1 February 2007 - To help citrus growers manage the crippling disease called citrus greening, two Agricultural Research Service (ARS) units in Fort Pierce, Fla., are investigating a range of strategies. One involves developing controls for the Asian citrus psyllid (Diaphorina citri Kuwayama), which is a major carrier of the disease.
The tiny brown psyllid was first spotted in Florida in 1998. Discovery of citrus greening--also known as Huanglongbing, or HLB disease--followed in August 2005. The characteristic yellow shoots, mottled leaves, and degeneration of vein phloem--part of trees' vascular system--are caused by the Candidatus Liberibacter asiaticus bacterium that is spread by psyllids as they feed.
With HLB attacking all kinds of Florida citrus, several strategies are being developed by scientists in the ARS Subtropical Plant Pathology Research Unit, led by plant pathologist Tim Gottwald, and in the Subtropical Insects Research Unit, led by entomologist David Hall.
The ARS researchers believe that an integrated HLB management program incorporating cultural, chemical and biological control strategies will have the best chance of success. Program components will include the removal of infected trees and special replant strategies, along with active psyllid control. To develop an integrated program, scientists will conduct epidemiological experiments testing the different management components, as well as pursue an array of approaches to maximizing biological control of the psyllid.
The psyllid is already known to be subject to natural control by lady beetles, syrphid flies, lacewings, spiders and one parasitoid species, Tamarixia radiata. Another parasitoid species has been found to attack the psyllid in Asia, and ARS is working with the Florida Department of Agriculture and Consumer Services to import this promising candidate for testing.
Another intriguing approach is trying to create citrus trees that are partially deciduous during some periods of the year. Since psyllids need leaves to feed on and reproduce, eliminating leaves at a certain time of year would eliminate the psyllids as well. Researchers are also working with Vietnamese and Australian scientists, who have indicated that interplanting citrus with guava almost entirely negated infestations by citrus psyllids, the vector of the disease, and as a consequence the citrus trees remained free of HLB.