We'd like to understand how you use our websites in order to improve them. Register your interest. Surveys of Impatiens and Verbena species in local nurseries in Fredericton, Canada and Verbena species in New Delhi, India showed widespread infection of Citrus exocortis viroid CEVd in vegetatively-propagated and seed-grown plants. To determine viroid seed transmission, samples of eight varieties of Impatiens and 11 varieties of Verbena were obtained from four commercial sources.
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Contents - Previous - Next. The exocortis disease of citrus, caused by the citrus exocortis viroid CEV , was first reported and described by Fawcett and Klotz as a bark-shelling disorder of trifoliate orange rootstock. Benton et al. They recommended that budwood for propagation be taken only from older trees on trifoliate rootstock showing no scaling of the butt or trunk. The disease is present in almost all citrus-growing regions of the world. Although many of the commercial citrus cultivars are symptomless carriers, trees may be stunted to some degree on rootstocks normally considered tolerant.
In countries where trifoliate orange is the primary rootstock, bud selection over many years has avoided CEV, but other citrus viroids may be present. Shell bark of lemon is sometimes confused with exocortis. Although CEV or a citrus viroid CV may be factors in enhancing shell bark symptoms, they may not be causal. Incidence of shell bark is greatly diminished by use of virus and viroid-free nucellar or shoot-tip grafted lemon bud selections.
Gummy pitting and gum pocket have been reported from Australia and South Africa and may be associated with certain citrus viroids.
Symptoms are gum-impregnated pits or pockets in the trifoliate rootstock, which can be seen on the wood surface if the bark is removed.
Gum impregnated areas range from few to numerous and from small to large Fraser and Broadbent, and are more prevalent in older trees. As stated above the causal organism of exocortis disease is the citrus exocortis viroid Semancik and Conejero-Tomas,; Semancik, It is a low-molecular-weight RNA consisting of nucleotides.
It can exist as either linear or circular molecules and is highly mechanically transmissible by tools from tree to tree. CEV is transmissible into Gynura aurantiaca, , petunia or tomato plants, and causes distinct and characteristic severe epinasty symptoms on leaves of these hosts. Nucleic acid extraction can be made from these hosts or from young symptomatic citron shoots and analysed by polyacrylamide gel electrophoresis PAGE. CEV will migrate in a band on the gel and can be visualized by staining with silver or ethidium bromides.
Recent studies indicate that there are a number of citrus viroids of a molecular weight lower than CEV that can also induce symptoms in citron. Some of these viroids induce mild bark cracking in trifoliate orange Figure 37 distinct from the severe bark shelling associated with CEV.
These citrus viroids should be considered as independently transmitted and distinct pathogens. Schlemmer, Roistacher and Semancik were the first to report that citron-variable viroid reacted only in citron but not in Gynura or other herbaceous hosts in which CEV reacts and multiplies.
Duran-Vila et al. These viroids induced specific mild reactions in citron, and were found in field trees of citrus in pure form or in various combinations. Currently there are some ten to 12 viroids in the CV complex, some of which appear closely related, based on nucleotide number and nucleic acid hybridization assays. However, they may induce different reactions in field trees.
For example, the cachexia viroid will migrate on a polyacrylamide gel and form a band IIb containing approximately nucleotides.
It very closely resembles another citrus viroid IIa that contains about nucleotides. However, symptoms induced in citron and other citrus cultivars and rootstocks are clearly and strikingly different. In any indexing programme citrus, grape or stone fruit with its object of producing disease-free primary stock, it is important that all viroids be recognized and, if possible, removed from propagative budwood.
Citrus viroids are distributed primarily by the introduction and propagation of infected budwood and subsequently by mechanical transmission. The viroids are transmitted mechanically by hedging equipment, tools and knives, especially from lemon to lemon.
Mechanical transmission of CEV was first demonstrated by Garnsey and Jones , who showed that contaminated tools could be disinfected by a mixture of 2 percent sodium hydroxide plus 2 percent formaldehyde. Roistacher, Calavan and Blue demonstrated that low dilutions of sodium hypochlorite are readily available, less toxic and a very efficient disinfectant for CEV. Although mechanical transmission from orange, mandarin or grapefruit is less efficient than from lemon Garnsey, , once the viroid is present it will spread from tree to tree throughout an orchard over a period of time by hedging, pruning, clipping of fruit or collecting of budwood.
Neither CEV nor CVs are known to be vector- or seed transmitted, and root transmission, though possible, would be overshadowed by mechanical transmission. All of the citrus viroids appear to be readily eliminated by shoot-tip grafting or by use of nucellar budlines. They are extremely tolerant to heat and have not been successfully eliminated from budwood by thermotherapy. At first the diagnosis of exocortis was a long-term process in which trifoliate orange or Rangpur lime as a rootstock, under a vigorous growing scion such as lemon, was inoculated and placed in the field.
Symptoms would appear in the rootstock after two to six years or longer, depending on the severity of the viroids present. The use of citron as a rapid indicator was first proposed by Salibe and Moreira , tested by Frolich et al. Roistacher et al. The detection of CEV and related CVs by graft-transmission and by PAGE requires the production of the highest quality plants growing under ideal conditions of nutrition and temperature.
The need for an exacting type of plant laboratory as outlined in Part II becomes apparent when indexing for these viroid-induced diseases. Recent important developments in the detection of a whole range of new citrus viroids are reviewed by Duran-Vila et al. Principles relating to these viroids may be applicable to similar viroids now being found in grapes and stone fruit. Field symptoms range from mild bark cracking to very severe bark scaling, primarily on trifoliate and Rangpur lime rootstocks, accompanied by various degrees of stunting of the tree see Figures and For more detailed descriptions and photographs see Weathers and Wallace Both Rangpur lime and trifoliate orange twigs and branches may show a chlorotic stem blotching which is symptomatic Figure Citrons are highly susceptible and, when inoculated, may show bark cracking Figure 41b , leaf epinasty and necrosis of the leaf veins Figure 41a.
