Abstract
Citrus concave gum-associated virus (CCGaV) is a negative-stranded RNA virus, first reported a few years ago in citrus trees from Italy. It has been reported in apple trees in the USA and in Brazil, suggesting a wider host range and geographic distribution. Here, an anti-CCGaV polyclonal antiserum to specifically detect the virus has been developed and used in a standard double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) that has been validated as a sensitive and reliable method to detect this virus both in citrus and apple trees. In contrast, when the same antiserum was used in direct tissue-blot immunoassay, CCGaV was efficiently detected in citrus but not in apple. Using this antiserum, the first apple trees infected by CCGaV were identified in Italy and the presence of CCGaV in several apple cultivars in southern Italy was confirmed by field surveys. High-throughput sequencing (HTS) allowed for the assembling of the complete genome of one CCGaV Italian apple isolate (CE-c3). Phylogenetic analysis of Italian CCGaV isolates from apple and citrus and those available in the database showed close relationships between the isolates from the same genus (Citrus or Malus), regardless their geographical origin. This finding was further confirmed by the identification of amino acid signatures specific of isolates infecting citrus or apple hosts. Analysis of HTS reads also revealed that the CE-c3 Italian apple tree, besides CCGaV, was simultaneously infected by several viruses and one viroid, including apple rubbery wood virus 2 which is reported for the first time in Italy. The complete or almost complete genomic sequences of the coinfecting agents were determined.
Highlights
Citrus concave gum-associated virus (CCGaV) was first identified in citrus trees in southern Italy [1], where it was found to be closely associated with symptoms of concave gum (CG), a citrus graft-transmissible virus-like disease which has been known for more than eighty years [2]
In Brazil, CCGaV was detected in “Royal Gala” (RG) and “Mishima” (MI) grown in the field [6]. In both the USA and Brazil, CCGaV-infected apple trees were simultaneously infected by other viruses including apple rubbery wood virus 1 (ARWV-1) and ARWV-2, which are two negative ssRNA viruses classified in the genus Rubodvirus and first reported in Germany, where they were found to be associated with wood rubbery disease [7]
Twenty trees were infected by Citrus virus A (CiVA), four of which were simultaneously infected by CCGaV, while 19 trees tested negative to both viruses (Table 1)
Summary
Citrus concave gum-associated virus (CCGaV) was first identified in citrus trees in southern Italy [1], where it was found to be closely associated with symptoms of concave gum (CG), a citrus graft-transmissible virus-like disease which has been known for more than eighty years [2]. Wright and colleagues [5] reported the presence of CCGaV in “Honeycrisp” apple trees grown in two orchards of central Washington State, and in several accessions from Spain, Italy, France, Australia and USA held at a germplasm collection of the Washington State University. In Brazil, CCGaV was detected in “Royal Gala” (RG) and “Mishima” (MI) grown in the field [6]. In both the USA and Brazil, CCGaV-infected apple trees were simultaneously infected by other viruses including apple rubbery wood virus 1 (ARWV-1) and ARWV-2, which are two negative ssRNA viruses classified in the genus Rubodvirus (family Phenuiviridae) and first reported in Germany, where they were found to be associated with wood rubbery disease [7]. CCGaV was identified in apple-decline-affected trees, the presence of other coinfecting viruses in these plants hampered the drawing of any conclusions on its role in eliciting any disease in apple [5,6]
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