Citrus Leaf Blotch Virus: A New Citrus Virus Associated with Bud Union Crease on Trifoliate Rootstocks

Abstract

Citrus leaf blotch virus (CLBV) was first detected in a Nagami kumquat, clone SRA-153 from Corsica (France), showing bud union crease when propagated on Troyer citrange. Citranges are important rootstocks in Spain, presently being used for about 50% of all commercial citrus trees. Therefore, dispersal of a graft-transmissible pathogen causing bud union crease on this rootstock could potentially cause important economic losses. CLBV has filamentous particles about 900 × 14 nm in size, with a single-stranded, positive sense, genomic RNA (gRNA) of 8,747 nt, and a coat protein about 41 kDa. The gRNA contains three open reading frames (ORFs) and two untranslated regions of 73 and 541 nt at the 5' and 3' termini, respectively. Biological and molecular properties of CLBV support its inclusion in a new virus genus. The virus can be detected by graft-inoculation onto Dweet tangor seedlings, in which it induces chlorotic blotching in young leaves, but transmission is sometimes erratic and at least six indicator plants should be used in each test. We developed a quick detection procedure using RT-PCR with two sets of primers derived from sequences in ORF1 (a region containing motifs characteristic of an RNA poly-merase) and ORF3 (the coat protein gene). Results with both sets of primers were similar. CLBV was readily detected in young leaves of infected Nagami kumquat or in Nules Clementine, Owari Satsuma, Eureka lemon, Marsh grapefruit or Newhall navel orange inoculated with kumquat SRA-153, but not in Pineapple sweet orange, a host that yielded more than 80% false negatives.