Unraveling the Mechanisms Leading to Prolonged Tree Health by Combining IPCs and Brassinosteroids

By Fernando Alferez, Divya Aryal and Saoussen Ben Abdallah

University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) research in the last three years has found that the combined use of individual protective covers (IPCs) and brassinosteroids (BRs) enhances protection of citrus trees against HLB and other diseases that limit citrus production in Florida.

The research has been conducted on different varieties, including Tango and Early Pride mandarins as well as Hamlin and Valencia oranges grafted on sour orange or US-942 rootstocks. Trees were covered immediately after planting with IPCs. The covers remained on the trees for three years before removal. All trees tested negative for HLB.

Then, monthly foliar spray applications of BRs were made at a rate of 6.2 fluid ounces of product in 100 gallons of water. This treatment resulted in trees remaining HLB-negative for about six months without the IPCs. An increase in tree height and canopy volume resulted in an increase in biomass. Yield increased by 30% on average in BR-treated trees. This was consistent across variety and rootstock combinations.

UF/IFAS researchers are now focused on studying the mechanisms underlying this prolonged protection that seems to result in improved tree health. They have found consistently that activation of salicylic acid biosynthetic pathway and downstream responses are early effects of the BR treatment in young citrus trees, regardless of the variety, rootstock or combination used. Interestingly, these effects are not seen in mature trees (six years or older).

Spraying BRs during flushing reduces psyllid population, oviposition and number of nymphs significantly. Researchers are working now to understand this effect. In any case, this observation opens the possibility of applying brassinosteroids only at the time of flushing for protecting new shoots. This would reduce the frequency of application of this plant growth regulator.

Fernando Alferez is an associate professor, Divya Aryal is a Ph.D. student, and Saoussen Ben Abdallah was a postdoctoral associate, all at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.