Huanglongbing

Are the bottoms of your oranges staying green? If so, your tree (and citrus growers everywhere) are in big trouble.

Huanglongbing on mandarin oranges (T.R. Gottwald and S.M. Garnsey - USDA APHIS) Public Domain

Huanglongbing (HLB), or citrus greening, is a vascular disease of citrus trees that makes the fruit inedible.

The HLB story

In 1998, Florida citrus growers saw Asian (Diaphorina citri) and African citrus psyllids (Trioza erytreae), or two-spotted citrus psyllids, for the first time. Insects the size of aphids, these citrus psyllids can carry a bacteria (Liberibacter spp.) fatal to citrus trees. 

American scientists didn't identify the disease until 2005. [China's first case occurred in 1943.] By 2014, HLB had caused $4.6 billion in damage to Florida's citrus crops. As of 2023, HLB has spread to 33 countries, plus California and Texas.

HLB: tree killer

HLB can kill a tree within five years of infection. Because this disease threatens the entire citrus industry, trees infected with HLB must be destroyed and removed by professional arborists. [You may be able to get assistance from your local Dept. of Agriculture on this.] To do your bit to save the world's oranges, lemons, limes, and grapefruits, start by learning what HLB looks like.

Symptoms of HLB

The obvious symptom of huanglongbing is the fruit remains green on the bottom. By this time, the disease may have spread far and wide. Watching for earlier signs of HLB can prevent that spread.

The first symptom of huanglongbing is yellowing veins. Neighboring tissues may also turn yellow. After that, leaf mottling, early leaf drop, and twig dieback occur. If you scrape away the soil, you will also see the feeder and lateral roots decomposing.

Other signs of HLB include:

• Tiny yellow eggs tucked into new leaf growth
• Deformed or blotchy new leaf growth
• New shoots look burned
• Thin, white waxy tubes seen on new leaf growth
• Stunting
• Flowers may appear at the wrong time of year and then fall off
• Yellow shoots
• Misshapen internal structure
• Corky veins
• Small, hard fruit that tastes bitter

HLB may be mistaken for a nutrient imbalance. But nutrient deficiencies create symmetrical symptoms, while HLB symptoms are asymmetrical.

Preventing the spread of HLB

Scientists are looking at oak leaf sprays, antibiotic and peptide injections, and other treatments, but there isn't a cure yet. So prevention is critical. 

At the government level, quarantines are in place that prohibit moving citrus fruit trees, leaves, and stems out of affected areas. Contact your county officials to see if you are in a quarantine zone. Looking at the maps below, you can see how a quarantine zone can change in just one year. Since the information to create these maps takes time to assemble, you should assume if you live near a quarantine zone, you should act like you are in it.

2015 ACP quarantine map (SCC CA)

2016 ACP quarantine map (SCC CA)

Managing psyllids is one way to slow the spread of HLB. Psyllids are jumping plant lice. They feed on sap. You can control psyllid populations with sticky sheets and insecticidal soap. Do not use dish soap.

Like other psyllids, Asian citrus psyllids produce honeydew. Honeydew attracts ants and provides a growth medium for sooty mold. If you see ant trails around citrus trees, take a closer look. And block ant traffic with sticky barriers.

Asian citrus psyllid (David Hall, ARS) Public Domain

You can also help prevent the spread of this disease with these tips:

• Do not accept tree cuttings from uncertified sources. Infected grafts can spread HLB.
• If requested, allow the State Dept. of Food and Agriculture to hang a trap on your property.
• Do not transport leaves or stems out of quarantine areas.
• Wash fruit thoroughly before removing it from a quarantine area.

​While commercial growers use pesticides to control HLB disease vectors, backyard growers are less likely to do so, creating a significant threat to citrus trees everywhere.

This threat is not to be taken lightly.

If you suspect or see signs of the Asian citrus psyllid or Huanglongbing disease, immediately contact the CALIFORNIA DEPARTMENT OF FOOD AND AGRICULTURE HOTLINE: 1‐800‐491‐1899.

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UPDATE: 3/5/2016
Bloomberg reports that Florida citrus growers are now using dogs trained to sniff out the disease to help with early detection and to reduce the spread of this disease.

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UPDATE: 4/1/2018
According to Farm Week Now, half of all Florida citrus growers have failed because of this threat. They predicted that the 2018 Florida citrus crop would be one-fourth of what it was ten years ago.

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UPDATE: 3/24/2023
According to Progressive Farmer, the annual production of oranges has dropped from 200 million boxes to 20 million boxes due to huanglongbing. All citrus crops are the smallest on record since 1929-1930.