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Natural viroid infections by members of the families Avsunviroidae and Pospiviroidae occur throughout the Caribbean, North, Central, and South America. Of the 31 viroid species currently recognized by the International Committee on Taxonomy of Viruses, at least 22 have been reported to occur in this region. This chapter provides a review of current literature on the geographic distribution of viroids in the Americas and the hosts from which these viroids are reported.

The mission of APHIS and its predecessor agencies within the USDA is consistent with a key goal of conservation biology: preventing the introduction of harmful nonindigenous organisms. Realizing this goal requires informed decisions by regulatory risk managers regarding importations of agricultural commodities. The technical basis for these informed decisions is most often a plant pest risk assessment. Past plant pest risk assessments and, indeed, the majority of current assessments have used qualitative measures of the risk posed by commodity pathways. Increasingly, probabilistic assessments have emerged as the risk assessment method of choice to support difficult regulatory decisions. The United States has taken the lead in using this type of assessment for agricultural trade. But assessments of this type are becoming more common, and New Zealand, Australia, and Canada have conducted risk assessments of this type in support of decisions on international trade in agricultural commodities. Biological systems in general and plant pest-commodity interactions in particular are difficult to assess both because of their inherent complexity and the frequent dearth of data available to describe them. Probabilistic assessments provide the best available tools to account for the variability and uncertainty that biological complexity and data gaps introduce into the plant pest risk assessment process.

Rooted cuttings of `Halford' and `Redhaven' peaches [Prunus persica (L.) Batsch] and `Stanley' (Prunus domestica L.) and `Marianna 2624' (P. cerasifera × P. munsoniana) plums were planted in soil containing ≈38 tomato ringspot virus-(TomRSV) infested nematodes (Xiphinema americanum sensu lato Cobb) per 100 cc. Test- and control-plant sap extracts were made from root and leaf tissues after 10, 22, and 34 weeks. Aliquots of these samples were assayed by mechanical inoculation to Chenopodium quinoa Willd. Total nucleic-acid extracts prepared from the remainder of each sample were analyzed by dot blot hybridization using a cRNA probe for TomRSV. The bioassay identified one `Stanley' and two `Redhaven' infected plants. Hybridization results indicated that two of two `Stanley', three of three `Halford', five of five `Redhaven', and zero of six `Marianna 2624' were infected. Our results demonstrate the sensitivity of molecular hybridization for TomRSV detection in Prunus and substantiate the TomRSV resistance of `Marianna 2624'.

We have synthesized and mapped a library of cDNA clones representing the RNA genome of a strain of blueberry scorch carlavirus (BBScV) associated with a disease known locally, in New Jersey, U.S.A., as Sheep Pen Hill disease. The nucleotide sequence of that strain was determined to be 8514 residues, excluding the poly(A) tail. In addition, cDNA clones representing the 3′ terminus of another strain of the virus from the same field were synthesized, mapped and sequenced. The overall identity between sequences of these two strains was approximately 90% spanning the 1634 residue overlap, confirming their identity as distinct strains and not simply different isolates of a single strain. Finally, the coat protein gene of a distinct strain of the virus, isolated from plants with blueberry scorch disease in the Puyallup Valley in Washington State, U.S.A., was cloned from total cDNA by PCR. Sequence analysis revealed that the strain from Washington was more divergent from the two New Jersey strains than they were from each other. Comparisons of these sequences with other carlavirus sequences indicated that BBScV is more closely related to lily symptomless virus and potato virus S than to potato virus M, Helenium virus S, carnation latent virus or poplar mosaic virus. BBScV and potato virus M shared approximately 54% nucleotide sequence identity overall.

A chemiluminescent molecular hybridization protocol was compared to 32P autoradiography for detecting potato spindle tuber viroid (PSTVd) and apple scar skin group viroids (ASSVd). Labeled cRNA probes for PSTVd and ASSVd were synthesized by SP6 RNA polymerase transcription using digoxigenin-11-UTP or α-[32P]UTP. Dot blot hybridization of purified viroids and sap extracts from infected plants showed that chemiluminescent detection using digoxigenin-labeled probes was as sensitive as autoradiography using 32P probes. A minimum of 2.0–2.5 pg purified viroid was detected. ASSVd could be detected in as little as 0.4 ng of total nucleic acid extract from infected tissue or in sap extracts diluted to 10−3 with healthy extracts. Tissue blots of PSTVd-infected potato tubers and tomato roots, stems and leaves and ASSVd-infected apple fruit, stems and petioles, gave positive reactions when hybridized with the digoxigenin probe. No reaction with similar tissues from healthy plants was observed.