Historical document
Flower manipulation with Ralex to increase fruit size and even out alternate bearing of citrus (CT03026)
Publication date: September 1, 2005
Delivery Partner: NSW Department of Primary Industries
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Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.
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:
This historical project was a strategic levy investment in the Hort Innovation Citrus Fund
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