Pockets of “survivor trees,” or trees that have been infected with citrus greening yet continue to produce healthy fruit, are rare but exist throughout the state, including pockets discovered in Polk, Highlands and Hendry counties, Merritt Island, and the 40-year-old Donaldson tree in Groveland.
While research has yielded some information about how these survivors continue to thrive despite being infected with HLB, specific, replicable benefits that can help other infected trees have been few and far between.
Now, a researcher and prominent grower are teaming up for a field trial that will try to harness a biostimulant that they have identified through their lab work with survivor trees as a pathway that will allow HLB-infected trees to cope better.
At the March Citrus Research and Development Foundation meeting, Dr. David Norman, an associate professor of plant pathology at the University of Florida, and Pat Schirard of Citrus Extracts LLC were awarded a grant to field Test a way for infected trees to functionally react to the HLB like survivor trees do.
An HLB-infected tree produces excessive amounts of a compound called callose as a response to stress. The callose builds up in sieve pores and acts as a plug, effectively blocking the transport of the vascular tissue phloem, eventually causing phloem collapse. The collapse leads to the typical yellowing, nutrient starvation, and reduced fruit quality that is typical of a tree infected with greening. For the tree, it is akin to an overactive immune system that eventually becomes a danger to the whole body.
In laboratory trials, Norman identified an endophytic Streptomyces species isolated from HLB survivor trees in Lake County, Florida, that produces a callase-specific enzyme capable of effectively dissolving callose.
“Our theory, and it has shown results in the lab, is that the callase enzyme breaks down the callose, which frees the phloem to do its work in transporting nutrients,” says Norman. “The tree is still infected, but we have removed the symptoms, therefore allowing the tree to produce healthy fruit.”
It appears as if the tree recognizes that the microbes being introduced into the root system are beneficial and it does not try to kill them.
There are many transgenic trials with rootstocks and research trying to come up with a truly HLB-resistant tree that will become the future of the citrus industry in Florida. However, most of those trees are still a decade away from becoming a reality. This research now, if proven effective in the field, would allow growers to continue to keep their current trees in production and continue to produce quality fruit until that day comes.