First Report of Tomato chlorotic dwarf viroid in Greenhouse Tomatoes in Arizona

Abstract

Tomato chlorotic dwarf viroid (TCDVd), a member of the genus Pospivroid, family Pospiviroidae, was first identified on greenhouse tomato (Solanum lycopersicum) in Canada (2). Since then, it has also been reported elsewhere, e.g., on tomato in Colorado (4). During 2006 in Arizona, tomato plants in a large greenhouse facility with continuous tomato production exhibited viroid-like symptoms of plant stunting and chlorosis of the young leaves. Symptomatic plants were often located along the edge of the row, indicating the presence of a mechanical transmissible agent. Approximately 4% of the plants in this greenhouse were symptomatic in 2008. Symptoms were distinctly different from those caused by Pepino mosaic virus (PepMV), a virus that was generally present in this greenhouse and also in our test samples. Other commonly occurring tomato viruses were ruled out by serological, PCR, or reverse transcription (RT)-PCR tests in multiple laboratories. RT-PCR with two sets of universal pospiviroid primers, PospiI-FW/RE and Vid-FW/RE (4), yielded amplicons of the expected sizes of 196 and 360 bp in three samples collected from symptomatic plants. Direct sequencing of the amplicons revealed that the genome was 360 nt and 100% identical to the type TCDVd from Canada (GenBank Accession No. AF162131) (2). Mechanical inoculation with leaf tissue extract from four samples to plants of the tomato 'Money-Maker' resulted in the same viroid-like symptoms and TCDVd was confirmed in these plants by RT-PCR and sequencing. In both 2007 and 2008, 18 samples were tested using primers PSTVd-F and PSTVd-R (1), which are capable of amplifying the full TCDVd genome. Analysis of the sequences from the amplicons revealed two genotypes of TCDVd. The first genotype (GenBank Accession No. FJ822877) was identical to the type TCDVd and found in 11 samples from 2007 and one from 2008. The second genotype (GenBank Accession No. FJ822878) was 361 nt, differing from the first by nine nucleotide substitutions, 2 insertions, and 1 deletion. This second genotype was found in 7 and 17 samples from 2007 and 2008, respectively, and showed the highest sequence identity (97%) to a Japanese tomato isolate (AB329668) and a much lower sequence identity (92%) to a U.S. isolate previously identified in Colorado (AY372399) (4). The origin of TCDVd in this outbreak is not clear. The genotype identified first could have been introduced from a neighboring greenhouse where the disease was observed before 2006 and where this genotype also was identified in 2007. The second genotype may have been introduced from infected seed since TCDVd has recently been shown to be seed transmitted in tomato (3). To our knowledge, this is the first report of natural occurrence of TCDVd in Arizona.