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Posted 17 July 2006. PMN Crop News.

Scientists Create a Defense Plan Against Citrus Greening

Citrus greening causes new foliage to twist or die.

University Of California, Davis.

Davis, California (June 21, 2006) Citrus greening is one of the most devastating diseases of citrus in the world, stunting trees and causing small, bitter fruit. With the recent discovery of citrus greening in Florida, an educational effort by a team of scientists to stop the Asian citrus psyllid from becoming established in California is especially timely.


The psyllid is an efficient carrier of the bacterium that causes the disease called citrus greening or "Huanglongbing" because the fruit develops a bitter taste and does not color properly, leading to the name greening. Entomologist Beth Grafton-Cardwell from University of California (UC) Riverside organized a team of researchers from the University of Florida and California Department of Food and Agriculture to develop a brochure (pdf), Web site, and slide presentation to educate California citrus growers, the ornamental nursery industry, and regulatory agency staff about Asian citrus psyllid and greening disease. The UC Exotic/Invasive Pests and Diseases Research Program funded the project.

The Asian citrus psyllid was first discovered in Florida in 1998 and now infests most of the citrus growing regions in Florida. The educational program is an effort to keep the pest from becoming established in California. There were 170 interceptions of Asian citrus psyllid at U.S. ports on plant material from Asia during 1985 to 2003.

In 2001, the psyllid was accidentally introduced into the Rio Grande Valley of Texas on potted nursery stock from Florida. The psyllid could invade California at any time, with most likely sources of infestation being imports from Florida, Mexico, Hawaii, or Asia.

The psyllids are small, brownish insects that usually feed on the underside of leaves. They feed with their heads down almost touching the surface of the leaf and, because of the shape of their heads; their bodies are lifted at about a 45-degree angle. Adults can live for one to two months. A single psyllid nymph feeding for less than 24 hours on a citrus leaf causes permanent malformation of the leaf. Adults survive over the winter and collect on newly forming citrus leaf buds where they feed and mate.

The disease, Huanglongbing, is found throughout Asia, the Indian subcontinent and neighboring islands, the Saudi Arabian peninsula, and in the Sao Paulo State of Brazil. In 2005, Huanglongbing was discovered in backyard citrus in southern Florida. "This discovery greatly increases the risk of the disease making its way into California. The citrus greening bacterium is transmitted by Asian citrus psyllid, grafting, and possibly by citrus seed," says Grafton-Cardwell.

Symptoms of citrus greening include yellow shoots and mottling and yellowing of leaves due to lack of the green pigment chlorophyll. Infected trees are stunted, sparsely foliated, and may bloom off-season. In addition, there is twig dieback, leaf and fruit drop, production of small, lopsided, hard fruit, and small, dark aborted seeds.

In areas of the world where citrus greening occurs, citrus production is greatly reduced. The disease could arrive in infected citrus trees or budwood or in infected psyllids. The best prevention for the disease is to use only certified budwood in commercial and homeowner plantings, and for people to follow quarantine rules and not import citrus.

"The psyllids are most likely to arrive on citrus or closely related plants such as orange Jessamine, a common ornamental in Florida," says Grafton-Cardwell. "The best prevention of establishment of the psyllid is to prevent movement of these favored hosts from infested areas into California and to inspect all plant material that arrives from out of state."

The research team members are: Kris Godfrey, California Department of Food and Agriculture; Michael Rogers and Carl Childers, University of Florida, Citrus Research and Education Center; and Philip Stansly, University of Florida, Southwest Florida Research and Education Center.

The UC Exotic/Invasive Pests and Diseases Research Program funded this project. The program targets research on exotic pests and diseases in California. Its aim is not only to improve our knowledge and management of pests that are already here, but also to reduce the potential impact of those pests and diseases that pose a threat to the state. The program is a collaboration between the UC Statewide Integrated Pest Management Program and the UC Riverside Center for Invasive Species Research. U.S. Department of Agriculture, Cooperative State Research Education and Extension Service, funds the program.


Beth Grafton-Cardwell
Entomologist University of California Cooperative Extension
(559) 646-6591