Citrus Viruses and Virus-Like Diseases 1

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1 Circular 1131 December 1993 Citrus Viruses and Virus-Like Diseases 1 J.J. Ferguson and S.M. Garnsey 2 IMPORTANCE Virus and virus like diseases have a major impact on citrus production world-wide and often become the dominant yield-limiting factor in grove management. The most obvious impact of these diseases is their lethal effect on specific hosts. Tristeza alone has destroyed approximately 50 million trees in the past years. A less obvious but economically important effect is the variety of non-lethal, subtle responses that reduce tree size, tree vigor and fruit yield. In many cases, growers suffer significant losses from virus diseases without recognizing the nature of these diseases. Finally, citrus virus and virus-like diseases may restrict the grower s choice of scions and rootstocks that can be grown profitably in certain areas. For example, tristeza-sensitive cultivars must often be avoided in areas where severe forms of the virus are prevalent, even though all other factors would predict profitable use of these cultivars. NATURE OF VIRUSES Structure Viruses are extremely small, infectious particles. Plant viruses are composed of an outer protective sheath of protein and an inner core of nucleic acid, the genetic messenger material found in all living organisms. Plant viruses differ in shape and size but are generally either threadlike or spherical. They range in size from 1/12,000 to 1/1,250,000 inch and are considerably smaller than other plant pathogens. Although viruses are agents of disease, as are bacteria, fungi and nematodes, and can transmit genetic information, they cannot grow or reproduce by themselves outside the host plant as can most living organisms. Viruses can replicate only inside suitable host cells by converting the reproductive machinery of the host cell to reproduce the components of the virus. Viroids are even smaller and contain only nucleic acid without any protein coat. Hosts A multitude of living organisms, from single-celled plants or animals to large trees and mammals, can be attacked by viruses. Some viruses attack man and other animals and cause diseases such as influenza, polio, rabies, smallpox and warts. Other viruses cause plant galls, flower break, stunting and decline. Still other viruses attack microorganisms such as bacteria and fungi. Over a thousand viruses are 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 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 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

2 Citrus Viruses and Virus-Like Diseases Page 2 Symptoms Plant viruses can cause a variety of disease symptoms, most of which result from systemic infection (infection spread throughout the plant). Stunting and reduction in yield are probably the most common effects of viral diseases of plants but specific foliage, fruit, wood and bark symptoms also occur. These symptoms include mosaics, yellowing and ringspots. A mosaic pattern consists of light-green, yellow or white areas mixed with the normal green color of leaves. Depending on the intensity of infection or the particular pattern of discoloration, mosaics can be described as mottling, streaking, vein clearing, vein banding, chlorosis and spotting. Yellowing refers to a uniform discoloration or chlorosis of foliage without any spotting patterns. Ringspot is a distinctive pattern of a brown, red-brown or yellow border surrounding a green center. Transmission Plant viruses can be spread in various ways, including propagation (budding and grafting), contaminated plant sap, seed or pollen, insects, mites, nematodes, certain fungi and parasitic plants (dodder). The principal means of transmission of citrus viruses are infected propagative material, insects and contaminated tools. The primary purpose of budwood certification programs has been to prevent the spread of viruses by infected budwood and contaminated seed. Despite various quarantines against the movement of potentially diseased plant material, including budwood, man has frequently moved citrus viruses long distances and established them in new production areas. Seed transmission of psorosis has been observed in Troyer citrange, Carrizo citrange and trifoliate orange. No other citrus viruses are known to be transmitted through seed. Pollen transmission of citrus viruses has not been demonstrated. Inoculation of healthy plants with sap from infected plants (mechanical transmission) is important in the field spread of pathogens, such as citrus exocortis viroid which can be easily spread as a contaminant on pruning and cutting tools. The use of virus-free budwood and budwood certification programs are highly effective for controlling viruses spread mainly by propagation such as much of the psorosis complex, exocortis and xyloporosis. Insects are significant vectors of several important virus and virus-like pathogens of citrus. Although tristeza is spread by several species of aphids, the most effective aphid vector of tristeza, Toxoptera citricida, the brown citrus aphid, has not yet occurred in Florida, but has been recently discovered throughout the Caribbean. Three major virus-like diseases (stubborn, greening and citrus