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Stopping a Killer Before It Starts At first sight, it looks like a small tick. Yet this dark-colored aphid has the potential to cause considerable damage to, if not wipe out, the entire citrus industry in Arizona. The brown citrus aphid (BCA) is a vector of the citrus tristeza virus (CTV), which, depending on its strain, can kill a citrus tree in one to two years. (“Tristeza” is a Spanish word meaning sadness.) Millions of citrus trees have died in Brazil, Spain and Argentina, resulting from the BCA invasion, since the 1930s. Its ability to efficiently transmit CTV from one tree to another makes the aphid’s potential to destroy the Arizona citrus industry very real, according to Zhongguo Xiong, an assistant professor in the Department of Plant Pathology. He is working with other University of Arizona College of Agriculture faculty and personnel from the Arizona Department of Agriculture, hurriedly testing thousands of citrus trees to detect the amount and strains of the tristeza virus that may already be here. In particular, Xiong is studying the effectiveness of several tests to detect the incidence of CTV in Arizona’s citrus. In November 1995, Florida citrus officials found nearly 40 colonies of the brown citrus aphid in fruit fly traps during routine inspections. The discovery of the aphid came nearly five years earlier than most of the Florida and even the Arizona citrus industry had suspected. Thus Xiong believes that it will not be long before the aphid makes its way into Arizona. “When BCA was first introduced from South Africa into Argentina and Brazil,” Xiong states in a written article, “the severe strains of CTV transmitted by BCA nearly wiped out the entire citrus industry.” The citrus industry in those two countries took years to rebuild, according to Xiong and other scientists. With the recent discovery of the aphid in Florida, concern for the BCA’s arrival to Arizona continues to grow. “The tristeza virus is a very serious disease,” Xiong says. “Since the virus was detected, it has changed the nature of the citrus industry. The brown citrus aphid is an efficient vector of this virus, meaning that the insect can spread the disease more effectively than aphids that are already common to citrus.” Researchers know that the BCA’s potential as a vector can produce explosive results. “This pest has a tremendous ability to transmit the tristeza virus,” says Mark Wilcox, agriculture agent with the Yuma County Cooperative Extension office. “The virus is particularly destructive to citrus grown on sour orange rootstock, but other rootstocks are also susceptible to varying degrees. CTV is the single most important economic disease of citrus. Its host range is limited to citrus and citrus relatives.” “The severe strain can kill trees on sour orange rootstock within one to two years,” he states. “The mild strain can affect the yield of fruit from a tree and tree growth. The severe strain also results in stem-pitting on grapefruit and sweet orange trees where the virus destroys the vascular system which is the blood vessels of the tree.” Often, CTV is called the HIV/AIDS of the citrus industry. Given the right conditions, much like the HIV/AIDS virus, BCA can transmit the citrus tristeza virus from an infected tree to a non-infected tree. One assay test Xiong is using to diagnose CTV is the enzyme-linked immunosorbent assay test, commonly called ELISA. The methods involved in using the ELISA test is similar to tests used to find the HIV antibody in human blood samples. “We put ground-up samples of citrus leafs on a microtitre plate,” Xiong states. “We then mix the sample with antibodies that will recognize the viral protein from the leaf samples and bind to it. The antibodies are also linked to an enzyme that can turn a colorless chemical to a yellowish color.” It is this color that reveals the presence of CTV. With antibodies that recognize different CTV strains, researchers can determine the strain of tristeza found in that particular tree. “Traditional diagnosis of CTV relies on a plant indicator assay. It takes a long time for the indicator plant to show any signs of the virus in it,” Xiong says. “This is one of the primary methods, though, being used for the Budwood Certification program in Arizona. By grafting budwood from a citrus tree to a series of indicator plants, the virus will usually produce visible signs of itself within three to six months. With the ELISA method, the diagnosis can be completed within twenty-four hours.” According to Xiong, the ELISA test can be used to help identify the distribution of mild and severe strains of CTV found in Arizona. Xiong is also interested in other tests that can be used to detect which particular strain of CTV can be found in the tree. The Immuno Dotblot test produces results like the ELISA test but by a different method. The ELISA test relies on ground up samples of the tree’s leaf. With the Immuno Dotblot test, surveyors retrieve a small sample of the leaf by a simple procedure. They slash a leaf petiole and press the fresh cut surface to nitrocellulose, which is likened to a thin paper. “We have to slash the leaf petiole to obtain saps from the leaf. Once the saps are transferred to the nitrocellulose, they can be processed in laboratories,” Xiong explains. “In this test, antibodies that recognize CTV are linked to an enzyme which emits light under certain conditions. When we put an x-ray film on top of the paper-thin nitrocellulose, the light generated by the enzyme produces some dark spots in the x-ray film. The dark spots indicate the presence of CTV in particular samples. This test is easy because samples can be collected and processed in the field, which allows us to process a larger number of samples.” Monoclonal antibodies, which are essential to determining if CTV is in a citrus tree, can differentiate between severe and mild strains. Xiong says that other antibodies can also detect viral proteins within a citrus leaf. All of this detection work is needed to identify sources of the virus and eliminate them. Once the disease enters a tree, nothing can be done for it. “Once a citrus tree has been infected with the tristeza virus, there is no way to cure the disease from the tree,” he states. “You either live with it or destroy the tree.” The tristeza virus has been found in several citrus trees in Arizona within the last year. “Although we have found severe strains of the virus in Arizona, we are not sure how widely spread the tristeza virus is,” Xiong says. “It will probably be the urban trees that have the most severe strain in them.” Xiong says there is a need to identify the source of the virus, whether it is from a grove or backyard, in order to take the appropriate action before the brown citrus aphid arrives. Article Written by Crystal Renfrow, ECAT, College
of Agriculture Researcher:
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The
concern is not over which strain the aphid will carry with it from
Florida, but how much of the local virus the aphid can spread when
it comes to Arizona. There are currently two strains of the virus
found in Arizona, primarily from Meyer lemon trees. One is severe,
the other is mild. Many Arizona citrus trees have scions and rootstocks
that are quite susceptible to the severe strain of CTV. Tristeza occurs
in many forms, known as strains or isolates: there are approximately
150 known strains of the virus recorded at USDA/ARS Laboratories,
according to Xiong.