New study maps risk of citrus greening establishment

Four oranges affected by the Candidatus Liberibacter asiaticus bacteria compared to a healthy fruit.
Four oranges affected by the Huanglongbing bacteria, at right, compared to an unaffected fruit. Citrus greening disease leads to mottled green fruits that are small and of poor juice quality.

A new study by an international research team, including UF medical geographer and EPI investigator Sadie Ryan, identifies global regions most at risk of — and most resilient to — citrus greening. There is no cure for infected trees, and the disease has wiped out millions of acres of citrus crops worldwide.

Orange juice is a breakfast staple, but the future availability of citrus products is threatened by the global spread of citrus greening disease. The disease prevents commercially viable fruit from forming.

A new study by an international research team, including UF medical geographer and Emerging Pathogens Institute investigator Sadie Ryan, PhD, identifies global regions most at risk of — and most resilient to — citrus greening.

“Translating these models into maps helps communicate our findings to citrus stakeholders, and creates a baseline for thinking about potential climate change impacts,” says Ryan, who holds joint positions with UF’s EPI and the College of Liberal Arts and Sciences’ geography department.  

The disease has devastated Florida’s citrus industry, and led to a nearly 75 percent decline in boxed orange production in 2018 – the lowest since World War II, according to the Tampa Bay Times. Jobs in the state’s citrus industry have declined by 59 percent in the past decade; and Brazil has emerged as the global producer of orange juice, supplanting the Sunshine State’s position as a top producer. This has cost the Sunshine State an estimated $2 billion in lost economic impact.

The global maps, produced by Sadie Ryan’s lab, illustrate the number of months throughout a calendar year that fall within the temperature suitability zone for Huanglongbing bacterium transmission, the causal agent of citrus greening. The work provides a framework to not only visualize which citrus-producing regions of the world are at risk, but also where citrus production may move in the future as the climate shifts warmer.
Yellow mottling in citrus leaves is a hallmark of the emerging disease.

Citrus greening is caused by the Huanglongbing bacteria which is transmitted by the Asian citrus psyllid insect. Both the pathogen and the insect vector have spread in recent years, devastating regions famous for high citrus production and threatening the citrus industry’s future. As citrus greening menaces growers worldwide, the industry’s future may depend on identifying locations most resilient to production collapse.

Knowing which environmental conditions are suitable for disease transmission, and where those conditions occur, is vital for crop management both now and in the future. Ryan’s new paper, published today in Journal of Applied Ecology, investigates the temperature-driven comfort zone for citrus greening transmission.

Led by Rachel Taylor of the United Kingdom’s Animal and Plant Health Agency, the research team modeled how citrus greening transmission depends on temperature, and then mapped how this translates into areas where the disease could become established.

Infection with the Huanglongbing bacteria causes citrus trees to produce smaller fruits that often include misshapen lobes, as pictured here, with a bitter quality.

“Our suitability maps can be used to underpin risk-based surveillance and prevention, to ensure resources to fight citrus greening are applied in the best locations,” Taylor says.

Disease transmission dynamics largely depend on temperature, both for successful replication of the Huanglongbing bacterium and survival of the psyllid insects that spread it. The model was built with data collected under laboratory conditions, and directly incorporated the effects and limitations of environmental temperature into the suitability estimate.

The model predicts that successful infection of host plants can occur between 60.8˚F and 91.4˚F (16˚C to 33˚C), with peak transmission at around 77˚F (25˚C). By using the temperature limits for disease spread, the authors mapped global suitability and showed how many months of the year have temperature conditions that would place citrus groves at risk for infection with Huanglongbing. Many regions with nearly year-round suitability for citrus greening include some of the citrus-growing areas hit hardest by the disease, including Brazil and South-East Asia.

Asian citrus psyllid vector which transmits the bacteria that causes citrus greening disease.

This work provides critical information for citrus production and crop management moving into the future. Some locations identified by the model as suitable for transmission for half of the year, such as California and the Iberian Peninsula, are currently free of citrus greening. In these areas known for high citrus production, preventing the establishment of the disease vector through increased surveillance and management may help prevent the devastating effects that citrus greening has had on other growers.

“Although the approach is fairly simple, we’ve shown in other systems that we can make surprisingly accurate predictions,” says coauthor Leah Johnson, from Virginia Tech.

By: DeLene Beeland