variegated chlorosis) are caused by specialized types of bacteria and are spread by several different leafhoppers and 2 species of citrus psyllids (insects the size of planthoppers). IDENTIFICATION AND DETECTION The classic means of identifying virus and viruslike infections in citrus is based on identification of symptoms in the field or by inoculating citrus indicator plants (herbaceous and woody plants that are sensitive to a particular virus). Herbaceous indicator plants have been used 1) to identify some mechanically transmitted viruses and 2) to detect unsuspected latent infections (infection without the development of visible symptoms) not revealed by tests using citrus indicator plants. Reliable herbaceous indicator plants have been found for exocortis, but not for tristeza, xyloporosis and most forms of psorosis. Although improvement in the use of citrus indicator plants has been made, plant procedures still require extensive periods of time, good plant material and facilities, and skilled personnel. Interpretation of results for mild strains (virus strains that cause only mild symptoms) and for mixed infections (infection of a plant by more than one virus at the same time) may depend on temperature, plant vigor and the experience of the observer. Considerable progress has been made in developing serological detection procedures for citrus viruses. Serology refers to the use of a portion of animal blood, the serum, in procedures that are used to identify viruses. The first step in the serology procedure is to inoculate animals, usually rabbits or mice, with purified virus. The virus acts as an antigen or foreign substance that stimulates the immune system of the animal to produce specific antibodies that react only with the virus injected. These antibodies are collected from the serum portion of the animal s blood and are used in various laboratory

3 Citrus Viruses and Virus-Like Diseases Page 3 tests to identify viruses. One such assay referred to as ELISA (Enzyme Linked Immunosorbant Assay), has been used around the world to test millions of citrus trees for tristeza. Serology procedures are available for three citrus viruses which occur in Florida (tristeza, citrus variegation, and citrus leaf rugose viruses). CONTROL Control strategies for plant viruses include 1) avoidance or exclusion of the disease, 2) the use of certified budwood, 3) eradication or suppression, 4) the use of tolerant or resistant cultivars, 5) cross protection, and 6) cultural practices that reduce infection. The success of each of these control strategies depends on a number of factors, including knowledge of the causal agent of the disease, symptoms and mode of dissemination. MAJOR VIRUSES AND VIRUS-LIKE DISEASES Significance Tristeza Citrus tristeza virus (CTV), the most destructive virus disease of citrus, has caused disastrous losses in many citrus-producing areas worldwide. "Tristeza," which means sadness or melancholy in Spanish, actually refers to several disease symptoms, many of which occur in Florida. Strains of tristeza exist which differ in the symptoms they produce. The tristeza symptom most familiar to Florida growers is a quick decline of orange, grapefruit and mandarin trees on sour orange rootstock. CTV can also cause stem pitting in limes, some sweet oranges, grapefruit and in some root stocks like Citrus macrophylla. Severe stem pitting, which can be very damaging, is present in major citrus growing areas such as Brazil, South Africa, Australia and Asia. Severe stem pitting due to tristeza is not present in Florida but would cause severe problems in the future if introduced. Severe strains of tristeza may cause seedling yellows, a leaf chlorosis and stunting of sour orange, lemon, grapefruit, and citron seedlings. The leaves on affected plants are small and yellow and growth usually ceases after the first few leaves are formed. Seedling yellows is an experimental reaction that occurs in laboratory indicator plants and is usually not encountered in the field. The significance of seedling yellows is that it indicates the presence of severe strains of tristeza. Symptoms Decline Field diagnosis of tristeza can be based on observation of the following symptoms: 1) tristeza decline, associated primarily with trees on sour orange or Citrus macrophylla rootstocks, 2) affected trees lack feeder roots and do not maintain normal amounts of starch in roots, 3) declining trees frequently, but not always, show honeycombing and thickening of the inner bark just below the bud union. Tree decline usually results from reduction of the feeder root system or necrosis of the phloem tissue near the bud union. Decline symptoms can be associated with a number of diseases including foot rot and root rot, nematode diseases like spreading decline and slow decline, blight, a number of viruses, and root weevil and water damage. In tristeza decline, the phloem (conducting tissue located inside the bark through which sugars produced in the leaves by photosynthesis are moved to roots) is affected, thus girdling the tree. This girdling is associated with leaf chlorosis and wilting, heavy fruit set, starch depletion in the root system and destruction of feeder roots. Without feeder roots the tree wilts, producing quick decline