Biological Control of Citrus Huanglongbing with EB92-1, a Benign Strain of Xylella fastidiosa

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus, is currently the most destructive disease of citrus and threatens production in all affected areas. There is no cure. Benign Xylella fastidiosa strain EB92-1 provided good control of Pierce's disease of grapevine by inducing host resistance; therefore, we evaluated the strain for the biological control of HLB in citrus. Treatment was by injection into the trunk of the trees. Strain EB92-1 was shown to colonize citrus and provided control of HLB by reducing both the incidence of symptomatic trees and the percentage of trees with severe symptoms in three separate trials. All trees were positive by quantitative PCR (qPCR) for Candidatus Liberibacter asiaticus at the start of the trials, and treatment did not eliminate the bacterium. In a trial of asymptomatic mature trees, a single treatment with EB92-1 reduced the incidence of trees with symptoms through 18 months after treatment and reduced the incidence of trees with severe symptoms through 3 years. In mature trees that had 60% incidence of mild HLB symptoms at trial initiation, percentage of trees with symptoms and trees with severe symptoms continued to develop in both the untreated and in the EB92-1-treated trees for 12 months. However, retreatment at 9 and 20 months prevented the development of additional severe symptoms in the EB92-1-treated trees throughout the remainder of the 5-year trial. In 2-year-old trees, incidence of trees with HLB symptoms was higher in the untreated trees than in the treated trees throughout the 6 years of the trial, reaching 90% in the untreated versus 50% in the treated. After 6 years, severity of symptoms was much lower in the EB92-1-treated trees, only 3% of the treated trees had become unproductive compared with 18% of the untreated. In 2020, the reduction in HLB severity resulted in approximately twice as much yield in the treated trees as in these untreated 8-year-old trees. X. fastidiosa strain EB92-1 proved to be efficacious for the control of HLB symptoms in both mature trees and newly planted young trees, but trees may need to be retreated yearly for at least the first 2 to 3 years.