated when Ralex® was applied around late May or early June periods at a 50 per cent skin colour stage. There was an indication of effect on fruit firmness when fruit was observed visually, but these effects needed further investigation as they were not part of this project. Some of the results were published in scientific journals and conference proceedings. Results of the project were extensively reported at Cittgroup meetings, field day, farm walks and in national and international conferences.

Key outcomes:

1. Ralex® could be applied at 150 ml/100 L around mid June (approx. 4-6 weeks) before bud break. Therefore, the use of Ralex® in first sensitivity peak was recommended.
2. Investigations on fruit colour trials revealed no skin colour delay affects at harvest with Ralex® use if applied during late May-June periods or at 50 per cent colour stage.
3. Ralex® gave positive effects on skin colour with higher percentage of yellow coloured fruit compared to untreated fruit. Ralex® effects for rind quality needs urgent attention for further research.
4. Ralex® application could eliminate alternate bearing problem across two growing seasons. Return fruit size data indicated the potential benefits of Ralex® treatment. Final yields were very similar across two years.
5. Results of this project did not suggest any benefits of fruit size increases at harvest after Ralex® application in an “off-flowering” year. Therefore, Ralex® application was not recommended in an anticipated “off-flowering” year.
6. A permit (PER8269) was put in place (22 June 2005- 30 September 2007) for a limited use of Ralex® for research purposes.