symptoms. Trees declining from tristeza usually have reduced starch levels in their root systems. Starch can be detected by dipping cut ends of roots in an iodine potassium iodide mixture. Roots turn blue-black when starch is abundant but the roots of a tristeza-affected sour orange tree usually do not stain. However, since declining trees affected by other diseases like blight often have low root starch levels, this procedure is not an infallible test for tristeza. Trees with "quick decline" may die rapidly, with the progression of symptoms from leaf wilting to tree death occurring within several weeks. Such trees may also fruit heavily because of starch accumulation above the budunion. A yellow or brown stain may occur on the inner face of the bark, just below the budunion. In Florida, trees commonly decline over a period of several years. Trees that decline slowly usually develop honeycombing symptoms (a massing of numerous small pinholes on the inside of the bark

4 Citrus Viruses and Virus-Like Diseases Page 4 just below the budunion). In addition, the inner bark of affected trees on sour orange rootstock often thickens just below the budunion. When budwood infected with severe isolates of citrus tristeza virus is propagated on sour orange rootstock, young trees grow weakly, often set fruit prematurely, are chlorotic and may remain stunted and unthrifty. Stem Pitting Stem pitting, a condition associated with tristeza and several other citrus virus diseases, consists of depressions in the outer wood with corresponding pegs or projections on the inner face of the bark. This pitting can be seen by peeling the bark off small twigs. Tristeza-induced stem pitting can vary according to virus strain and host. In severe cases of stem pitting, the trunk and scaffold limbs have a gnarled or rope like in appearance. Citrus macrophylla seedlings frequently show moderate stem pitting from Florida strains of tristeza. Severe strains of tristeza may cause stem pitting in sweet orange cultivars regardless of rootstock. Affected trees are usually stunted or have a more compact growth habit than normal trees. Sweet orange stem pitting strains of tristeza occur in Brazil, Peru, Japan and California. However, no severe stem pitting strains have been reported in Florida. Plants Affected With few exceptions, most citrus cultivars can become infected with citrus tristeza virus (CTV), but many (mandarins, sweet oranges, rough lemon and Rangpur lime) often do not show obvious symptoms). Others (lime, lemon, grapefruit, citron and Citrus macrophylla) show symptoms of varying intensity. Trifoliate orange and some trifoliate orange hybrids such as Carrizo citrange are resistant to tristeza infection. That is, the virus does not multiply in these plants even when they are grafted with CTV-infected scion material. Transmission Tristeza is spread in Florida by propagation and by aphids. By 1968, extensive natural spread of tristeza by aphids forced the Budwood Registration Program to abandon hopes of maintaining the Citrus Budwood Foundation Grove as virus-free. Valuable scion sources could not be maintained free of tristeza in the field and newly propagated nursery trees would become infected eventually in any case. A 1980 survey indicated that about 90% of the registered budwood sources of sweet orange propagated on sour orange were infected with tristeza. Many of these trees presumably carry mild strains, but since most nurseries have propagated their budwood sources on tristeza-tolerant rootstocks the severity or virulence of the strain cannot be determined from the condition of the tree from which the budwood was taken. Recent observations clearly show that some budwood source trees are, in fact, infected with severe strains that produce stunted trees on sour orange rootstock. Budwood from trees infected with severe strains produce vigorous, normal trees on tristeza-tolerant stocks. However, these trees only serve to increase the reservoir of severe tristeza strains that can be spread by aphids to adjacent plantings of trees on sour orange rootstock. Three species of aphids (Toxoptera aurantii, the black citrus aphid, Aphis gossypii, the melon aphid, and Aphis spiraecola, the spirea aphid) can vector tristeza in Florida. Fortunately, the most effective vector, Toxoptera citricida, the brown citrus aphid, is not yet present. Different strains of tristeza can be spread by different aphids at different rates. Aphid spread of citrus tristeza virus is also affected by environmental conditions and by the host itself. Some host cultivars apparently are better tristeza sources and some are better receptor plants than others. In general natural spread is more rapid in orange and grapefruit than in other citrus. Control Tristeza Resistant Rootstocks Several conditions affect control strategies for citrus tristeza virus (CTV) in Florida: 1) the virus is already widespread and most budwood sources are infected, 2) CTV is naturally spread by aphids from the large inoculum source already present. The grower is faced with the choice of either accepting the risks of tristeza and planting trees on sour orange rootstock or of choosing another rootstock with other inherent disease susceptibilities and horticultural characteristics.