Sweet limes and certain lemon varieties may show elongated bark cracks. Certain mechanically inoculated herbaceous hosts, i. Gynura, petunia and tomato, will show a distinct leaf curl, epinasty and vein necrosis.
Trees on trifoliate hybrid rootstocks can be stunted to varying degrees Figure 43 and some of these hybrid rootstocks will show bark cracking. Trees on trifoliate rootstock affected with both exocortis and tristeza can be very severely stunted. Trifoliate orange rootstock infected with individual or combinations of CVs excluding CEV may show mild to moderate bark cracking Figure If the rootstock is trifoliate orange, certain citranges or Rangpur lime, typical bark cracking as shown in Figures 35 and 36 is symptomatic and diagnostic for CEV.
Citrus viroids I to IV excluding cachexia do not induce the severe bark cracking typical of CEV, but may cause a mild bark cracking in trifoliate orange stock Figure Twigs and branches of CEV-infected trifoliate orange or Rangpur lime may show a yellow blotch as seen in Figure The use of trifoliate orange as a seedling or rootstock for long-term field indexing is no longer recommended. However, it may be useful for detecting and classifying the milder reacting CVs.
Studies under way show that certain CVs will crack trifoliate orange rootstock in the field and cracking will vary, depending upon combinations of viroids present in the inoculum Roistacher and Semancik, unpublished. Severe stunting of trees on trifoliate orange rootstock, even in the absence of bark scaling, may indicate a viroid infection. The use of sensitive clonal citron selections such as , or S-I budded to a vigorous rootstock such as rough or Volkamer lemon, or propagated as cuttings, is the preferred and recommended index method.
It is relatively rapid, sensitive and highly diagnostic Caravan et al. Citron seedlings may be used for detection of CEV, which causes strong symptoms. However, when indexing for the milder reacting citron viroids of groups I to IV, S-I budded to a vigorous rootstock is the recommended indicator. Collection of budwood. In a routine index, a minimum of four budsticks should be collected, one from each quadrant of the tree. However, for periodic critical reindexing of foundation block trees, where they may have been subject to possible contamination by infected tools via mechanical transmission, eight budsticks should be collected from around the tree.
At times CEV infection has been discovered in only one of the eight sectors of the tree. Whenever collecting budwood, tools must be dipped or sprayed with a 1 percent sodium hypochlorite solution when going from tree to tree. This practice is extremely important and necessary as a sanitary precaution, and should be incorporated as standard sanitary procedure in the field as well as in the plant laboratory. Inoculum tissue. The best inoculum tissue for indexing is the "bud" bud, blind bud or chip bud.
A minimum of two inoculum buds are recommended per test plant. Leaf tissue should not be used. Studies by Blue et al. The preferred indicator scion is S When obtained, a bud of S-1 is propagated on rough lemon or other lemon type stock and maintained as a source plant. Extreme care should be taken to disinfect clippers when collecting budsticks from the source plant for use as scion indicators. The S-1 citron bud is then grafted to a vigorous seedling rootstock, such as rough or Volkamer lemon, at about 25 cm above the soil surface.
Other rootstocks should be tested for compatibility, since citron does not grow well on all rootstocks. When wrapping the citron bud with budding tape, the "eye" of the bud may be exposed for forcing, or it can be completely wrapped, and unwrapped two or three weeks after budding. Inoculation can be done at the same time as grafting the citron scion bud. Two inoculum "buds" are grafted anywhere below the citron bud and completely wrapped.
The seedling is then bent just above the citron bud and the top portion of the seedling tied to the base of the plant to aid in the forcing of the citron bud Nauer and Goodale, This is the same procedure used for forcing the Parson's Special mandarin bud in the cachexia index Cachexia. Figures 47 and However, if rough lemon seedlings are used as the rootstock, the seedlings can first be cut back at the time of inoculation to 25 or 30 cm above the soil surface and the citron bud inserted near the top of the cut seedling and wrapped.
The inoculum buds are then inserted below and wrapped. The compatibility of citron with rough lemon is excellent and cutting back at the time of inoculation favours the forcing of the citron bud. This procedure is more convenient than bending and has been successfully used. Number of indicator plants. Indicator plants should be grown one per container for CEV indexing. The milder symptoms associated with citrus viroids are best expressed and observed in plants grown one per container.
A minimum of four inoculated plants is recommended for each index test, and the inoculum collected from the various sectors of the field tree to be tested should be uniformly distributed among the test plants. After inoculation, the used inoculum budwood should be refrigerated and saved.
EPPO Global Database
Citrus exocortis is a disease of citrus plants, caused by the Citrus exocortis viroid CEVd. It can cause stunted growth and reduced yields in affected plants. The disease is also sometimes called "scalybutt". The resulting disease is sometimes called "tomato bunchy top disease. Other symptoms include leaf epinasty, stunting, and necrosis of the leaf midvein. Affected trees will show rootstock "shelling" where the bark peels off of the rootstock — the lower tree that the main orange cultivar was grafted onto.
Contents - Previous - Next. The exocortis disease of citrus, caused by the citrus exocortis viroid CEV , was first reported and described by Fawcett and Klotz as a bark-shelling disorder of trifoliate orange rootstock. Benton et al. They recommended that budwood for propagation be taken only from older trees on trifoliate rootstock showing no scaling of the butt or trunk. The disease is present in almost all citrus-growing regions of the world.