By Vincent Lazaneo.
Huanglongbing (HLB), also known as citrus greening, is a devastating bacterial disease of citrus worldwide. The pathogen is primarily spread from tree to tree by a small, sap feeding insect, the Asian citrus psyllid (ACP). Every variety of citrus is susceptible to HLB. There is no cure for citrus with the disease. Infected trees decline and die, usually within five years.
ACP and HLB were initially detected in the United States in Florida – ACP in 1998 and HLB in 2005. Closer to home, ACP spread from Mexico into California and it was initially detected in San Diego County in 2008. The insect is now established throughout southern California and has spread as far north as the San Francisco bay area and Sacramento. In California, HLB was initially detected in Los Angeles County in 2012, on a single citrus tree in a residential area of Hacienda Heights. A second infected citrus was found in the same area in August 2015. In San Gabriel, about 10 miles from the initial find, several citrus with HLB were detected in July 2015 and more diseased trees were detected in the same area in 2016. In January 2017, a single citrus with HLB was found in Cerritos near the border of Orange County. In Mexico, HLB was detected in early 2016 in Ensenada and in October 2016 in Mexicali, only 23 miles from the U.S. border. HLB could spread into San Diego County any time, but it may not be detected here for a while since it takes up to two years after infection with the bacteria before a tree exhibits symptoms of HLB.
An early symptom of HLB in citrus is the yellowing of leaves on an individual limb or in one sector of a tree’s canopy. Leaves that turn yellow from HLB show an asymmetrical pattern of blotchy yellowing or mottling of the leaf with patches of green on one side and yellow on the other side. Citrus leaves may also yellow due to a deficiency of zinc or other micronutrients, however, the pattern of this discoloration typically occurs symmetrically between or along leaf veins. It only takes a few months before ACP can spread the disease from a newly infected tree. Monitoring citrus for ACP and keeping its population under control can help slow the spread of HLB.
The best time to check citrus for ACP is when trees produce new growth. The psyllid’s tiny eggs are laid in the folds of tender new leaf flush. They hatch into nymphs that are wingless, flattened, and yellow or orangish to brownish in color and only 1/100” to 1/14” long (yes, you need a magnifying lens to see them). The nymphs feed on sap and discharge a large quantity of sugary liquid which is excreted through a waxy tubule that is curly with a bulb at the end. Our Columnists This structure is unique to the ACP. Nymphs molt four times as they grow, then become adults which are brownish, winged insects about the size of an aphid. The adult psyllid is about 1/8” long with a pointed front end, red eyes, and short antennae. It feeds with its head down almost touching the leaf and the rest of its body is raised at about a 45° angle with its tail end in the air.
Agricultural officials do not have the resources to monitor every citrus tree in the state. They need your help. Learn to recognize the symptoms of HLB, know how to check your citrus for ACP, and learn what to do if the insect is present. See the UC IPM Pest Note Asian Citrus Psyllid and Huanglongbing Disease. If you suspect a citrus tree has HLB, call the CDFA exotic pest hotline at 1-800-491-1899 to have the tree checked. Visit californiacitrusthreat.org to learn more.
Vincent Lazaneo is UC Urban Horticulture Advisor Emeritus. He has a master’s degree in horticulture and a teaching credential in vocational agriculture from UC Davis. In 1983, Vince began the Master Gardener program in San Diego. Vince frequently contributes to the San Diego Union-Tribune and other publications and he enjoys growing specialty plants in his home garden, reading, hiking, and fishing.