An Outbreak of a Necrosis Disease of Tomato in California in 2008 was Caused by a New Ilarvirus Species Related to Parietaria mottle virus

During the 2008 early-summer growing season, virus-like necrosis symptoms, most similar to those induced by Tobacco streak virus (TSV), were observed in leaves, stems, and petioles of processing tomato plants in the Central Valley of California. Symptoms were observed in numerous fields in Merced, San Joaquin, and Yolo counties, though the incidence of the disease in most fields was not high (not more than 5% but over 20% in some areas). Antibody-based tests of representative samples of the disease for infection with Tomato spotted wilt virus, TSV, and Tomato apex necrosis virus, which cause similar symptoms, were negative. A putative virus-like agent was sap- and graft-transmitted to tomato plants and induced necrotic spots in leaves and stem and petiole necrosis symptoms that were similar to those observed in the field. Eventually, these plants recovered from these symptoms. In sap-transmission experiments, the virus-like agent induced systemic symptoms in Chenopodium quinoa and C. amaranticolor (stunted growth and leaf curl and necrosis), Nicotiana benthamiana (necrotic leaf and stem lesions), N. tabacum cvs. Havana and Turkish (stunted growth and necrotic etching and ringspots followed by recovery for cv. Havana but not for cv. Turkish), and Datura stramonium (mild mottle and ringspots in newly emerged leaves followed by recovery); no symptoms were observed in inoculated common bean (cv. Topcrop), pumpkin (cv. Small Sugar), pepper, and N. glutinosa plants. Virus minipurification was performed with leaves from noninfected and infected D. stramonium plants, and polyacrylamide gel electrophoresis analyses revealed a protein band of ~29 kDa in infected but not noninfected plants. This protein was purified and subjected to liquid chromatography-mass/mass spectrometry analysis. Four peptides, obtained from the trypsin-digested protein, each had the highest match (score of 118) with the capsid protein (CP) of Parietaria mottle virus (PMoV), an ilarvirus that induces leaf and stem necrosis in tomatoes in Europe (1). Using sequences of PMoV and other ilarviruses, a single primer was designed from the 3' nontranslated region and paired with primers designed from conserved regions of ilarvirus RNAs 1, 2, and 3. In reverse transcription-PCR analyses, these primer pairs directed the amplification of the expected-sized fragments for ilarvirus RNAs 1, 2, and 3 from RNA extracts prepared from leaves with the unusual necrosis symptoms. Sequence analyses confirmed these were ilarvirus fragments. Partial RNA 1, 2, and 3 sequences were 81, 84, and 82% identical, respectively, with those of PMoV and 80, 77, and 69% identical, respectively, with those of TSV. The amino acid sequence of the CP gene (GenBank Accession No. FJ236810) was 86 and 61% identical to those of PMoV and TSV, respectively. Together, these results indicate the necrosis disease of tomato is caused by a new ilarvirus species, tentatively named Tomato necrotic spot virus, although further studies are needed to confirm this. The mode of transmission of this new ilarvirus to tomatoes in the field is unknown, but it may involve thrips feeding on infected pollen, a known method of transmission for TSV (2). References: (1) L. Galipienso et al. Plant Pathol. 54:29, 2005. (2) R. Sdoodee and D. S. Teakle. Plant Pathol. 36:377, 1987.