Understanding citrus greening and Asian citrus psyllid
To get an understanding of the disease and the threat it poses, a citrus industry team of two New Zealanders and six Australians recently spent 10 days in both Florida and California. Brad Davies, NZCGI (New Zealand Citrus Growers Inc.) Mandarin Product Group chairman and Kerikeri citrus grower, and Nikki Johnson, executive manager of NZCGI, were the New Zealand delegation.
Citrus Australia initiated the trip and invited the New Zealand citrus industry to be part of it. Most of the Australians in the team were at a State Government level involved in biosecurity response planning along with one Federal Government representative and the tour leader from Citrus Australia. “With the scale of impact to the citrus industry in Florida and potentially California, we need to be informed and prepared,” Brad says. “Everyone involved in the trip was open and obliging with information and very generous with their time.”
Citrus greening is also known as huanglongbing (HLB), a Chinese word that means “yellow shoot disease”. Brad explains that HLB is a pathogen that gets into the phloem of the plant, blocks the vital passageways, and is fatal to the tree. The main way it is spread is by the Asian citrus psyllid – “a different psyllid from the one we have here affecting the potato family.”
The Florida citrus industry has been fighting citrus canker in recent years, so they didn't realise that in the meantime citrus psyllid was there spreading HLB. By the time they realised, 40% to 50% of the trees were already infected. “It's a huge issue in Florida and we drove past big orchards with trees in various stages of decline. Infected trees continue to produce, but in less volume, and often the fruit aborts before maturity. There has been 30% reduction in production there year on year. Eighty percent of the Florida citrus industry is based around juice, so the flow-on affect is the loss of 70,000 jobs.”
Citrus production costs in Florida have doubled in the bid to combat the psyllid, and returns are a lot less because of falling production volumes. Some growers have walked away. “We left Florida feeling like they may be fighting a losing battle.”
Citrus greening, spread by Asian citrus psyllid, has devastated 60,000 acres of citrus trees in Florida since it was detected in 2005 and has taken 70,000 jobs with it. “With the scale of impact to the citrus industry in Florida and potentially California, we need to be informed and prepared.”
The extent of damage in Florida has alerted the citrus industry in California. California has had time to see the scale of the impact so is being pro-active. They are putting a huge effort into managing (mostly spraying) the Asian citrus psyllid population to reduce the risk of spread of HLB. It's a stopgap measure to buy time until a greater solution is found. “When we were in California, only one tree in the State (in Los Angeles) had been found to be infected with HLB, but there is a huge residential programme in place managing people's backyard citrus. The source of the infection in that single tree was likely uncertified plant material. This tree had been budded some15 times by a home gardener with many unknown varietal types. HLB could be out there in random citrus trees that die, but unless it is partnered with Asian citrus psyllid, it won't spread – other than by infected tree material being spread.”
Solutions to HLB and Asian citrus psyllid are so far elusive. Brad says there don't seem to be any resistant varieties yet, but obviously research is being done to find or develop these. While most of the current pest and disease management effort is going into spraying the psyllid, they have identified a bio-control parasite Tamaraxia and have a major breeding programme for it in place to help manage the psyllid population, however its effects are limited.
Obviously the Australian/New Zealand team was assessing readiness and response options for the citrus industry down- under if HLB and the Asian citrus psyllid appeared here. “Comments from the American professors made it clear that conditions favourable for growing citrus are the same conditions favourable for HLB and Asian citrus psyllid. Any incursion of HLB here would be through movement of infected plant material. Material imported through quarantine is tested before release so the biggest risk is an incursion through illegally imported plant material. So again this highlights that we need to be absolutely vigilant around our biosecurity.”
“We were staggered at the resources being invested in HLB and its spread in Florida. The disease is obviously a huge risk to the California industry and they are investing heavily in detection and preparedness. In New Zealand we would have to be prepared to have a fighting fund to allow early detection and eradication. The team's response after this trip though, was that if the disease and psyllid established here, the citrus industry would struggle to survive.”