Citrus greening
Class: Bacteria
Common Name
Citrus greening, greening, Huanglongbing (HLB), and citrus vein phloem degeneration (CVPD)
Causal agent
Candidatus Liberibacter spp.
Potential Hosts
Citrus trees
Who am I?
Citrus greening is a bacterial disease, exclusive on the citrus genus. The bacteria is vectored and transmitted by Asian citrus psyllid (Diaphorina citri) and African citrus psyllid (Trioza erytreae).
Citrus greening can kill trees, which severely reduces production. The bacteria is spread from tree-to-tree by psyllid vectors. Nothing can be done once a tree is infected; there is no cure for citrus greening.
Symptoms
Affected Plant Parts
Citrus green symptoms vary depending on the citrus species and the environmental conditions. The symptoms may initially appear on a single shoot. The most common symptoms to appear in early stages are blotchy, mottled leaves. Leaf mottling, asymmetric chlorosis, and yellowing of veins and midribs will likely show up as the disease progresses. Infected trees will produce fewer, smaller, bitter-tasting fruits that oftentimes remain green in color. In later stages of the disease, symptoms on leaves might resemble nutritional deficiencies and may be misdiagnosed. Another symptom seen with citrus greening is vine corking. Though, vine corking could also be due to a citrus tristeza virus infection.
Asian Psyllid
The Asian citrus psyllid is a tiny, white-brown, mottled insect that is about the size of an aphid. The adult psyllid feeds with its head down, almost touching the leaf, and the rest of its body is raised from the surface at an almost 45-degree angle with its tail in the air. No other citrus pests position their bodies like this while feeding. Adults typically live 1-2 months. Females lay tiny yellow-orange, almond-shaped eggs in the folds of developing citrus leaves. Each female can lay several hundred eggs during her lifespan. Psyllid produce white waxy tubules around their rear end. Stages of the psyllid can live anywhere on the tree, but the immature stages and eggs will be found on young leaves. With time, psyllid presence on trees give rise to the formation of sooty mold.
Control measures
Citrus trees are magnificent; they produce large amounts of tasty fruits while staying green all year round (evergreen). This makes them popular in areas outside farming such as private gardens and backyards.
Community responsibility
To help slow the spread of citrus greening and buy time for researchers to find a cure, the general public is being asked to take an active part. Citrus owners should check their trees regularly for the presence of the Asian citrus psyllid. Look for the symptoms described above and become involved by reporting contaminated places.
Fighting the Vector
Eliminate the Asian citrus psyllid before it has a chance to infect more trees. Make it a routine to monitor your orchard, backyard, and even your neighborhood by searching citrus trees for the presence of the Asian citrus psyllid.
Don’t use products based on the same active ingredient in consecutive treatments. Use of insecticides belonging to different groups to prevent pathogens from developing resistance to a specific chemical.
The following is a list of generic names for insecticides, sorted by groups (with respect for their mode of action), that are used in one or more parts of the world:
Group 1: Imidacloprid, acetamiprid, and thiamethoxam; Group 2: Abamectin (in combination with thiamethoxam); Group 3: Dimethoate; Group 4: Fenpropathrin; Group 5: Diflubenzuron;
Organic: Sprayable products containing one or more of the following ingredients: azadirachtin, neem oil, pyrethrins, and potassium salt of fatty acids.
Important notes
No naturally immune citrus cultivars have been identified
Citrus greening disease is best controlled through an integrated strategy involving the use of healthy planting material, removal of infected trees and branches, and controlling the Asian citrus psyllid
Keep in mind that when dealing with organic, biologic, and to some extent a small number of conventional chemical products a complete eradication of the pest or disease will often require several iterations of a specific treatment or a combination of treatments
Did you know ?
In some places, small parasitic wasps have been imported from overseas to reduce the number of Asian citrus psyllid. These are natural enemies that are very specific to the Asian citrus psyllid. Small parasitic wasps are extremely small, and the females lay their eggs inside the psyllid. Their eggs hatch and newborns feed on the guts of the psyllid, eventually killing it. Although, it won't be enough to fully control of the Asian citrus psyllid alone.
Caution and careful notice should be taken when using any plant protection products (insecticides, fungicides, and herbicides). It is the grower’s sole responsibility to keep track of the legal uses and permissions with respect to the laws in their country and destination markets. Always read the instructions written on labels, and in a case of contradiction, work in accordance to the product label. Keep in mind that information written on the label usually applies to local markets. Pest control products intended for organic farming are generally considered to be less effective in comparison to conventional products. When dealing with organic, biologic, and to some extent a small number of conventional chemical products, a complete eradication of a pest or disease will often require several iterations of a specific treatment or combination of treatments.