Growers of seedless Clementine mandarins in California commonly apply gibberellic acid (GA3) to counter problems with low fruit set and small sizes.

Until now, there has been a lack of California-specific research on the proper use of GA3. And without clear label instructions, many growers have been left guessing about how to best apply the plant growth regulator, particularly in light of the alternate bearing nature of California Clementines. At the same time, research has shown that improper applications can actually be detrimental and result in low fruit set, canopy damage and a reduction of flower formation in following years.

Now, after a three-year study on the effects of various rates and timing of GA3 applications in Nules and other seedless Clementine cultivars, University of California researchers have developed guidelines for using GA3 to increase fruit set and the production of commercially desirable sizes without negative impacts on the tree or crop.

UC Riverside Plant Scientist Carol Lovatt says results from commercial trials show the best strategy for improving fruit set and production of more marketable large-sized fruit is to apply high rates of GA3 early in the bloom stage only during off-crop years.

“We found that the best strategy is to apply 15 to 25 mg GA3 per liter four times a year starting early at 60 percent bloom to increase the yield of large and jumbo-sized fruit,” Lovatt said. “In on-crop years, applying GA3 gives a reduction in yield so it's better not to apply GA3 in on-crop years.”

Lovatt's research confirmed that applying GA3 during on-crop year cycles actually reduced total yield and the yield of larger sized fruit.

The research was done in commercial San Joaquin Valley orchards from 2004 through 2006, which included two on-crop years, with yield of approximately 1,200 fruit per tree, and an off-year with yields of about 550 fruit per tree. Rates ranged from 10 mg/liter to 25 mg/liter with timing at 60 percent bloom, 90 percent full bloom, 75 percent petal fall and 10 days after 75 percent petal fall.

In both on-crop years, Lovatt found it was better not to apply any of the studied rates, as all treatments either reduced both total yield and yield of larger sized fruit, or had no effect at all. She said applying higher rates of 15 to 25 mg GA3/liter, starting early at 60 percent bloom, and frequently in four applications improved the commercial size structure of the crop during off-crop years and had no negative impact on yield the following year.

During her presentation at the UC's spring citrus meeting in Tulare, Lovatt said she plans to further study the alternate bearing nature of Clementine mandarins in California to determine the factors that contribute to uneven productivity and may affect the ability of GA3 to increase fruit size during on-crop years.

Growers in California commonly find they have a large crop of small fruit during on-crop years and too few fruit with sometimes excessively large sizes and poor fruit quality during off-crop years, creating income and market instability.

“Alternate bearing results in a loss of stable income and market share to the industry and a loss of market share for growers often not recovered during the on-crop year in subsequent seasons,” Lovatt said.

Environmental conditions, such as too little chilling, adverse climate during fruit set or excessive June drop can cause alternate bearing. Lovatt said that once an alternate bearing cycle is initiated in trees it becomes entrained within the tree as fruit set affects flowering the following year.

Lovatt is exploring the mechanisms within the tree that lead to alternate bearing cycles so that growers can take steps to even out production from one season to the next.

Her studies have shown that increasing the number of summer and fall shoots during on-crop year cycles and removing summer/fall shoots on trees during off-crop years can help even out alternate bearing.

Lovatt also will be looking at the application of low-biuret urea and other foliar fertilizers to increase fruit set and yield. Studies already have shown that December applications of urea work to increase large-size fruit of Nules mandarins, and Lovatt will be looking at rates and timing of urea and other foliar applications including potassium, phosphate, zinc and boron, to develop recommendations for those foliar fertilizers that can produce a return for growers.

Lovatt also shared a word of caution about the fairly new practice of using 2,4-D to increase fruit size of mandarins. Lovatt noted that the product, which will be registered under a 24C supplemental label as Citrus Fix, works well to increase fruit size of mandarins, but should be used carefully to avoid problems with fruit quality, particularly with cultivars such as Nules that are prone to the condition.

Lovatt said previous research indicates 2,4-D treatments should not be used when yield is low in an off-crop year; in combination with potassium nitrate sizing sprays; or under conditions that might make fruit prone to dryness or granulation.

“This is a very good strategy, but you do have to use it carefully,” she said.