Citrus Greening, also known as Huanglongbing (HLB), is a devastating bacterial disease that threatens citrus production worldwide. It is caused by a phloem-limited bacterium, most commonly Candidatus Liberibacter asiaticus, which attacks the tree’s vascular system, preventing the transport of sugars and nutrients. Once a citrus tree is infected, there is no effective cure to restore its health or productivity. A proactive, multi-faceted prevention strategy is the only viable means for protecting orchards from decline and eventual tree death.
Controlling the Vector: Managing the Asian Citrus Psyllid
The bacterium causing HLB is transmitted exclusively by the Asian Citrus Psyllid (ACP), a tiny, sap-sucking insect. Because the disease cannot spread without this vector, control strategies must focus intensely on suppressing psyllid populations. This is primarily achieved through a combination of chemical and biological controls managed under an Integrated Pest Management (IPM) program.
Chemical control relies on the strategic use of insecticides, requiring rotation of products with different Modes of Action (MOA) to prevent psyllid resistance. Systemic insecticides, such as neonicotinoids, are applied as soil drenches. They are absorbed by the roots and translocated throughout the tree, offering long-lasting protection against feeding nymphs. These treatments also act as anti-feedants, discouraging psyllids from feeding and transmitting the bacteria.
Foliar sprays, typically broad-spectrum insecticides like pyrethroids, provide a quick knockdown effect against highly mobile adult psyllids. The timing of these applications is important and must coincide with the production of new tender leaf growth, or “flush.” Psyllids only lay eggs on this new flush, and the resulting nymphs can only survive on this soft tissue for about 15 to 20 days before the leaves harden off.
Biological control integrates natural enemies of the psyllid into the orchard environment to provide sustained, long-term suppression. The parasitic wasp Tamarixia radiata is a widely used ectoparasitoid that lays its egg underneath the psyllid nymph. The larva then feeds on the nymph, killing it and leaving behind a distinct “mummy” shell.
The adult female Tamarixia wasp also engages in host feeding, killing additional psyllid nymphs to obtain protein for egg production. For biological control to be effective, growers must manage ant populations. Ants feed on the psyllid’s honeydew excretions and aggressively defend psyllid colonies from the predatory wasps.
Horticultural Exclusion and Sanitation Practices
Preventing the introduction and movement of the disease requires strict physical and cultural practices, focusing on clean plant material and the immediate removal of infection sources. The first step for any orchard is to plant only certified, disease-free nursery stock. These trees are typically grown in insect-proof screenhouses to guarantee they are free of HLB and psyllids upon planting.
For small orchards or high-value trees, physical exclusion can be extended to the field through the use of Citrus Under Protected Screen (CUPS) structures. These large, permanent screenhouses utilize a fine mesh, typically 40 to 50 threads per inch, to physically block the tiny psyllid from reaching the trees. This structural exclusion ensures that the trees remain disease-free throughout their productive lives.
Sanitation involves the immediate removal of infected host material, a process known as roguing, to eliminate the source of the bacterial inoculum in the local environment. If a tree tests positive for HLB, it must be removed completely, as the bacteria are systemic and cannot be pruned out. Removing the infected tree prevents psyllids from acquiring the bacteria and spreading it to neighboring healthy trees.
Any pruning or cutting tools used on a symptomatic or infected tree must be sterilized before being used on a healthy tree to prevent mechanical transmission. A bleach or isopropyl alcohol solution can be used to disinfect the blades between cuts or trees. Managing the surrounding environment also involves controlling alternative host plants in the Rutaceae family, such as Orange Jasmine or Indian Curry Leaf. These plants can harbor the psyllid when citrus trees are not producing new flush.
Systematic Monitoring and Symptom Recognition
Constant vigilance is necessary because the presence of the psyllid or the disease can go unnoticed in the early stages. Recognition of the Asian Citrus Psyllid relies on identifying its unique physical characteristics and behaviors. The adult insect is small, only about three millimeters long, and is most easily identified by its characteristic 45-degree feeding posture, where its head is lowered almost to the leaf surface while its body is raised.
When disturbed, the adult psyllid will quickly jump or fly a short distance, distinguishing it from other common citrus pests. Psyllid nymphs are yellowish-orange, move slowly, and are always found on new flush. They produce distinct, curled white waxy tubules that are excretions of honeydew, which is a tell-tale sign of a psyllid infestation.
Recognizing the visual symptoms of HLB is complicated because they can often mimic nutritional deficiencies. The most reliable diagnostic sign is asymmetrical leaf mottling, often called blotchy mottle, where the yellowing pattern is uneven across the leaf mid-vein. This contrasts with a zinc or iron deficiency, where the chlorosis is usually symmetrical on both sides of the leaf.
Other symptoms include the production of small, lopsided, or partially green fruit, often with a bitter taste, and general dieback of shoots. Scouting techniques involve visual inspection of new flush using a hand lens and tap sampling, where a branch is struck sharply above a white surface to dislodge adults. Any suspicious leaf or insect sample must be sent to a certified laboratory for Polymerase Chain Reaction (PCR) testing to confirm the presence of the Liberibacter bacteria.