The Australian Centre for International Agricultural Research (ACIAR) funded Nerida Donovan (NSW Department of Primary Industries) to travel to Brazil to participate in the 13th International Citrus Congress (ICC). More than 1000 delegates from 25 countries attended the conference, which gave Nerida the opportunity to gather the latest knowledge on citriculture, build useful information networks and strengthen existing collaborations.
Huanglongbing (HLB) is a bacterial disease spread by an insect vector (mainly the Asian citrus psyllid or ACP) that has significantly affected worldwide citrus production in recent years. Symptoms associated with HLB include leaf yellowing and mottling, shoot dieback, reduced yield and fruit quality, tree decline and eventual death. Infection leads to reduced root volume and blocked phloem vessels, disrupting the movement of water, nutrients and hormones throughout the plant. Roots of affected trees last only around four months, compared with 9–12 months in healthy trees. A second stage of root loss occurs simultaneously with canopy reduction (leaf drop and twig dieback). Detection rates of the causal bacteria are highest in root tissue.
HLB infection leads to an off-flavour (bitterness) in juice from affected trees due to a decrease in sugars and increases in acids, bitter limonoids and astringent flavonoids. The off-flavour has been observed to be worse early in the harvest season. In trials, the bitter flavour could be detected when 25–50% of juice from HLB-affected trees was mixed with juice from healthy trees.
In the absence of a cure, the key to managing HLB is prevention. In addition to strict quarantine measures to limit the movement of infected material and the vector, HLB management in countries living with the disease is based upon:
It is difficult to eradicate HLB because efforts focus on symptomatic trees. Every 50 days, a psyllid can colonise a new flush and start a new infection, but symptoms might not be seen for 3–4 years.
No citrus varieties with natural genetic resistance to HLB have been identified to date. However, citrus species exist that do have a level of tolerance to HLB infection, depending on the age of the plant when infected. Some existing mandarin cultivars (e.g. Clementine and Dancy) were found to have substantial tolerance to HLB when mature trees were infected.
Researchers in Brazil and Florida are also trying to introduce resistance or tolerance to citrus varieties by genetic engineering. Progress has been made in recent years with identifying and releasing potential varieties for commercial use in Florida. Industry has allowed the process to be fast tracked with cultivars released before their horticultural characteristics have been fully evaluated. Sugar Belle® is a mandarin hybrid for which symptom development is limited in trees that are carefully managed, with particular attention paid to nutrition.
In Florida, the year-round growing conditions create an ideal environment for diseases that rely on flush foliage or fruit. The industry has been decimated by HLB in recent years:
Juice quality has been affected as Florida fruit is becoming noticeably symptomatic with the increased and advanced infection status of orchards. Industry focus is now on conserving tree health and productivity on the remaining 60 million trees, most of which are infected. Research efforts (approximately $120 million to date) include short-term solutions to buy time until long-term solutions are found. A significant investment has focused on breeding and selecting potentially tolerant or resistant varieties.
In Florida, a commercial company is providing a thermal therapy service, whereby individual orchard trees are treated with steam and hot water to lower the amount of bacteria in infected trees. This short-term solution is likely to be cost prohibitive in Australian citrus production systems.
In Brazil, HLB appears to have had less of an impact on production. Researchers and industry have successfully worked together to slow down progression of the disease. Brazilian growers were already biosecurity aware after dealing with outbreaks of citrus variegated chlorosis (Xylella fastidiosa) and citrus canker (Xanthomonas citri subsp. citri) in previous years. Research efforts (costing approximately $36 million) have focused on improving the elements of the current ‘three pronged system’ – planting healthy nursery trees, removing symptomatic trees and controlling the insect vector. But it was suggested at the congress that all strategies are simply buying time until long-term solutions are developed.
In California the timeframe between the first reports of ACP (2008) and HLB (2012) was quicker than in other countries because they were actively looking via a comprehensive surveillance and testing program. HLB-infected plants are destroyed. Attempts were made to eradicate the psyllid but psyllid eradication has now been abandoned except in commercial production areas. Using pathogen-tested propagation material is now mandatory so the system has been flooded with healthy buds from the Californian Citrus Clonal Protection Program (CCCPP), which has helped to combat disease spread. Nurseries had to quickly adapt to new legislative requirements, which meant the number of nurseries in the state fell from 40 in 2010 to 15 in 2015. Industry, federal and state agencies are working together to enforce the 53,000 square mile ACP quarantine zone, which contains 50 million commercial citrus trees and more than 30 million backyard citrus trees.
HLB is also a priority area for research in China, with HLB and ACP occurring in 11 of 18 citrus provinces. The pomelo psyllid (Cacopsylla citrisuga) has been reported to be another vector of HLB in China. HLB was also reported as the major cause of citrus decline in India.
It was acknowledged at the congress that no amount of money can solve a problem if you do not have coordinated efforts and collaboration between industry and government. And yet, the solution has not been found after more than $150 million in research investment in the US and Brazil.