Citrus greening, also called huanglongbing, or HLB, is the most destructive disease of citrus in the world. It has no cure.
Infected trees decline over a few years, produce small, misshapen, bitter fruit, and eventually become unproductive.
In Florida, the orange harvest has fallen by more than ninety percent since the early two thousands, largely because of this disease. It is established across much of Asia, Africa and the Americas,
and every citrus region that does not yet have it is working hard to keep it out.
In this video: what causes HLB and how it spreads, how to recognise it, including the one leaf symptom that sets it apart, the psyllid insects that carry it, and the three pillars of management that slow it down: clean planting material, psyllid control, and removing infected trees.
The advice is general and global.
Follow your national plant protection rules, because HLB is a regulated disease in many countries.
HLB is caused by bacteria that live in the phloem, the tree's food-conducting tissue. The most widespread is Candidatus Liberibacter asiaticus, found in Asia and the Americas.
A related species occurs in Africa, and another in Brazil.
The bacteria block the flow of sugars, so the roots starve and die back, and the whole tree declines.
The bacteria are spread mainly by two small sap-sucking insects.
The Asian citrus psyllid carries the Asian form,
and the African citrus psyllid carries the African form. A psyllid that feeds on an infected tree picks up the bacteria and can then infect healthy trees for the rest of its life. The disease is also spread by grafting, when budwood from an infected tree is used to propagate new plants. That is why moving plants and budwood is so strictly controlled.
There is a long hidden period.
A tree can be infected for months, or even more than a year, before symptoms appear, and during that time psyllids can already pick up the bacteria from it.
The most reliable leaf symptom is blotchy mottle. The leaf shows patches of yellow and green that cross the veins and do not match on the two sides of the midrib.
Fold the leaf along the midrib: if the yellow pattern on one half does not mirror the other half, think HLB.
Nutrient deficiencies, especially zinc and manganese, also cause yellowing, but their patterns are usually symmetrical and follow the veins.
Other signs build up over time.
Single shoots or sectors of the canopy turn yellow, which is where the Chinese name, yellow dragon disease, comes from.
Leaves become small, thick and upright, sometimes with corky, swollen veins, and twigs die back.
Fruit is small and lopsided, stays green at the bottom end while the top colours, tastes bitter, and drops early, often with dark, aborted seeds.
Because many problems look similar, and the hidden period is long, a laboratory test, usually PCR, is needed to confirm infection.
If you suspect HLB in a region where it has not been reported, contact your plant protection authority.
The Asian citrus psyllid is a small, mottled brown insect,
about three to four millimetres long.
Adults feed with their head down and their body raised at an angle of about forty-five degrees, almost like a thorn on the leaf.
The yellow-orange nymphs live only on young, tender flush, where they produce white, waxy threads,
and they cause new leaves to curl. Besides citrus, the psyllid breeds on a few relatives,
especially orange jasmine, Murraya paniculata, a common hedge plant.
Because nymphs develop only on new flush, the flush is where you scout.
During each flush, inspect tender shoots for eggs, nymphs and waxy threads, and tap branches over a white board or tray to count adults.
Yellow sticky traps also catch adults and show when numbers rise.
The first pillar is to plant only healthy trees.
Buy from certified nurseries that produce trees from tested budwood under insect-proof screening.
A tree infected in the nursery brings the disease straight into a new orchard,
and it may not show symptoms for a long time.
Never bring in budwood, plants or cut branches of citrus or orange jasmine from areas with HLB, and follow the quarantine rules of your country.
Avoid planting orange jasmine near citrus, because it supports psyllid populations.
The second pillar is psyllid control, and it works best when it is coordinated across a whole area, so that neighbouring groves spray in the same window and psyllids cannot simply move from one grove to the next.
Time sprays to the flush cycles, when psyllids breed, and consider a spray in the cool season, when adults are not breeding, to reduce the population before the spring flush.
Insecticides used against the psyllid come from several IRAC groups, for example the neonicotinoids in group four A, often applied to the soil around young trees;
the pyrethroids in group three A and organophosphates in group one B for adults; and the diamides in group twenty-eight, the tetramic acids in group twenty-three, and abamectin in group six for nymphs and adults. Horticultural mineral oils also suppress psyllids.
Rotate groups, because psyllid populations have become resistant to some of them, and follow the label and local registrations.
Natural enemies help too. A tiny parasitic wasp, Tamarixia radiata, has been released in several countries and parasitises psyllid nymphs. Lady beetles, lacewings and spiders also feed on them, so avoid unnecessary broad-spectrum sprays outside the planned windows.
The third pillar is removing infected trees.
An infected tree is a source of bacteria for every psyllid that feeds on it. Where HLB is still rare, surveying regularly and removing confirmed infected trees quickly, root and all, slows the spread a great deal,
and replanting with clean trees keeps the orchard productive.
Where the disease is already widespread, many growers try to keep infected trees producing for longer with careful irrigation,
balanced and frequent fertiliser, including micronutrients, and good root health. In some places, regulated treatments such as antibiotic trunk injections are permitted. These measures can slow decline, but none of them cures the tree,
and the trees remain a source of the disease.
To recap.
HLB is caused by phloem-limited bacteria spread by citrus psyllids and infected budwood. It has no cure, and trees can carry it for a long time before symptoms show. Look for blotchy mottle, a yellow pattern that does not match on the two sides of the midrib, along with yellow shoots, small upright leaves, and lopsided, bitter fruit that stays green at the bottom.
Confirm with a laboratory test. Then manage with three pillars: clean, certified trees;
area-wide psyllid control timed to the flush, with IRAC group rotation and natural enemies;
and prompt removal of infected trees.
For more on citrus and every other crop, visit laisha.ai. Thank you for watching.
