HLB update
In case you haven’t heard, HLB is a killer.
HLB (or huanglongbing) is a bacterial disease spread by an insect vector (mainly the Asian citrus psyllid or ACP) and there is no cure. Whilst not found in Australia, HLB is considered the biggest threat to citrus production worldwide. HLB and associated vectors are found in countries close to our northern borders, increasing the threat. Australian scientists need to develop and maintain international linkages to stay up to date with the latest knowledge on HLB and other citrus pathogens to reduce the potential effects of diseases on our industry.
Dr Nerida Donovan and Grant Chambers from the NSW DPI Citrus Pathology team attended the Joint conference of the International Organization of Citrus Virologists (IOCV) and International Research Conference on Huanglongbing (IRCHLB) held in Riverside, California in 2019. The 21st IOCV conference was attended by more than 200 people and there were more than 550 participants in the 6th IRCHLB from 23 countries; the largest conference held with a focus on one plant pathosystem. Leading international citrus researchers presented their work on the detection and management of graft-transmissible citrus diseases, including HLB. Nerida was an invited speaker at the IRCHLB, giving an enlightening account of how the industry and government are working together to keep HLB out of Australia and to be prepared for its arrival if it were to occur.
Knowledge and international linkages gained from this conference and tour will assist the NSW DPI citrus team to work with industry and government to maintain the high health status of Australian citrus.
Country updates presented at the conference
In the United States, HLB is widespread in Florida and also found in Texas, South Carolina, Louisiana, Georgia and California (limited area). Regulatory changes in California have greatly increased Citrus Clonal Protection Program (CCPP) users, flooding the system with disease-free trees to combat the disease. At the conference, it was stated that more than USD400 million has been spent on HLB research since 2005, yet few management options are available and no cure has been found. However, there have been considerable knowledge gains about the HLB pathosystem.
Brazil has lost an estimated 52 million citrus trees since HLB was detected in 2004. However, proactive management has meant that HLB incidence in the major growing regions of Sao Paulo and Minas Gerais has remained at an estimated 18% in recent years – although this varies from 20–60% in other affected regions. Incidence is significantly higher on smaller farms due to the psyllid vectors concentrating around the edges of blocks.
In China, HLB is found in over 300 counties and the epidemic is moving north. An estimated 50 million infected citrus trees have been removed from Jiangxi province since 2013 and growers are reportedly spraying up to 30 times per year to manage ACP vector populations. Producing and planting diseased nursery trees is proving to be the greatest challenge in China as the supply of health-tested material from formal propagation schemes cannot meet national demand.
South Africa has been managing the African strain of HLB for many years but the detection of ACP (the vector of the more aggressive Asian strain of HLB) in Mozambique, has heightened their threat level.
The causal agents of HLB and associated vectors have not been reported in Europe, except for the African citrus psyllid (Trioza erytreae). Efforts to eradicate the psyllid failed. Only Spain has a formal scheme for producing disease-free citrus trees and there are no regulations for producing high health status citrus nursery trees in protective structures. Four million Euros have been invested by the EU to evaluate the risk and increase preparedness for many insect-borne prokaryote-associated diseases of perennial crops (e.g. palm, grapevine and citrus), including HLB.
Recommendations from the conference for Australia to consider
- Australia must continue its efforts to prevent exotic citrus diseases entering Australia. This is of particular importance for HLB and associated insect vectors. Management strategies employed in affected regions overseas may be cost prohibitive in an Australian environment.
- Pathogen-tested citrus propagation material is essential for a sustainable and profitable citrus industry in countries with HLB, but its use in Australia is currently voluntary. Mandatory use of pathogen-tested material from Auscitrus should be introduced before HLB and associated vectors enter the country.
- The introduction of a mandatory nursery registration system would aid tracing investigations during an emergency response.
- New diagnostic methods, including emerging technologies, for detection of endemic and exotic citrus pathogens, require evaluation and validation through the Hort Innovation industry funded project CT17007 ‘Improving diagnostics and biosecurity for graft-transmissible diseases in citrus’ led by NSW DPI.
