Citrus disease threatens orange crops

California homeowners can help stave off exotic citrus disease

Thursday, August 27, 2009 8:03 PM CDT

California citrus growers and scientists, gravely concerned by confirmation this week that the Asian citrus psyllid is making its way northward, are asking all Californians to be on the look out for signs of the pest on backyard, apartment complex, community garden and public park citrus trees.

Helping authorities put the brakes on Asian citrus psyllid is in the best interest of the entire state. The psyllid imperils California's $1.1 billion citrus industry because it can transmit Huanglongbing (HLB, also known as citrus greening disease), considered the world's most devastating citrus disease. Trees with the disease are stunted, sparsely foliated, and may bloom off-season. There is twig dieback, leaf and fruit drop. The fruit that remains becomes bitter, lopsided, small and hard. Once a tree is infected with HLB, it cannot be saved. To date, none of the psyllids captured in Southern California was carrying the bacteria that cause HLB.

Asian citrus psyllid was first detected in California in early 2008 in San Diego and Imperial counties. The pest is believed to have been introduced from northern Mexico. The adult psyllids recently found in Orange and Los Angeles counties were in traps hanging in backyard trees. The University of California, California Department of Food and Agriculture, USDA and the citrus industry have worked closely to quarantine the affected areas and try to eradicate the pest.

Californians can help by monitoring for the Asian citrus psyllid. According to UC citrus entomologist Beth Grafton-Cardwell, the insect's presence isn't too hard to spot during the late summer and fall citrus flush.

Pin-head-sized psyllid adults, mottled brown in color, can live anywhere on the tree, typically on the underside of leaves. But the earlier stages - known as nymphs - congregate on the light green, tender new citrus leaves and shoots. These tiny, bright yellowish-orange flightless insects leave behind tell-tale signs of their presence. While feeding, the nymphs extract large amounts of plant sap. They excrete the sap as honeydew, which can turn leaves and fruit black from sooty mold, or they make white waxy tubules that look like bits of instant noodles. The leaves become twisted and curled and, in severe cases, the shoots die back.

"If homeowners take a close look at the new growth on their citrus trees, if there is an Asian citrus psyllid infestation, they will be able to see signs with the naked eye," Grafton-Cardwell said. "But it's easier to see with a magnifying glass."

A suspected Asian citrus psyllid infestation should be reported immediately to the county agricultural commissioner or the CDFA hotline at (800) 491-1899.

Asian citrus psyllid is established in parts of Florida, Georgia, South Carolina, Louisiana, Texas, Hawaii and Mexico. It also occurs in Belize, Brazil, Cuba, the Caribbean, China, India, Afghanistan, Saudi Arabia and other parts of the world. The pest is typically introduced into new areas when it is carried by people on fruit or plant material.

"People in California can help us fight Asian citrus psyllid, and prevent the introduction of other pests, by not bringing citrus trees or other plants or fruit into the state from other parts of the United States or other countries," Grafton-Cardwell said.

In Florida, the Asian citrus psyllid was found before symptoms of HLB were observed. Authorities believe numerous illegally imported citrus trees infected with HLB were already growing in Florida backyards. The disease went unnoticed for years because there was no insect to spread it from tree to tree. However, in 1998, Asian citrus psyllid made its way to the Sunshine State. In short order, the pest and the disease swept into all of Florida's 31 citrus-growing counties. Currently, HLB is killing 10 percent of Florida trees every year.

The potential for very rapid spread of HLB isn't the only thing the California industry has learned from the Florida experience. When trees started dying, Florida didn't have enough disease-free citrus stock available to replace them.

In California, all new trees are produced by nurseries with disease-free budwood from the UC Riverside Citrus Clonal Protection Program, which maintains a block of disease-free trees at the UC Lindcove Research and Extension Center in Exeter. (Budwood is small citrus cuttings that nurseries use to propagate new trees.) For decades, the parent trees were maintained in a field at Lindcove, but the increasing threat of another citrus disease, citrus tristeza virus, prompted UC to move true-to-type, disease-free trees into a screenhouse in the late 1990s to protect the trees from aphids, which spread tristeza. Three years ago, as tristeza virus pressure intensified, plans were made to expand the screenhouse, a move especially fortuitous now that another pest is threatening California citrus trees.

The original screenhouse houses 500 potted and 100 in-ground trees representing 379 commercially important varieties. More than 200 of these citrus varieties are registered with the CDFA for budwood distribution.

The expansion, which is under construction, will hold an additional 250 in-ground trees, according to Georgios Vidalakis, director of UC Riverside Citrus Clonal Protection Program.

"To keep out Asian citrus psyllid, the new screenhouse will have finer mesh than the old one and the existing screenhouse will be rescreened," Vidalakis said.

For more ANR news, visit http://news.ucanr.org