The citrus industry in California is now on high alert for Huanglongbing (HLB or citrus greening) and the citrus psyllid that can transmit the disease. With infestations of the insect in southern California and the Los Angeles urban area, it now seems only a matter of time before it becomes established in California’s citrus production areas.
“Australia is one of the few countries outside the Mediterranean area with a citrus industry that is free from HLB and citrus psyllids,” said Pat Barkley, Technical Advisor to Citrus Australia Ltd.
“While both agents are needed for a full blown incursion, there’s no room for complacency, something that citrus growers in Florida have learned the hard way,” said Mrs Barkley.
HLB is a highly destructive disease of citrus. It occurs when trees are infected with forms of the bacterium ‘Candidatus Liberibacter’
from Asia, Africa or South America.
HLB is spread either by infected plant material (eg budwood, cuttings or marcotts), Asian citrus psyllid (Diaphorina citri) or African citrus psyllid (Trioza erytreae). The psyllid feeds on the nutrients in the plant sap, infecting plants as it moves from one plant to the next.
Mrs Barkley said: “Infection with HLB is a death sentence for a tree because there is currently no way to treat the disease. Trees need to be destroyed to prevent the disease from spreading. The psyllid population also needs to be controlled to limit further spread of the disease.”
“We must ensure that the psyllid does not enter or become established in Australia. The Asian citrus psyllid is present in East Timor, Indonesia, New Guinea, Guam and American Samoa, and could conceivably blow across to Australia in cyclonic winds.”
“More disturbing is the possibility of citrus psyllids being present on illegally imported citrus or citrus relatives such as curry leaf (Bergera),” Mrs Barkley added.
Growers in Florida were largely unconcerned when the Asian citrus psyllid arrived in 1998. Now, the Florida citrus industry is contracting at a rate of 10-15 per cent per year since 2005 when HLB was first identified. From 5 to 12 insecticidal sprays are required per year to keep the infection rate at 4-5 per cent of trees per year. But that has increased production cost significantly, something that the Australian citrus industry could well do without.
“While a contingency plan has been developed for Australia, inspection for the Asian citrus psyllid is our first line of defence and citrus tree owners should inspect often. Any orange jasmine (Murraya exotica or M paniculata, a common garden plant) in the area should be inspected as well, since Murraya is the preferred host of the Asian citrus psyllid,” advised Mrs Barkley.
Adults are 2.5 to 3 mm long, yellowish-brown, have grey legs and transparent wings with spots or bands. Eggs are less than 0.5 mm in size, light yellow to orange, with distinct eye spots at maturity. The abdomen is a pointed shape when viewed from above and they have a distinctive feeding posture (see images and fact sheets below for more details).
Symptoms of HLB in citrus include: yellow shoots; asymmetric, mottled (across veins) leaves; small upright chlorotic leaves; out of phase flushing; and branch dieback.
Flower and fruit symptoms include: unseasonal and heavy flowering on diseased branches; small, lopsided, bitter-tasting fruit with small, brown, aborted seeds and uneven colouring at maturity; and excessive fruit drop.
“The disease can go unnoticed because symptoms can be confused with mineral deficiencies and other endemic pathogens,” advised Mrs Barkley.
HLB is the primary cause of losses in citrus production in Asia. The presence of HLB in the state of Sao Paulo in Brazil led to the elimination of an estimated 3 million diseased trees between 2004 and 2008. It is estimated in Florida that post-HLB, production costs have increased by approximately 40% compared to pre-HLB production costs.
“In the presence of the citrus psyllid, similar losses and increased production costs could be expected from HLB in Australia if the pest became endemic. Some of our native vegetation such as desert and finger limes would also be under threat,” said Mrs Barkley.
You have an important role to play in protecting your orchard and the Australian citrus and nursery industries from biosecurity threats.
1. Be aware of biosecurity threats
Make sure you and your orchard workers are familiar with the most important citrus pests, and particularly the Asian citrus psyllid.
2. Use high health status, true to type propagation material
Ensure all propagation material and other orchard inputs are pest-free. Obtain nursery trees from reputable nurseries which use Auscitrus budwood and seed.
3. Keep it clean
Practicing good sanitation and hygiene will help prevent the entry and movement of pests onto your property. Workers, visitors, vehicles and equipment can spread pests, so make sure they are clean before they enter and leave your orchard.
4. Check your crop
Checking your crops frequently for pests will help you and your staff to notice anything new or unusual. Keep records of all observations.
5. Abide by the law
Support and be aware of legislative regulations established to protect the citrus industry from biosecurity threats. It is illegal to obtain nursery trees or budwood from Queensland. Do not illegally import budwood from overseas.
6. Report anything unusual
If you suspect a new pest, report it immediately to the Exotic Plant Pest Hotline, 1800 084 881.
More information is available from: the Asiatic citrus psyllid fact sheet; the HLB and Asian citrus psyllid spotting guide; and the Orchard Biosecurity Manual for the Citrus Industry.
Photos: courtesy of Andrew Beattie, University of Western Sydney