Citrus Greening Disease: How to Spot, Stop, and Prevent HLB

For anyone who loves citrus, the thought of a disease that can wipe out entire groves is truly terrifying. This is the reality of citrus greening disease, a formidable foe that has devastated citrus industries around the world, leaving behind a trail of dead trees and lost livelihoods. It’s a silent killer that works from the inside out, making early detection and prevention absolutely critical. This guide is here to arm you with the knowledge you need to identify, manage, and ultimately protect your citrus trees from this catastrophic disease.

What is citrus greening disease & why is it so destructive?

Citrus greening, known scientifically as Huanglongbing (HLB), is one of the most destructive diseases of citrus on the planet. It is not caused by a fungus or a virus, but by a species of bacteria called Candidatus Liberibacter that colonizes the tree’s vascular system (the phloem).

Once inside, the bacteria act like a clot, blocking the flow of nutrients throughout the tree. This starves the roots and leaves, leading to a slow, inevitable decline. The reason it’s so devastating is simple: there is no cure. Once a tree is infected, it will stop producing usable fruit and will eventually die. Its relentless spread has crippled citrus production in major growing regions, posing a severe threat to the global supply of oranges, lemons, grapefruit, and other citrus fruits.

How does citrus greening spread?

The disease does not spread through wind or rain on its own. It relies on a carrier, known as a vector, to move from tree to tree. Understanding how it travels is key to stopping it.

• The main vector: The primary culprit for spreading citrus greening is a tiny, gnat-sized insect called the Asian citrus psyllid (Diaphorina citri). These insects have piercing-sucking mouthparts that they use to feed on the sap of citrus leaves and stems.
• The transmission process: When a healthy psyllid feeds on an infected tree, it picks up the HLB bacteria. The psyllid then flies to a healthy tree to feed, injecting the bacteria into its vascular system and starting a new infection. The psyllid remains infectious for the rest of its life, turning it into a tiny, mobile disease-spreading machine.
• Infected plant material: The disease can also be spread over long distances through the movement of infected citrus plants or grafts. This is why it is absolutely critical to only purchase citrus trees from certified, disease-free nurseries.

Citrus HLB (caused by CLas) can infect nearly all citrus varieties and their relatives. The disease is spread by the Asian citrus psyllid (ACP), which can use any citrus species as a host. After a latent period of several months, infected trees begin showing mild to severe symptoms.

What are the key symptoms of citrus greening in trees and fruit?

One of the most insidious things about HLB is that a tree can be infected for months or even years before showing obvious symptoms. When they do appear, they are often mistaken for nutrient deficiencies. Here’s what to look for:

• Asymmetrical blotchy mottling: This is the most reliable visual symptom. The leaves will show a blotchy, irregular yellowing pattern that is not symmetrical and often crosses the veins. In contrast, a nutrient deficiency usually causes a symmetrical pattern on both sides of the leaf’s central vein.
• Yellow shoots: The tree may produce yellow shoots and twigs that stand out against the green canopy.
• Fruit deformity: Infected trees produce fruit that is small, lopsided, and fails to color properly, often remaining green at the bottom (hence the name “citrus greening”).
• Poor fruit quality: The juice from affected fruit tastes bitter and salty, making it completely unusable. The fruit also tends to drop from the tree prematurely.
• Overall decline: As the disease progresses, the tree will show thinning canopy, twig dieback, and eventually, whole-tree death.

In India, Huanglongbing symptoms include:

(a) Yellow shoots – Nagpur mandarin, Maharashtra
(b) Blotchy mottle – Khasi mandarin, Arunachal Pradesh
(c) Blotchy mottle – Sathgudi sweet orange, Andhra Pradesh
(d) Blotchy mottle – C. indica, Meghalaya
(e) Blotchy mottle – Mosambi, Maharashtra
(f) Blotchy mottle – Gol nimbu, Assam
(g) Blotchy mottle – Nagpur mandarin, Maharashtra
(h) Blotchy mottle – Assam lemon, Assam
(i) Blotchy mottle – Lisbon lemon, Nagaland
(j) Stylar-end greening – Kinnow, Punjab
(k) Stylar-end greening – Valencia, Maharashtra
(l) Lopsided, blotchy fruit – Sathgudi, Andhra Pradesh
(m) Aborted seeds – Nagpur mandarin, Maharashtra
(n) Lopsided pink-fleshed Pomelo with poor coloration – Gujarat

Which environmental conditions favor the disease?

The spread of citrus greening is heavily influenced by the environment, as both the psyllid vector and the bacteria have preferred conditions. The disease is most active and spreads fastest in warm, humid climates.

Research has shown that disease transmission can occur between 16°C and 33°C (61°F to 91°F), with the peak for transmission occurring around 25°C (77°F). This is why regions like Florida, Brazil, and Southeast Asia have been hit so hard. The Asian citrus psyllid also thrives in these warm conditions, allowing its populations to build up and spread the disease more efficiently.

How can you manage and prevent citrus greening?

Management strategy	Action steps
Control the psyllid	Implement a rigorous insecticide program targeting the Asian citrus psyllid. This can include foliar sprays, systemic soil drenches, and trunk injections applied by certified professionals. Organically, this involves releasing natural predators like the parasitic wasp Tamarixia radiata, ladybugs, and lacewings, and using organic-approved pesticides like neem oil.
Remove infected trees	Once a tree is confirmed to have HLB, it must be removed immediately to eliminate it as a source of bacteria for psyllids to pick up and spread to other healthy trees.
Plant certified trees	Only purchase citrus trees from reputable, certified nurseries that sell disease-free stock. This prevents you from unknowingly introducing the disease to your property.
Use protective covers	For young, newly planted trees, use protective mesh covers to physically block psyllids from feeding on them while they are most vulnerable.
Nutritional support	While it won’t cure the disease, providing infected trees with enhanced nutrition programs can help support their health and may extend their productive lifespan for a short time.

What is the economic impact of citrus greening on growers?

The economic toll of citrus greening has been catastrophic, particularly for the Florida citrus industry, which has been the epicenter of the U.S. outbreak. Between the 2006-2007 and 2010-2011 seasons alone, HLB was estimated to have reduced the value of Florida’s citrus output by $4.51 billion.

Overall, the disease has reduced Florida’s citrus production by 75% and has more than doubled the cost of production for growers, who must now spend heavily on insecticides and nutritional programs.

The effects are threefold: increased tree death, reduced marketable fruit per tree, and soaring production costs. This has forced many multi-generational family farms out of business and has fundamentally reshaped the entire industry.

Conclusion

In the end, fighting citrus greening disease is one of the most significant agricultural challenges of our time, requiring immense dedication from growers and researchers. It’s a battle fought with vigilance and science. 

For home growers, early detection is your best weapon, and the Planteyes app can be an invaluable partner in this effort, helping you identify potential symptoms from a simple photo. The in-app chat also connects you with experts for personalized advice, giving you confidence and a clear path forward. Download it today and join the fight to protect our beloved citrus trees.

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