Pathogen Profile #11

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inboxes.

Today, we're diving into the world of Huanglongbing (HLB), also known as citrus greening disease. If you live in the Americas, you’re probably familiar with the devastation it has been causing for citrus growers.

Huanglongbing, which translates to "yellow dragon disease", is caused by the bacterium Candidatus Liberibacter asiaticus (CLas). This phloem-limited pathogen is transmitted by the Asian citrus psyllid (Diaphorina citri Kuwayama) which typically feeds on the veins on young leaves. The transmission of CLas from psyllid to tree can occur in as little as 15 minutes of feeding. Once inside the host, CLas infection results in excessive callose deposition which interferes with the transport of sucrose from leaves to fruits. This ultimately causing fruit starvation, resulting in smaller, sour fruits.

The impacts of HLB have been felt most acutely in the citrus growing regions in the Americas. 85% of the global supply of orange juice comes from Florida in the US and São Paulo in Brazil with both regions being hard hit by HLB. Brazil is set to experience their smallest crop in 36 years as a result of HLB. The impacts are also being felt on the West Coast of the US where HLB was first detected in California in 2012, a state with over 3,000 citrus growers with farmland covering 320,000 acres. HLB did not establish itself in commercial orchards until 2023, prompting the establishment of quarantine zones. In March this year, USDA expanded the quarantine area for HLB in the state to slow the spread of the disease.

As the spread of HLB relies on controlling the psyllid vector and removing infected trees, insecticides and labour add substantial cost to production. In Florida, it is estimated to cost producers over $660 per acre (about 4047 square meters) to control it. Despite production volume reducing by nearly 80% in Florida, and increased costs, the citrus industry still contributed nearly $7 billion to the state’s economy in the 2020 - 2021 season. The impacts of HLB extend beyond the economic impacts. In Mexico, nearly 70,000 families depend on the citrus industry and the rapid spread of HLB has led to significant job losses.

For countries like Australia which are fortunate enough to be free of HLB, strict biosecurity measures are in place to keep the country disease free. With the threat looming with the Torres Strait as a potential pathway of entry into Australia, restrictions are in place to limit the movement of plants, fruit, or soil out of the Torres Strait Protected Zone. ​

If you’re interested in delving into the science of HLB, I’ve summarised a few articles below. Robigalia subscribers can scroll down to access my online database, which includes full summaries of these and all other featured Robigalia articles.

Huanglongbing papers

Economic and Social Impact of Huanglongbing on the Mexico Citrus Industry: A Review and Future Perspectives

• This review examines the impact of HLB on Mexico's citrus industry. The review highlights the significant economic losses and job cuts in a sector that supports 69,000 families. The authors discuss the current status of HLB in Mexico, detailing how existing management strategies have failed to control its spread or restore crop yields to pre-infection levels. They also explore the rising production costs faced by growers as they attempt to combat the disease.

A gap-free and haplotype-resolved lemon genome provides insights into flavor synthesis and huanglongbing (HLB) tolerance

• Researchers have produced a gap-free, haplotype-resolved lemon genome, revealing insights into flavor synthesis and Huanglongbing (HLB) tolerance. The study identified expanded gene families associated with pathogen resistance and phloem regeneration, which contribute to lemon's HLB tolerance. These findings provide valuable resources for breeding HLB-resistant citrus varieties and enhancing lemon's nutritional and medicinal value.

Field performance of ‘Valencia’ orange trees on diploid and tetraploid rootstocks in different huanglongbing-endemic growing environments

• This study on revealed that while diploid rootstocks produced larger trees and higher yields, tetraploid rootstocks offered better yield efficiency and fruit quality, making them ideal for high-density plantings. Despite not showing increased HLB tolerance, the promising performance of tetraploid rootstocks could enhance citrus production sustainability in HLB-endemic regions.