Huanglongbing (HLB), better known as citrus greening disease, is a bacterial disease carried by the Asian citrus pysllid (Diaphorina citri), which can quickly spread it through an entire grove. It can also be transmitted by grafting diseased budwood. It cannot be spread by any other organism, by contact with contaminated tools, or through the air. The disease can lie latent in the tree for up to two years before it shows any symptoms of infection. It has no known cure.
This disease has been known in China since the late 1800s. In the United States, HLB was first detected on pummelo leaves and fruit in September 2005, although the Asian citrus pysllid had been observed in the United States for nearly a decade before then. Since then, it has spread through most of southern Florida. As a whole, Florida accounts for 68% of the total orange crop of the United States, with a base value of over one billion dollars.
To protect as much of this crop as possible, current strategy is to contain and destroy infection. At present, 31 counties in Florida are under quarantine, with all infected trees to be destroyed. All host plants of the citrus pysllid must be treated prior to moving them out of quarantined areas, and they may not be moved into any American territory which has a citrus industry. Specifically, these protected regions are Alabama, Arizona, California, Louisiana, and Texas, as well as the islands of Guam, American Samoa, Northern Mariana Islands, Hawai’i, Puerto Rico, and the U.S. Virgin Islands. Movement of all plants vulnerable to citrus greening disease is banned altogether.
However, this quarantine may be too little, too late. The Asian citrus pysllid has already been observed in 32 Texas counties, where it is believed to have arrived on mock orange landscaping ornamentals. It has also been found in the Jefferson and Orleans parishes of Louisiana, as well as in Guam, Hawai’i, and Puerto Rico. Although Texas citrus groves show no sign of disease as yet, citrus greening disease has been observed just across the border in Mexico. Given the two-year latency period, it is likely just a matter of time before citrus greening disease shows up in Louisiana and Texas.
Virtually all citrus plants are susceptible to citrus greening disease, although sour citrus fruits such as lemon and grapefruit are less susceptible than the sweet oranges. As well, several host plants for the citrus pysllid, such as mock orange (also known as orange jasmine), cannot be infected by citrus greening disease, although these plants can help spread the infection by supporting the movement of the citrus pysllid between vulnerable citrus trees.
Citrus greening disease is characterized by yellowed leaf veins followed by blotchy yellow leaf mottling, which gives it its Chinese name of ‘yellow dragon disease.’ This mottling may appear on a single shoot or on the entire tree, and is sometimes misdiagnosed initially as zinc or manganese deficiency. New growth will have small, narrow leaves and short stems, giving the plant a bunched look. Stems begin to die back, and feeder roots decay. The flowering cycle is disrupted, and most flowers will fall off without bearing fruit. Younger trees may never bear fruit. Any fruit that does grow is misshapen and has a pale peel which remains at least partly green, especially near the stem. The pulp of the fruit tastes bitter and medicinal, making it useless even for juicing. In time, the tree will die.