5 Citrus Viruses and Virus-Like Diseases Page 5 Tristeza-tolerant or resistant rootstocks are available, and tristeza decline can be avoided by using rough lemon, Volkamer lemon, Rangpur lime, Cleopatra mandarin, Carrizo citrange, trifoliate orange, sweet orange or Swingle citrumelo (see Table 1). These alternatives generally have other drawbacks such as susceptibility to blight, foot rot or cold. New hybrid rootstocks are being developed continuously, but their value cannot be determined without years of field testing. For the moment, the grower must evaluate all risks carefully and choose the rootstock that poses the fewest risks for his own situation. The tristeza situation in Florida will not improve and growers should not use sour orange without considering the long term risks. If sour orange rootstock is used, it should not be propagated with budwood containing a severe isolate of CTV. At present this can best be done by ascertaining that the budwood source has previously performed well on sour orange and by observing that the young budlings grow off well in the nursery and in young plantings. Beware of budwood sources that perform markedly less well on sour orange than on Cleopatra mandarin or Carrizo citrange rootstock. Screening The Budwood Registration Program is screening some of the most heavily used registered scion trees and information on the specific citrus tristeza virus (CTV) content of individual source trees should soon be available. Since biological screening tests are slow and difficult to conduct on a large scale, means to rapidly screen trees for strain severity remains a high research priority. It is important to all growers that the reservoir of severe CTV isolates not be increased needlessly by propagation. Yet with the current production of over 14 million budded nursery trees per year, the potential is obvious. Growers who choose tristeza tolerant or resistant stocks are less likely to be concerned than those who choose sour orange. Quarantine efforts should be maintained and increased to prevent entry of more severe CTV strains from overseas and from other citrus producing areas and to prevent entry of the brown citrus aphid which is the most efficient vector. Smuggling of prize new horticultural selections from overseas is always a temptation and cannot be entirely prevented by regulations. Growers should therefore strongly support agricultural quarantines and inspections at ports of entry. Mild Strain Cross Protection Mild strain cross protection, whereby the mild strain is deliberately introduced into a tree and prevents the subsequent expression of severe strains which are inoculated later, has been proposed as a control strategy for tristeza. Healthy-looking trees on sour orange carrying mild strains of tristeza may, in fact, have a natural cross protection already. The protective ability of naturally occurring mild strains in Florida has not been proven although many tests are in progress. Preliminary experimental data suggest caution in that some mild isolates do not protect well against aphid reinfection with severe quick-decline isolates. Mild strain cross protection is used on a commercial basis in Brazil to protect against the stem pitting effects of citrus tristeza virus (CTV) in Pera orange grafted on CTV-tolerant rootstocks, primarily Rangpur lime. Only very limited field data for cross protection in trees on sour orange rootstock are available from Florida, California and Australia. Mild strain cross protection may eventually be used to help deter CTV decline effects on sour orange but it will not be a complete long term solution. Long Term Approaches A long term approach to tristeza control is to develop resistant scion cultivars and citrus tristeza virus (CTV) tolerant sour orange types. The resistance found in trifoliate orange can be transferred by hybridization and preliminary results suggest that development of CTV resistant scion cultivars with acceptable fruit quality is possible. Unfortunately, incorporating the needed horticultural properties will be more difficult than incorporating CTV resistance. Another approach is to reduce natural spread by aphid vectors, either by direct control of the vector at critical periods or by developing cultivars that are repellent to aphids.

