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Viroids: New and continuing risks to agriculture.
R. W. HAMMOND (1). (1) USDA ARS MPPL, Beltsville, MD, U.S.A.

Viroids are single-stranded, covalently closed, circular, highly structured non-coding RNAs that cause disease in several economically important crop plants. Viroids replicate autonomously and move systemically in host plants with the aid of the host machinery. In addition to symptomatic infections, viroids also cause latent infections where there is no visual evidence of infection in the host, as is the case in many ornamentals; however, transfer to a susceptible host can result in devastating disease. Viroid infections have their origins from natural vegetation and infected seed and propagation materials. Once introduced, they are easily mechanically transmissible and have exhibited an expanding host range into susceptible crops. Viroid infections occur more often than widely appreciated and recent interceptions and outbreaks have prompted discussions regarding the expansion of import/export regulations of germplasm. Emerging viroid diseases in the <i>Solanaceae </i>(e. tomato and pepper) caused by seed-transmitted viroids (<i>Potato spindle tuber viroid</i>, <i>Columnea latent viroid</i>, <i>Pepper chat viroid</i>, <i>Tomato chlorotic dwarf viroid</i>, among others), and in vegetatively-propagated crops, are a threat to agriculture worldwide. Challenges facing industry and regulatory agencies include the use of appropriate detection, reporting, and control measures to maintain healthy crops.

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