Can Brassinosteroids Help Citrus Cope With Disease and Extreme Weather?

Josh McGillDiseases, Research, Weather

By Fernando Alferez and Muhammad A. Shahid

Brassinosteroids (BRs) are a natural group of hormones that regulate physiological and biochemical attributes that include seed germination, flower development, root formation and resistance to various abiotic and biotic stresses. BRs have shown promise in reducing pests and diseases in a wide range of plants, including citrus.

A commercial formulation of BRs has been approved for use in Florida citrus. Although BR treatment increases resistance to a wide range of pathogens in plants, the role of BR in plant responses to pathogens is complicated, and the regulation of immunity occurs at multiple levels. In citrus affected by HLB, some genes associated with systemic acquired resistance are enhanced by BR treatment.

Several years of BR work have taken place at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center (SWFREC). Monthly foliar applications with BRs at a rate of 6.2 fluid ounces per 100 gallons of water significantly delayed HLB disease development and protected trees for several months after application, as compared to non-treated trees. BRs also reduced psyllid colonization in new flushes and significantly reduced the incidence of other pests.

Researchers monitored the Candidatus Liberibacter asiaticusinfection rate and colonization by psyllids and other pests, including rust mites. After six months, only 25% of BR-treated trees were HLB-positive. Psyllid colonization and rust mite incidence were significantly lower compared to the non-treated trees, where the HLB infection rate was about 80%. These effects were likely due to a strong immune response in the citrus trees.

BRs may enhance salicylic acid (SA)-mediated defense responses. It has also been proposed that a proper-sized endogenous BRs pool should be maintained over time to support SA responses and downstream effects. Preliminary data strongly supports this finding. An exogenous BR treatment on newly planted young citrus trees in the field induced SA biosynthesis. This effect was maintained for several weeks. Researchers monitored the expression of genes associated with immune response, and the genes associated with the production of SA were highly upregulated after BR treatment.

Following these findings, research started aimed at understanding how the immune response of young citrus plants changes in response to the exogenous application of BR. Researchers also wanted to know how long BR will protect young citrus trees.

The reason for doing this is cost related. It is believed that adjusting BR treatment to a minimum number of applications will result in savings for the grower. Since it was observed that BRs delay HLB in young trees, treatments began at the rates stated above once per month. Work is being conducted to determine how long the protection will last. This will allow for the minimum use of BR treatment necessary to achieve effective protection.

The research group at the SWFREC found that individual protective covers (IPCs) are an effective tool in protecting newly planted trees against HLB. However, this protective system needs to be removed after two to three years, leaving trees exposed to the psyllid vector of HLB. This leads to the disease developing in less than one year after the removal of IPCs. Once IPCs are removed, growers should try to keep trees healthy with additional means that are available and affordable. Growers want to ensure high yields and high-quality fruit once trees enter maturity.

A system that involves IPC installation until the tree enters production age and BR treatment once IPCs have been removed may result in prolonged protection. Ensuring that BR is applied only when necessary will result in cost savings. Typically, at least in trials with Valencia trees, three years of IPC protection greatly improved fruit yield and quality compared to non-protected trees. However, fruit quality declines as the trees get infected upon IPC removal, although at a lower rate than non-protected trees.

Adding a BR treatment when IPCs are removed may prolong the health of these trees while improving fruit yield and quality. This possibility is currently being researched thanks to a new grant from the U.S. Department of Agriculture, National Institute of Food and Agriculture, Emergency Citrus Disease Research and Extension program.

Extreme weather events are unfortunately common in Florida. This season, the state had two major events that are still causing uncertainty in two geographical areas with totally different citrus production practices. We are talking about Hurricane Ian on Sept. 28, 2022, which crossed most of the major citrus-growing areas for juice processing, and the freeze event faced by the North Florida citrus-growing area in the last week of December 2022.

Following Hurricane Ian’s impact, tree health and yield of SWFREC trials are being monitored on mature Hamlin and Valencia trees in which BRs are being applied to modulate fruit maturation. This work is possible thanks to a grant from the Citrus Research and Development Foundation. Although fruit loss was heavy, especially in Hamlin, some mild reduction (10% less) in fruit drop in the plots treated with BRs has been seen. It is not clear if this is a direct effect of BRs on fruit abscission or an indirect effect of better health in these trees. Researchers are currently investigating these possibilities.

The cold-hardy citrus industry has been expanding in North Florida with approximately 3,000 acres in 14 counties. A recent freeze event with five consecutive days (Dec. 22–26) below freezing caused damage to the citrus industry in North Florida (Figure 1). There is no freeze-protection approach except using a microsprinkler to protect the bud/graft union and the lower portion, particularly in young plants.

Figure 1. Mandarin tree damage in North Florida following the December 2022 freeze event

In the last years, however, evidence is mounting that BRs can improve cold-stress tolerance with few tradeoffs in terms of growth and yield. Research groups at the North Florida Research and Education Center (NFREC) and the SWFREC are planning experiments in a controlled environment to understand what BRs can do for cold protection in young citrus trees. They have just done a short preliminary study in programmed freezing chambers to determine if BRs affect cold hardiness in citrus. BRs did improve the cold tolerance when treated young plants were exposed to 25 degrees Fahrenheit for 24 hours (Figure 2). Detailed studies to understand the effect of BRs on physiological and biochemical indicators of stress and modulation of tolerance are forthcoming.

Figure 2. Young mandarin trees in controlled conditions were kept at 25 degrees Fahrenheit for 24 hours. The tree on the left was treated with distilled water. The tree on the right received brassinosteroid treatment.

Fernando Alferez is an assistant professor at the UF/IFAS SWFREC in Immokalee. Muhammad A Shahid is an assistant professor at UF/IFAS NFREC in Quincy.

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