6 Citrus Viruses and Virus-Like Diseases Page 6 Table 1. Susceptibility to Tristeza Decline. Rootstock Carrizo Citrange Citrus macrophylla Citrus volkameriana Cleopatra Mandarin Milam Rangpur Lime Rough Lemon Sour Orange Sweet Orange Swingle Citrumelo Trifoliate Orange Significance Psorosis Susceptibility more susceptible very susceptible Psorosis, a complex of diseases that is distributed worldwide, can affect most citrus cultivars. The causal agent of psorosis has not yet been identified, but it is assumed to be a virus or a group of viruses. The forms of psorosis that occur in Florida are psorosis A, psorosis B, blind pocket and concave gum. There is some evidence that the psorosis B strain may be identical to some strains of citrus ringspot virus. Psorosis had been well controlled by the Budwood Registration Program but can still be found in older plantings in many areas. Recently, interest in psorosis B or ringspot has increased because natural spread occurs by unknown means in Argentina, Texas, and, in one case, in Florida. Tree decline from natural psorosis infections is currently a major problem in Argentina and is regarded as a potential threat in Florida and Texas. Leaf and Fruit Symptoms All forms of psorosis can cause flecking symptoms in young leaves but these symptoms normally are seen only in early spring in the field and even then only on some leaves. Leaf symptoms characteristic of psorosis A consist of a flecking pattern arranged in faint bands 1/4-1 mm wide (1/101-1/25 inch) along the leaf veins. Leaf flecking arranged in an oak leaf pattern centered on the midrib is associated with concave gum form of psorosis. Leaf patterns fade as leaves mature and usually are not visible in fully mature leaves. Psorosis B leaf symptoms consist of flecking and chlorotic blotch patterns, often with some ringspot patterns that can persist in mature leaves. Fruit may show partial or complete rings of sunken tissue on the rind. The undersurface of mature leaves may also show brown swellings that resemble greasy spot. Since meticulous inspection is required to find leaf symptoms even on known infected trees, leaf flecking is generally not considered to be a good field diagnostic symptom. Bark Symptoms In Florida, psorosis A bark scaling usually develops approximately 6 years after the appearance of leaf symptoms. Scaling begins in isolated areas of the trunk or main limbs and consists of the separation from the bark of small, dry irregular flakes mm (1/127-1/84 inch) thick. Scaling involves more and more of the bark until the affected area encircles the entire trunk, severely affecting tree health, yield and susceptibility to freeze injury. Scales remain attached at the edge of the lesion and curl outward like a shingle. Conducting tissue just inside the bark becomes impregnated with brown gum. When lesions become advanced, normal transfer of nutrients is impeded by the gum formation and the portion of the tree above the lesion declines. The curling of the bark to produce an awning-like effect and the exudation of minor quantities of gum help to distinguish psorosis A from scaling induced by other causes. In psorosis B, scaling and wood symptoms are similar to psorosis A except that they develop more extensively. Blind pocket psorosis is characterized by steep-sided, narrow, channel-like parallel depressions running lengthwise in the trunk. A form known as eruptive blind pocket psorosis is further characterized by scaling of the bark and the flow of gum. Neither type of psorosis occurs frequently in Florida, but they are mentioned here because blind pocket psorosis is sometimes confused with folds that normally occur in cultivars such as tangerine, tangelo and trifoliate orange. Concave gum psorosis causes somewhat similar symptoms, but the depressions are less severe. Limbs may be flattened somewhat to create an "ax-handle" effect. In contrast to normal depressions,

7 Citrus Viruses and Virus-Like Diseases Page 7 gum impregnations are found in the wood with concave gum and blind pocket forms of psorosis. Plants Affected Most cultivars develop leaf symptoms, but bark scaling and tree decline occur primarily on sweet orange, grapefruit and some mandarins. Even cultivars which do not express distinct symptoms generally can be infected by the virus. Budunion effects have not been described, but different reactions can be seen readily in some scion-rootstock combinations (see Table 2). For example, when sweet orange scions are grafted on sour orange rootstock, bark scaling is confined to sweet orange. Transmission All strains of psorosis are transmitted by infected budwood. No vectors are known but they are suspected in some locations. Experimental, mechanical transmission of some strains has been demonstrated, but mechanical spread in the field has not been reported. Seed transmission of psorosis occurs in Carrizo and Troyer citranges and in trifoliate orange. Table 2. Susceptibility to Psorosis. Control Florida growers who purchase registered trees should have no psorosis problems. The observation of natural spread of a form of psorosis (ringspot) in Argentina and, to a lesser extent, in Texas, is triggering some additional research on psorosis. At least one case of natural spread has been observed experimentally in Florida but it apparently remains uncommon. Growers who are replanting in areas heavily infested with psorosis should watch for evidence of natural infection in young trees and contact their Extension office immediately if symptoms are observed. Significance Exocortis Citrus exocortis was originally considered to be caused by a virus, but is now known to be caused by a viroid; a small, infectious, self-replicating nucleic acid molecule without a coat protein. Viroids are extremely small and, even when highly magnified with special techniques, are difficult to visualize with an electron microscope. Since the viroid molecule has a special configuration, it is stable to heat and to other treatments which inactivate viruses. Rootstock Carrizo Citrange Citrus macrophylla Citrus volkameriana Cleopatra Mandarin Milam Rangpur Lime Rough Lemon Sour Orange Sweet Orange Swingle Citrumelo Trifoliate Orange Susceptibility susceptible The primary economic effect of citrus exocortis viroid (CEV) is dwarfing of trees grafted on susceptible rootstock. Scaling of the rootstock bark may also occur. The effects are rarely lethal and the degree of stunting varies with the strain of CEV. Major damage from exocortis has not occurred in Florida because exocortis-free budwood sources were developed before exocortis-sensitive rootstocks were used extensively, and the possibilities for spread on contaminated tools were recognized. Strains of exocortis vary greatly in severity. Mild forms of exocortis have been used to produce dwarfed citrus trees for high-density plantings. While this approach to tree dwarfing is promising, it may have limited application in Florida where exocortis-sensitive stocks (trifoliate orange, Carrizo citrange, Rangpur lime and sweet lime) are also susceptible to blight.

