Identification and Characterization of a Novel Tobamovirus from Tropical Soda Apple in Florida

Tropical soda apple (Solanum viarum Dunal) is a solanaceous weed, armed with prickles on stems, leaves, petioles, and calyxes. It is native to Argentina, Brazil, Paraguay, and Uruguay (26) and was introduced in the 1980s into Florida, where it thrives as a perennial, particularly in the southern part of the state (11). It has spread rapidly throughout Florida, now occurring on more than 405,000 ha (~1 million acres; 13). It is most commonly found in pastures but also occurs in citrus groves, sugarcane fields, and natural areas (23). Since the foliage is unpalatable to cattle, the weed has rapidly infested pastures (Fig. 1A), where the cattle ingest the aromatic fruits and perpetuate its spread (Fig. 1B). Based on 1993 data when tropical soda apple covered only 200,000 ha (~494,000 acres) in Florida, losses to cattle ranchers in the state were estimated at $11 million (22). Tropical soda apple was added to the Florida Noxious Weed List in 1994 and was declared a federal noxious weed in 1995 (19). It has now been reported from Alabama, Georgia, Louisiana, Mississippi, North Carolina, Pennsylvania, South Carolina, Tennessee, Texas, and Puerto Rico (21,31,38), although many of these states have effective eradication programs underway. The occurrence of the weed in several of these states can be linked to the movement of cattle, bahiagrass seed, or composted manure from Florida (38). Based on ecological range studies, Patterson and colleagues (29) determined that temperature and photoperiod were not likely to limit the spread of this weed throughout the southern United States and into adjacent regions. Tropical soda apple has a measurable economic impact on bahiagrass productivity. In a study by Mullahey and colleagues (24), a high density of tropical soda apple plants reduced the number of grazing days by more than half when compared to a bahiagrass stand free of the weed. Previous reports document viruses in several genera and families known to naturally infect tropical soda apple, including potyviruses (Potato virus Y [PVY] and Tobacco etch virus [TEV]), a begomovirus (Tomato mottle virus), a cucumovirus (Cucumber mosaic virus), and a tobamovirus (Tomato mosaic virus [ToMV]) (17,18,37). Tropical soda apple has also been shown to be an experimental host for a variety of viruses (8). Another tobamovirus, Tobacco mild green mosaic virus (TMGMV), has recently been demonstrated to have potential as a biocontrol agent for tropical soda apple due to its induction of a rapid and lethal hypersensitive response following inoculation (30). Foliar symptoms suggestive of virus infection were recently observed on tropical soda apple in Florida. An agent was mechanically transmitted to Nicotiana benthamiana from which a putative tobamovirus was isolated. In this paper, we present biological and molecular characterization of this tobamovirus, for which the name Tropical soda apple mosaic virus (TSAMV) is proposed. The host range of this virus, coupled with analysis of the nucleotide sequences of the coat protein (CP) and movement protein (MP) genes and the deduced amino acid sequences of the CP and MP, suggests that it is a novel member of the Solanaceae-infecting subgroup of the genus Tobamovirus.