During the 2007 growing season, melon (Cucumis melo) samples from the state of Guerrero in Mexico showing mosaic and other virus-like symptoms were collected for analysis. Electron microscopic examination of negatively stained leaf-dip extracts revealed the presence of abundant virus-like particles with features characteristic of the family Bunyaviridae. No other viral particles were observed in these preparations. However, enzyme-linked immunosorbent assays (ELISAs) specific for the most common Tospovirus spp. gave negative results. Antibodies raised against purified nucleocapsids reacted specifically with the infected leaf extracts in Western blots and double-antibody sandwich ELISA. The viral RNA was used as a template for a cDNA library, and nucleotide sequence analysis identified cloned cDNAs representing sequences corresponding to the three Tospovirus genome segments. Sequence comparisons showed that the new virus had the highest similarity to Chrysanthemum stem necrosis virus (CSNV). Phylogenetic analysis of two genome regions confirmed that this virus, provisionally named Melon severe mosaic virus (MeSMV), is a previously undescribed Tospovirus sp. belonging to the “new world” clade of Tospovirus spp. An initial survey of various cucurbit crops in various states of Mexico confirmed the widespread occurrence of this virus.
A full-length curtovirus genome was PCR-amplified and cloned from peppers in Mexico with symptoms of curly top disease. The cloned DNA of this isolate, MX-P24, replicated in Nicotiana tabacum protoplasts and was infectious in N. benthamiana plants. Sequence analysis revealed that the MX-P24 isolate had a typical curtovirus genome organization and was most similar to beet mild curly top virus (BMCTV). However, sequence identities were at the threshold value for establishment of a new curtovirus species. To further investigate the biological properties of MX-P24, an agroinoculation system was generated. Agroinoculated shepherd’s purse plants developed typical curly top symptoms, and virus from these plants was transmissible by the beet leafhopper (Circulifer tenellus). The host range of MX-P24 was similar to that of BMCTV, with curly top symptoms induced in common bean, pepper, pumpkin, shepherd’s purse and tomato plants and mild or no symptoms induced in sugar beet plants. Together, these results indicate that MX-P24 is a highly divergent strain of BMCTV associated with an outbreak of curly top disease in peppers in Mexico.
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