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Citrus Viruses and Virus-Like Diseases1 Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Circular 1131 December 1993 Citrus Viruses and Virus-Like Diseases1 J.J. Ferguson and S.M. Garnsey2 IMPORTANCE sheath of protein and an inner core of nucleic acid, the genetic messenger material found in all living Virus and virus like diseases have a major impact organisms. Plant viruses differ in shape and size but on citrus production world-wide and often become the are generally either threadlike or spherical. They dominant yield-limiting factor in grove management. range in size from 1/12,000 to 1/1,250,000 inch and The most obvious impact of these diseases is their are considerably smaller than other plant pathogens. lethal effect on specific hosts. Tristeza alone has Although viruses are agents of disease, as are destroyed approximately 50 million trees in the past bacteria, fungi and nematodes, and can transmit 40-50 years. A less obvious but economically genetic information, they cannot grow or reproduce important effect is the variety of non-lethal, subtle by themselves outside the host plant as can most responses that reduce tree size, tree vigor and fruit living organisms. Viruses can replicate only inside yield. In many cases, growers suffer significant losses suitable host cells by converting the reproductive from virus diseases without recognizing the nature of machinery of the host cell to reproduce the these diseases. Finally, citrus virus and virus-like components of the virus. Viroids are even smaller and diseases may restrict the grower’s choice of scions and contain only nucleic acid without any protein coat. rootstocks that can be grown profitably in certain areas. For example, tristeza-sensitive cultivars must Hosts often be avoided in areas where severe forms of the virus are prevalent, even though all other factors A multitude of living organisms, from would predict profitable use of these cultivars. single-celled plants or animals to large trees and mammals, can be attacked by viruses. Some viruses NATURE OF VIRUSES attack man and other animals and cause diseases such as influenza, polio, rabies, smallpox and warts. Other Structure viruses cause plant galls, flower break, stunting and decline. Still other viruses attack microorganisms such Viruses are extremely small, infectious particles. as bacteria and fungi. Over a thousand viruses are Plant viruses are composed of an outer protective known and more than half of these viruses cause plant diseases. 1. This document is Circular 1131, a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.Publication date: December 1993. 2. J.J. Ferguson, Associate Professor, Citrus Specialist, Department of Fruit Crops, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611; S.M. Garnsey, Citrus Virologist, USDA, Agricultural Research Service, 2120 Camden Road, Orlando, FL 32803. The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Citrus Viruses and Virus-Like Diseases Page 2 Symptoms spread mainly by propagation such as much of the psorosis complex, exocortis and xyloporosis. Plant viruses can cause a variety of disease symptoms, most of which result from systemic Insects are significant vectors of several important infection (infection spread throughout the plant). virus and virus-like pathogens of citrus. Although Stunting and reduction in yield are probably the most tristeza is spread by several species of aphids, the common effects of viral diseases of plants but specific most effective aphid vector of tristeza, Toxoptera foliage, fruit, wood and bark symptoms also occur. citricida, the brown citrus aphid, has not yet occurred in Florida, but has been recently discovered These symptoms include mosaics, yellowing and throughout the Caribbean. Three major virus-like ringspots. A mosaic pattern consists of light-green, diseases (stubborn, greening and citrus variegated yellow or white areas mixed with the normal green chlorosis) are caused by specialized types of bacteria color of leaves. Depending on the intensity of and are spread by several different leafhoppers and 2 infection or the particular pattern of discoloration, species of citrus psyllids (insects the size of mosaics can be described as mottling, streaking, vein planthoppers). clearing, vein banding, chlorosis and spotting. Yellowing refers to a uniform discoloration or IDENTIFICATION AND DETECTION chlorosis of foliage without any spotting patterns. Ringspot is a distinctive pattern of a brown, The classic means of identifying virus and virus- red-brown or yellow border surrounding a green like infections in citrus is based on identification of center. symptoms in the field or by inoculating citrus indicator plants (herbaceous and woody plants that Transmission are sensitive to a particular virus). Herbaceous indicator plants have been used 1) to identify some Plant viruses can be spread in various ways, mechanically transmitted viruses and 2) to detect including propagation (budding and grafting), unsuspected latent infections (infection without the contaminated plant sap, seed or pollen, insects, mites, development of visible symptoms) not revealed by nematodes, certain fungi and parasitic plants tests using citrus indicator plants. Reliable herbaceous (dodder). The principal means of transmission of indicator plants have been found for exocortis, but citrus viruses are infected propagative material, not for tristeza, xyloporosis and most forms of insects and contaminated tools. psorosis. The primary purpose of budwood certification Although improvement in the use of citrus programs has been to prevent the spread of viruses by indicator plants has been made, plant procedures still infected budwood and contaminated seed. Despite require extensive periods of time, good plant material various quarantines against the movement of and facilities, and skilled personnel. Interpretation of potentially diseased plant material, including results for mild strains (virus strains that cause only budwood, man has frequently moved citrus viruses mild symptoms) and for mixed infections (infection of long distances and established them in new a plant by more than one virus at the same time) may production areas. Seed transmission of psorosis has depend on temperature, plant vigor and the been observed in Troyer citrange, Carrizo citrange experience of the observer. and trifoliate orange. No other citrus viruses are known to be transmitted through seed. Pollen Considerable progress has been made in transmission of citrus viruses has not been developing serological detection procedures for citrus demonstrated. viruses. Serology refers to the use of a portion of animal blood, the serum, in procedures that are used Inoculation of healthy plants with sap from to identify viruses. The first step in the serology infected plants (mechanical transmission) is important procedure is to inoculate animals, usually rabbits or in the field spread of pathogens, such as citrus mice, with purified virus. The virus acts as an antigen exocortis viroid which can be easily spread as a or foreign substance that stimulates the immune contaminant on pruning and cutting tools. The use of system of the animal to produce specific antibodies virus-free budwood and budwood certification that react only with the virus injected. These programs are highly effective for controlling viruses antibodies are collected from the serum portion of the animal’s blood and are used in various laboratory Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Citrus Viruses and Virus-Like Diseases Page 3 tests to identify viruses. One such assay referred to as of seedling yellows is that it indicates the presence of ELISA (Enzyme Linked Immunosorbant Assay), has severe strains of tristeza. been used around the world to test millions of citrus trees for tristeza. Serology procedures are available Symptoms for three citrus viruses which occur in Florida (tristeza, citrus variegation, and citrus leaf rugose Decline viruses). Field diagnosis of tristeza can be based on CONTROL observation of the following symptoms: 1) tristeza decline, associated primarily with trees on sour orange Control strategies for plant viruses include 1) or Citrus macrophylla rootstocks, 2) affected trees lack avoidance or exclusion of the disease, 2) the use of feeder roots and do not maintain normal amounts of certified budwood, 3) eradication or suppression, 4) starch in roots, 3) declining trees frequently, but not the
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