8 Citrus Viruses and Virus-Like Diseases Page 8 Symptoms The most obvious field symptoms of exocortis are bark scaling on the rootstock and general stunting of the tree. Bark scaling usually begins when trees are from three to eight years old. Infection by severe strains becomes apparent more quickly than infection by mild ones. The first symptoms are shallow, longitudinal splits in the bark below the budunion. In time, splitting occurs over the entire rootstock and strips of bark begin to scale off. In contrast to some other diseases, the bark tissue underneath the scales remains alive. Severe strains of exocortis can stunt trees badly, especially those on trifoliate orange rootstock. Fruit quality is normally not affected. Yield reduction is generally associated with reduced canopy size. A rapid indexing method is available for exocortis which utilizes the viroid-stimulated leaf distortion and stunting of Etrog citron and several herbaceous plants. Another rapid indexing procedure which involves purifying citrus exocortic viroid from infected tissue and subjecting it to laboratory analysis has recently been developed. Plants Affected Trifoliate orange, Carrizo and Troyer citrange and Rangpur and sweet lime are important commercial rootstocks susceptible to scaling; however, cultivars which do not express distinct symptoms generally allow replication of the viroid. Budunion effects have not been described, but different host reactions can be seen readily in some scion-rootstock combinations. For example, when sweet orange scions are grafted on trifoliate orange rootstock bark scaling is confined to the trifoliate orange tissue of the trunk. Tangelos and grapefruit are less susceptible to mechanical inoculation than sweet orange. If a tree is large enough to hedge and top, then exocortis infection should not be a problem even to trees on sensitive rootstocks. Some eventual reduction in growth could occur if mature trees were infected but these trees should continue to be productive. Accidental contamination of budwood source trees, nursery trees or young tree plantings via budding or pruning equipment can be a very serious problem and must be avoided. Growers should purchase trees propagated from exocortis-free budwood and should protect young trees against contamination. For example, Carrizo citrange has been frequently used as a replant rootstock in mature grove sites and can easily be contaminated during normal grove operations. Disease Control Since exocortis can be spread through propagation of infected budwood and via contaminated budding or pruning tools, budwood certification coupled with decontamination of cutting tools constitutes the primary control strategy. Two disinfectants that have been found effective against the viroid are an aqueous solution containing 2% sodium hydroxide plus 2% formalin or a freshly made 5-10% solution of common household bleach. Transmission Exocortis can be spread by budding and grafting and as a contaminant on pruning equipment. The Budwood Registration Program has largely eliminated the spread of exocortis through the use of registered nursery stock, but exocortis-infected citrus is still produced from nonregistered sources in Florida. Trees with tolerant scion-rootstock combinations will not show any obvious symptoms (see Table 3). Hedging equipment can probably spread the viroid, but the extent of the spread depends upon the number of trees that are infected, the cultivars that are infected and the cultivars that will be inoculated.

