“When a tree gets infected, there is generally no way to cure it,” University of Maryland virologist Anne Simon wrote in a recent white paper about invasive tree diseases. She explained that trees lack adaptive immune systems, so once infected they typically must tolerate the pathogen for the remainder of their lifespan.
“In some cases, immunity can be provided by planting resistant rootstocks or scions, potentially including genetically modified organisms (GMOs); however, even when this is possible, it requires replanting slow-growing trees on a massive scale,” Simon wrote.
There are many known anti-pathogenic agents that can destroy viruses, bacteria and fungi in plants, but the challenge has always been to effectively deliver these agents throughout a tree on a sustained, cost-effective basis, Simon stated. “Unlike mammals, plants lack efficient circulatory systems, so any introduced antipathogenic product gets diluted as it circulates through the tree and eventually gets degraded,” she wrote. “Another consideration is to avoid the need for decades of ongoing treatments.”
“To achieve lifetime resistance against certain diseases, the only viable option to date has been to develop genetically modified (GM) seeds,” Simon stated. But GM seeds can cost hundreds of millions of dollars for each seed variety and take over a decade to develop, she added. Using GM seeds for trees also requires 100% replanting of orchards, she wrote. Besides, only two GM seeds on the market are for tree varieties – papaya and apple – and even if successful, the GM approach would drastically reduce the varieties of citrus that could be grown.
“Another approach under consideration is to use anti-pathogenic sprays or injectables for trees,” Simon stated. But she added that those approaches are limited due to the reduced efficacy of the antipathogenic agent over time. Even if the sprays or injectables were successful, and a coating is used to extend the residence time, a tree could require hundreds of applications of expensive materials over its lifetime, she pointed out.
Simon and the company she founded, Silvec Biologics, have used a novel method to successfully vaccinate a model plant against citrus tristeza virus (CTV) in the laboratory. They are now focusing on HLB. Learn more about the vaccination efforts.
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