Epidemiology of citrus greening

Four oranges affected by the Candidatus Liberibacter asiaticus bacteria compared to a healthy fruit.
Four oranges affected by the Candidatus Liberibacter asiaticus bacteria, at right, compared to a healthy fruit. Citrus greening disease leads to mottled green fruits that are small and of poor juice quality. (IFAS/Mongi Zekri)

You might call it the COVID-19 of citrus. Huanglongbing, or citrus greening, is a disease that has severely affected Florida’s citrus industry over the past decade. It was first detected in 2005 in southern Florida and then spread rapidly throughout the state’s groves and nurseries. A new book chapter by EPI faculty member Burton Singer, in collaboration with coauthor Susan Halbert, reviews and consolidates the epidemiology of citrus greening and its vector, the Asian citrus psyllid. Their chapter appears in Asian Citrus Psyllid: Biology, Ecology, and Management, edited by J.A. Qureshi and P.A. Stansly (Centre for Agriculture and Bioscience International 2020).

Halbert studies vectored plant pathogens with the Division of Plant Industry in the Florida Department of Agriculture, and Singer uses mathematical modeling to characterize disease dynamics. The pair review literature covering transmission mechanisms, and the significance of testing psyllids for Candidatus Liberibacter asiaticus (CLas), the bacteria initiates a process that causes citrus fruits to green, malform and whither. Detailed maps document the geographic expansion of CLas-positive psyllids and plants in the state from 2005-2012. Halbert and Singer nimbly discuss the disease in single trees, groves, multiple groves and at the statewide landscape level.

But perhaps the most interesting material is their portrayal of how the insects and the disease may have initially spread. The introduced psyllids favored a host plant, orange jessamine, which was widely planted at this time, and which facilitated an established reservoir of psyllids by the time CLas-positive insects infected at a garden shop in Homestead, FL had spread northward and began to be documented. They synthesize research covering infective colonization events that spread the pathogen between multiple generations of psyllids, coupled with a highly variable incubation period — running from six months to six years — in which citrus plants can be infected but remain asymptomatic; and then they link these ideas to both the commercial nursery market, which shipped the ornamental host plants that psyllids prefer along with citrus stock across the state, and trucking routes for shipping citrus fruits that sometimes also ferried the psyllids long distances — setting up a network for rapid spread that far outpaced the psyllid’s ability to fly up to 70 km per year.

Written by: DeLene Beeland