9 Citrus Viruses and Virus-Like Diseases Page 9 Table 3. Susceptibility to Exocortis Viroid. Rootstock Carrizo Citrange Citrus macrophylla Citrus volkameriana Cleopatra Mandarin Milam Rangpur Lime Rough Lemon Sour Orange Sweet Orange Swingle Citrumelo Trifoliate Orange Significance Susceptibility more susceptible more susceptible very susceptible Xyloporosis/Cachexia Xyloporosis/Cachexia has generally been considered a virus disease but recent evidence suggests it is caused by a viroid. Xyloporosis is common in old-line trees and is a continuing problem where growers have top worked old-line trees with cultivars such as mandarin hybrids that are sensitive to xyloporosis. Xyloporosis-free budwood for nearly all cultivars is available, but in an attempt to cut costs, growers sometimes pay dearly for using unregistered budwood. Symptoms Xyloporosis was originally described in sweet lime and cachexia was described in Orlando tangelo, but the diseases are apparently identical. Fovea is a similar disease in Murcotts. Xyloporosis can cause stunting, chlorotic foliage and decline symptoms that can be similar to those caused by other agents. The definitive symptoms of xyloporosis are stem pitting and gum impregnation of the bark in sensitive cultivars. Pitting and gum development usually begin near the budunion, but may spread to the xyloporosis-sensitive portion of the tree. The pits and corresponding pegs are not as angular as those found in tristeza-affected trees and may result in a generally bumpy surface on both wood and bark faces. Pegs on the bark face usually contain gum. Once these symptoms become well developed, a flat cut about halfway into the bark will reveal gum pockets of varying size. Symptoms usually take months to develop in highly susceptible cultivars such as Orlando tangelo and Citrus macrophylla and longer in less sensitive cultivars. Satsumas may not show symptoms for 8 years or more. Indexing is usually done by inoculating an indicator side graft of Parson s Special mandarin on a vigorous rootstock such as rough lemon, or by grafting the test plant on Orlando tangelo or Ellendale mandarin seedlings. Plants Affected Xyloporosis can affect most tangelos, some mandarins, many mandarin hybrids, sweet lime and Citrus macrophylla. Rough lemon may also show mild symptoms. It is especially severe on Citrus macrophylla and Orlando tangelo. Oranges, grapefruit, lemons, acid limes, some mandarins, trifoliate orange and its hybrids are among those hosts which can be infected, but usually don t show obvious symptoms (see Table 4). Transmission Like exocortis, xyloporosis can be transmitted by infected budwood and contaminated cutting tools. A surplus of white grapefruit has led some growers to top work old-line trees with other cultivars that are in greater demand. Top working involves grafting or budding a new scion cultivar onto an existing rootstock-scion combination. The new scion material is set in the main scaffold branches after the tree has been pruned severely or is set directly in the trunk following removal of the entire top of the tree. When mandarin hybrids are propagated this way, they can become infected by the xyloporosis pathogen that was present but not expressed in the white grapefruit scion. Conversely, if Orlando tangelos are top worked, they may also become infected unless virus-free budwood is used for top working. Control Control of xyloporosis can be achieved by using registered budwood and by decontamination of cutting tools. Two disinfectants that have been found effective are an aqueous solution containing 2% sodium hydroxide plus 2% formalin or a freshly made 5-10% solution of household bleach. Top-working sensitive cultivars should be avoided when xyloporosis could be present in the rootstock or scion.

10 Citrus Viruses and Virus-Like Diseases Page 10 Table 4. Susceptibility to Xyloporosis. Rootstock Carrizo Citrange Citrus macropyylla Citrus volkameriana Cleopatra Mandarin Milam Rangpur Lime Rough Lemon Sour Orange Sweet Orange Swingle Citrumelo Trifoliate Orange Susceptibility very susceptible susceptible susceptible MINOR VIRUS DISEASES The current absence of extensive natural spread and the use of disease free registered nursery stock currently seems effective in containing the following diseases. Several of these diseases could be extremely damaging if they become widespread. Growers should be aware of these diseases and their destructive potential. Citrus Variegation Virus Citrus variegation virus (CVV) has been found in all major citrus producing regions, including Florida. Once considered a form of psorosis, CVV and the closely related crinkly leaf virus are now known to be distinct pathogens. CVV can be readily transmitted mechanically to non-citrus hosts. Several different isolates of CVV have been found in different locations in Florida. CVV can produce variegated leaf symptoms in citrons, lemons and grapefruit, and milder symptoms in many other hosts, including sweet orange. Striking leaf and fruit symptoms often appear in grapefruit after a severe winter, but the trees may subsequently grow normally with few visible symptoms. Normally, CVV can be detected in Etrog citron indicator plants while indexing for exocortis, but a recently discovered mild isolate produced very weak symptoms in this indicator plant. Fortunately, good serological techniques