Scientists create a defense plan against citrus greening threat
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 its name.
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, 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.
Continued >> 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. U.S. port personnel intercepted 170 Asian citrus psyllids 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 and 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.
See the Web site and publication at http://citrusent.cekern.ucanr.edu/asian_citrus_psyllidmain.htm
Latest research findings presented at UC Exotic/Invasive Pests and Diseases Research Workshop
Attendees heard about the newest research results on exotic invaders at the fifth annual UC Exotic/Invasive Pests and Diseases Research Program (EPDRP) Workshop, Oct. 3, at UC Riverside.
From the glassy-winged sharpshooter that can cause Pierce’s disease of grapes and oleander leaf scorch, to fire ants that have infested residential areas in southern California, scientists funded by UC EPDRP described their research to prevent, detect, and control or eradicate these and other invaders.
Launched in 2001 and administered by the UC IPM Program, EPDRP funds projects to address exotic pests and diseases, and invasive species in agricultural, urban, and natural environments. The pests include insects, mites, molluscs, nematodes, bacteria, fungi, viruses and other microorganisms, vertebrates, and weeds.
Since its inception, the USDA-supported project has funded more than 100 studies, allocating nearly $9 million. The EPDRP is a collaboration between the UC Riverside Center for Invasive Species Research and the UC IPM Program, with funding from United States Department of Agriculture—Cooperative State Research, Education, and Extension Service.
What’s up, Doc? Maybe less air pollution
Who would guess that changing pest management practices in carrots could
reduce ozone in the San Joaquin Valley? That’s because fumigation
is used to control nematodes and diseases that commonly plague carrots.
However, to further reduce ozone, the California Department of Pesticide Regulation (DPR) plans to impose stricter rules on the use of soil fumigants. Soil fumigants are polluting gases that account for about one-half of all pesticides applied on crops in the San Joaquin Valley. DPR has also asked manufacturers to begin reformulating more than 700 insecticides, herbicides, and other pest-killing chemicals to reduce their emissions.
Because of fumigant use, carrots are one of the leading application sites for pesticides that give off VOCs. This is because fumigants are used to protect carrots from highly damaging diseases and nematodes.
Root knot nematodes are major pathogens of vegetables throughout the United States and world, impacting both the quantity and quality of marketable yields. In addition, root knot nematodes interact with other plant pathogens, resulting in increased damage caused by other diseases.
Nematode resistant carrots
Phil Roberts, in the Nematology Department at UC Riverside, and Joe Nunez, farm advisor for UC Cooperative Extension in Bakersfield, are trying to reduce VOCs from fumigant use, and provide cheaper and more reliable pest management, by using root knot nematode-resistant carrots as an alternative to fumigation.
Root knot nematodes attack a wide range of plants, including many common vegetables, fruit trees, and ornamentals. They are difficult to control and can be spread easily from field to field in soil and plant parts. That’s why fumigation is used to control them.
During ongoing work under a long-term breeding and genetics project funded by the California Fresh Carrot Advisory Board, Roberts and carrot breeder, Phil Simon, USDA at the University of Wisconsin, identified excellent root knot nematode resistance in carrot germplasm, together with gene markers that are useful to breeders for advanced selection.
The resistance is effective against the common root knot species found in San Joaquin Valley and southern desert valley carrot fields, and it has been bred into high-quality fresh carrot breeder lines. These lines have been made available to commercial carrot seed companies in recent years. The seed companies are in the latter stages of incorporating the resistance into elite carrot varieties for commercial release.
In a complementary project funded by the UC IPM Program, Roberts and Nunez tested these nematode-resistant carrots on two trial sites in commercial carrot fields in Kern County last summer. Both sites had high levels of root knot nematode at planting. Fumigation reduced the amount of taproot forking and galling caused by nematode infection in each field. Similar trials in additional carrot fields are being conducted in 2006.
“The resistant carrots had significantly lower levels of nematode-induced carrot damage compared with the susceptible lines,” says Roberts. “We are now evaluating how nontarget soil pests and diseases might impact nematode-resistant carrots grown without soil fumigation and are assessing yield performance.”
The identity of the nematode species and its damage potential under controlled conditions is being assessed in greenhouse tests. All plots were monitored for occurrence of other pathogens and growth constraints including weeds that may be present under nonfumigation conditions. No secondary pathogens or weeds occurred in fumigated or nonfumigated plots in both fields.
“In the long run, the hope is that through the use of resistant varieties of carrots, along with cultural practices and applications of less-toxic materials, the need for soil fumigation to control nematodes may be decreased,” says Roberts.
Another research project funded by the UC IPM Program is tackling cavity spot in carrots. Mike Davis, plant pathologist for UC Davis, is also working with Joe Nunez to develop a model to assess the risk of cavity spot, a key disease targeted by fumigation.
Cavity spot is characterized by elliptical to irregularly shaped, depressed lesions oriented across the mature carrot taproots. Infections occur anywhere along the taproot, but lesions tend to be more abundant on the upper third of the root and are often found where lateral roots emerge from the taproot. Lesions begin as pinpoint, sunken spots and generally enlarge as roots mature.
“Almost all of the carrot acreage in California receives multiple applications of the fungicide Ridomil-Gold, but this material has lost efficacy in some fields,” says Davis. “Isolates of the fungus that are not responsive to the fungicide have been identified, and breakdown of Ridomil is enhanced to ineffective levels in fields with a long history of its use. If we can find a way to predict cavity spot based on indirect measurement of cavity spot buds or spores in the soil, we could limit pesticide usage in those fields with the highest risk.
“We’re also trying to develop a method for extracting fungal DNA directly from soil and combine this with detection of fungus-specific DNA sequences by the polymerase chain reaction. PCR is a common method of creating copies of specific fragments of DNA. It rapidly amplifies a single DNA molecule into many billions of molecules. We will then need to correlate DNA levels in the soil with the severity of cavity spot actually experienced in the field.” A model for doing this is from Queensland, Australia, where soil samples collected prior to planting were used to determine the levels of pathogens in the fields and correlate this with yield losses, addressing the routine use of fumigation for tomatoes.By a combination of these strategies, it should be possible to make carrot production less expensive and more reliable, all while making the air a bit easier to breathe.
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