Citrus greening disease is by far the biggest threat to the citrus industry. The disease has caused widespread damage in Asia, Africa, South America, and the United States, including billions of dollars worth of citrus fruit losses in Florida alone.
Also known as Huanglongbing (HLB), citrus greening is spread by the Asian citrus psyllid. HLB attacks the vascular tissue that citrus trees use to transport nutrients. The trees still appear healthy at first, but gradually become unproductive and die.
Previous research has been primarily focused on trees that were already infected with the disease, so the factors that promote the spread of citrus greening are not well understood.
In a new study published by the American Phytopathological Society, scientists in Texas surveyed commercial and residential citrus trees from 2007 to 2017. The team closely monitored the proportion of citrus trees and the Asian citrus psyllid (ACP).
Study co-author Dr. Olufemi Alabi is an expert in Plant Pathology and Microbiology at Texas A&M University.
“Unlike previous studies on citrus greening disease epidemics that were typically initiated in commercial orchards after the disease had been introduced or became widespread in the area, our study commenced 5-years prior to the first detection of the greening bacterium in Texas and continued for 5 additional years,” explained Dr. Alabi.
“This gave us the unique opportunity to obtain a holistic picture of the progression of the disease epidemics from its onset in both commercial and residential ecologies.”
Even though citrus greening first appeared in Florida in 2005, the infectious bacteria was not found in Texas until 2011, when scientists detected it in psyllids.
Citrus trees remained disease-free in Texas until 2012, which indicates that psyllids could be used for early detection of the HLB pathogen as it invades new areas.
During the decade-long study, the proportion of infected trees and psyllids increased exponentially over time, according to the researchers. By 2017, at least one citrus tree was infected in 26 percent of fields and in 40 percent of residential backyards.
The experts also identified seasonal fluctuations that will provide new insight into the ongoing citrus greening epidemic in Texas. The findings may also help to inform other regions that have not yet been affected by citrus greening.
“Our study suggests that a flatter progression of citrus greening disease epidemics could be achieved through the implementation of strategies to protect new plantings from infection and the continued implementation of the area-wide ACP management program,” said study lead author Mamoudou Sétamou.
The research will bring some relief to Texas farmers, who feared that the epidemic would overwhelm the state’s citrus industry just as quickly as it did in Florida.
“Surprisingly, our research showed that although an exponential growth was observed in progression of infected trees in Texas, the annual rate of increase was relatively slower than reported from Florida,” said Sétamou.
“This led us to conduct a series of analyses that enabled us to identify potential climatic and cultural factors that may be contributing to the relatively slow spread of citrus greening disease in Texas.”
The study is published in the journal Plant Disease.