are available and the Enzyme Linked ImmunoSorbant Assay procedure can be used for rapid detection of CVV. Use of registered budwood should prevent problems with CVV in new plantings. Citrus Leaf Rugose Virus Citrus leaf rugose virus (CLRV) is related to citrus variegation virus. CLRV was discovered by chance in an old variety planting near Eustis, Florida and is thought to be spread naturally, possibly through pollen. Subsequent indexing showed it to be present in a number of cultivars, including early introductions from California and several seedlings trees. Serological techniques are now available for the detection of CLRV. CLRV causes minor leaf flecking in lemons, some puckering in leaves of Mexican lime and a severe stunting in young grapefruit seedlings. It infects a large number of other cultivars without producing obvious symptoms. This disease is not yet widespread and has not been found in registered budwood sources. As with citrus variegation virus, the best control strategy for CLRV is to avoid the disease by using only registered budwood sources. Citrus Ringspot Virus The spread of citrus ringspot virus (CRSV) presents a classic illustration of the importation and spread of a virus disease on infected nursery stock. CRSV was first isolated in Florida from a young Star Ruby grapefruit tree that had been smuggled from Texas into Florida. Budwood cut from infected trees was used in a commercial nursery, further spreading the disease. Although some natural spread of ringspot has occurred in Texas, there is no good evidence of natural spread in Florida. The Texas strain of CRSV and subsequently discovered Florida strains produce similar symptoms but the viruses have not yet been fully characterized. The relationship of the naturally spread form of psorosis in Argentina to CRSV strains in Florida also needs clarification. The persistent observations of natural spread of this disease are ominous because CSRV strains appear capable of causing severe damage to mature trees. At present, the best control policy is to use registered budwood and to be vigilant against natural spread of the disease.

11 Citrus Viruses and Virus-Like Diseases Page 11 Strains of psorosis B commonly produce symptoms similar to those produced by CRSV. These psorosis strains are mechanically transmissible to herbaceous indicator plants, producing leaf lesions in these herbaceous plants and typical leaf ringspot symptoms in grapefruit seedlings. Numerous sources of the psorosis B strain have been found in old trees in Florida that exhibit bark-scaling symptoms. Tatterleaf-Citrange Stunt Virus In Japan and the United States, tatterleaf-citrange stunt virus (TL-CSV) has been recovered from citrus originally imported from China. Although it is of limited distribution and natural spread has not been reported in the US or Japan, it is of interest because of the devastating scion-rootstock incompatibility it causes in inoculated mandarin and sweet orange trees on trifoliate orange or citrange rootstocks. The virus is widespread in Meyer lemon trees in Florida. Meyer lemon as well as orange, mandarin and grapefruit can be symptomless carriers. The virus can be detected by graft inoculation to Rusk citrange or Citrus excelsa plants, or by mechanical inoculation to beans. The best control strategy is to reduce the number of TL-CSV infected trees. All new plantings of Meyer lemon should be from virus-free sources. Other cultivars should never be grafted on old-line Meyer lemon or on Meyer lemon grafted to other cultivars. Algerian Navel Orange Virus This virus was discovered in a navel orange cultivar imported many years ago from Algeria when that cultivar was screened for virus diseases using herbaceous indicator plants. This virus produces symptoms in several herbaceous indicator plants but it has not yet produced symptoms in any of the seedling citrus cultivars tested thus far. Host range tests on grafted trees have yet to be done. At this point there is no indication of a disease problem associated with this virus but its presence indicates that all the viruses affecting citrus have not yet been discovered. Some of these viruses may not become apparent until a sensitive host is infected.

12 Citrus Viruses and Virus-Like Diseases Page 12 Milam Stem Pitting In the mid-1970 s, a severe stem pitting condition was observed in some Milam lemon seed-source trees. Definite but less severe pitting was also observed in several mandarin hybrids, including Robinson, Lee and Page. The symptoms were very similar to those caused by tristeza, but tristeza was not found in all of the pitted trees. Milam trees with stem-pitting were top worked to different cultivars to determine if it would severely affect other cultivars. No striking symptoms occurred in any of the eight cultivars tested, including Valencia orange and Marsh grapefruit. Transmission tests have been inconclusive. Pitting has not been produced on small, container grown plants in the glasshouse or screenhouse. Pitting is generally confined to the trunk and major scaffold limbs on affected trees in the field and does not extend below ground on Milam seedlings. This pitting does not appear to affect tree vigor and has not yet become a problem. Growers, however, should be aware of this situation and propagate Milam, Page, Lee and Robinson from sources free of obvious stem-pitting.