In April 2009, a large number of tomato plants (Solanum lycopersicum L.) grown in a commercial greenhouse facility near Los Angles, CA exhibited general plant stunting (short internodes) and foliar symptoms that included distortion, chlorosis, and scattered necrotic spotting. Over time, the leaves began to exhibit a purple color and curling. Diseased plants were often elongated and frail with spindly shoots. The disease resulted in a significant yield loss due to reduced fruit size. Disease symptoms described above are generally different from those of Pepino mosaic virus (PepMV) infection, which causes yellow mosaic or patches on leaves and marbling of fruits. The disease was initially localized in certain areas in a greenhouse despite using a number of cultural management efforts including vigorous scouting, roguing of diseased plants, and strict hygiene and cleaning practices. The disease was also observed in neighboring greenhouses by the spring of 2010. A standard panel of tests for common tomato viruses and viroids were conducted using the appropriate serological or PCR assays. Reverse transcription (RT) PCR analysis of nine symptomatic plants with pospiviroid-specific primers, Pospil-RE and Pospil-FW (3), produced an amplicon of the expected size (~196 bp) while three healthy looking tomato plants did not. Subsequently, full viroid genomic sequences were obtained through RT-PCR with primer sets specific for Potato spindle tuber viroid (PSTVd), 3H1/2H1 (2), as well as for the pospiviroid genus, MTTVd-F and MTTVd-R (1). Sequences obtained from direct sequencing of amplicons or cloned PCR products from one isolate were identical and consisted of a full viroid genome of 358 nt, which was named PSTVd-CA1 (GenBank Accession No. HM753555). BLASTn queries of the NCBI database showed that this isolate had a high sequence identity (98%) to other PSTVd isolates (i.e., EF044304, X52037, and Y09577). The disease was reproducible upon mechanical transmission (1) on three tomato ‘Moneymaker’ plants, which expressed symptoms that were similar to those on the source plants. Recovery of PSTVd on the inoculated tomato plants was confirmed by RT-PCR and sequencing. Because of its susceptibility to viroid infection, tomato ‘Moneymaker’ plants are commonly used as indicators for the study of pospiviroids, including PSTVd. Natural PSTVd infection on greenhouse tomatoes has been reported in Europe (3) and New Zealand. Although a number of reports in the United States have been published on naturally occurring PSTVd infections of potatoes, to our knowledge, this is the first report of a natural PSTVd infection on tomatoes in the United States. The exact source of the PSTVd inoculum in the current disease outbreak is unknown, but it could have been introduced from infected potato or ornamental plants (4) or through infected tomato seeds. The disease epidemic might have been enhanced by frequent hands-on activities in greenhouse tomato production and the environmental conditions (high temperature and intense lighting) in the greenhouse that favor symptom expression.
References: (1) K.-S. Ling and W. Zhang, Plant Dis. 93:1216, 2009. (2). A. M. Shamloul et al. Can. J. Plant Pathol. 19:89, 1997. (3) J. Th. J. Verhoeven et al. Eur. J. Plant Pathol. 110:823, 2004. (4) J. Th. J. Verhoeven et al. Plant Pathol. 59:3, 2010.
