Citrus growers may have a new tool in the fight against citrus greening — brassinosteroids. Horticulturalists discovered brassinosteroids about 40 years ago but are continuing to discover potentially valuable new uses for them.
Fernando Alferez, Associate Professor and Citrus Horticulturist with UF/IFAS, describes brassinosteroids as “a unique class of plant steroidal hormones with important roles in regulating many physiological and developmental processes, including, but not limited to: seed germination, flowering tree growth, and plant senescence.”
The use of brassinosteroids in some fruit-bearing plants, such as strawberries, can accelerate fruit maturation. When applied to citrus, researchers have found that applying brassinosteroids at the right time may induce internal maturation (increasing sugar content and decreasing acidity).
Alferez discusses two applications of brassinosteroids in citrus trees, by age. “In young citrus trees (3 years or younger) that are not bearing fruit yet, applying brassinosteroids by foliar spray results in better tree growth and more resistance to disease. In mature, 8- to 9-year-old trees and older, applying brassinosteroids may result in better fruit quality and slightly more yield.”
So far, Alferez and his team have tested brassinosteroids on Hamlin, Valencia, Tango mandarins, and grapefruit, with consistent results for each variety. “The two main factors that seem to govern the response of the trees, in the case of mature trees, are the initial tree health and the time of application.”
Alferez has identified two contributing factors influencing the success of brassinosteroid treatment. First is tree health at the time of application, the second is the time of application. “The timing is important, and the right time for treatment application must be established for each variety and growing area.”
They are also exploring whether brassinosteroid treatment can be combined with other hormones or chemicals to make treatments quicker. “We know that they can be tank-mixed with gibberellic acid. We are currently assaying their administration together with copper materials (to get more effective protection against citrus canker) and in parallel to oxytetracycline injections as a way to improve even more fruit quality. These trials are starting now.”
Most importantly, with all of the benefits that brassinosteroids provide, thus far Alferez and his team have not documented any undesirable side effects.
“We have seen no toxicity in plants or any adverse effects of fruit. It is a very safe product,” he says.
The brassinosteroid formulation being used is approved for grower use in Florida citrus.
