By Ed Ricciuti
Scientists have found a more efficient way to infect experimental trees with citrus greening disease, boosting efforts to find citrus varieties that are able to resist the bacterium which causes the disease. At the same time, their research has produced an unexpected bonus: a new strategy that growers can use to protect their trees against insects that transmit the disease, which threatens groves nationwide.
The time when trees are most vulnerable to citrus greening disease — and therefore the best time to infect them and to spray them in the field — is during “flush,” a stage in growth from the emergence of leaves until they expand to full size, but before they become thick and leathery. Such is the conclusion of research by USDA scientists, which is described in the Journal of Economic Entomology.
Citrus greening disease, also known as Asiatic huanglongbing (HLB), is spread by the Asian citrus psyllid (Diaphorina citri). The disease mars leaves, shrinks and turns fruit bitter, and eventually kills trees. So far incurable, it has cost the Florida citrus industry alone $1.3 billion. Indeed, the USDA warns that HLB “threatens the survival of Florida citrus and is a potential threat to the entire U.S. citrus industry.” Growers lacking effective control strategies for HLB have resorted to removing infected trees in hopes of stemming its spread. Thus, the discovery of any new weapon against it could have profound economic significance.
HLB, caused by the bacterium Candidatus Liberibacter asiaticus (CLas), seems to have emerged in China during the early 1900s. It appeared in Florida in 2005 and since, along with the Asian citrus psyllid, has reached Texas and California. Mottled brown on body and wings, the Asian citrus psyllid lays its eggs on the tiny, feathery leaves that emerge from buds breaking open during the initial stage of flush. The psyllid nymphs and adults also feed on the leaves, and they pick up bacterium from infected plants and transmit it to others.
The reproductive cycle of the psyllid is closely tied to flush, according to the authors, and so is the rate at which the psyllids infect citrus with the bacterium.
“CLas transmission rates are increased when citrus flush is present,” they wrote.
Since trees are most vulnerable during flush, that is when growers should mount most vigorous control methods. The paper puts it succinctly: “Healthy citrus should be protected from ACP infestations throughout a flush.”
Finding the new approach to combating HLB was fortuitous, said lead Dr. David Hall of the U.S. Horticultural Research Laboratory, who presented his findings to Florida citrus growers at the end in January 2016. He and his colleagues are at the forefront of a USDA effort to identify and develop strains of citrus that resist the disease. He has even appeared on NBC national television news to discuss HLB.
“We were looking for the best ways to test varieties for susceptibility,” he said.
To do that, researchers need an efficient method of infecting experimental plants with the disease, because it is not a given that infection will always take place. The more plants in an experimental sample that can be infected, the better resistance can be gauged.
“We are trying to understand disease transmission by the psyllid to increase our success in infecting plants,” Hall said.
Trees can be infected with HLB by hand via the grafting of infected shoots. But instead of such a labor-intensive procedure, Hall and his team opted to let the psyllids do the work for them, mimicking nature in the process.
Since the psyllid lays its eggs on emerging leaves, Hall and his partners decided to see if flush played a role in disease transmission and if it would impact experimental infection.
“I actually thought that it would not matter if flush was present,” Hall said. “But I was wrong. We now know that our success in infecting plants will be much better if flush is present.”
Very much better, the results of experiments indicate. The percentage of seedlings with flush that developed HLB after exposure to infected psyllids was much higher than a control group that was not in flush. The results of one experiment were particularly vivid. Seventy-seven percent to 97 percent of one strain, a hybrid rootstock, was infected compared to just 40 percent in the control group.
The researchers simulated the levels of flush by trimming the trees, with those in the older stage of flush trimmed first and those in the youngest stage two weeks later. A group of trees without flush was used for comparison. After one week of exposure to psyllids, the trees were grown for six months and then evaluated for HLB. A similar, second experiment with Valencia sweet orange trees resulted in a 23 percent infection rate during young flush, 80 percent during old flush, and only three percent when the trees were not in flush. Overall, the scientists concluded that not only should growers protect trees throughout a flush, but that older flush promotes the highest infection rate.
In the end, said Hall, whether the goal is to infect citrus or protect from HLB, a flush beats all.
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Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.