A New Strain of Potato Carlavirus M

Pest categorisation of potato virus M (non-EU isolates)

Following a request from the EU Commission, the Panel on Plant Health has addressed the pest categorisation of non-EU isolates of potato virus M (PVM). The information currently available on geographical distribution, biology, epidemiology, potential entry pathways, potential additional impact compared to the current situation in the EU and availability of control measures of non-EU isolates of PVM has been evaluated with regard to the criteria to qualify as a potential Union quarantine pest. Because non-EU isolates of PVM are absent from the EU, they do not meet one of the requirements to be regulated as a regulated non-quarantine pest (RNQP) (presence in the EU); as a consequence, the Panel decided not to evaluate the other RNQP criteria for these isolates. Populations of PVM can be subdivided into two strains: the ordinary strain (PVM-O) is present in the EU, while the divergent strain (PVM-D) is absent from the EU or considered to have at most a limited distribution in the EU. Non-EU isolates of PVM-O are not expected to have an additional impact in the EU compared to EU isolates and therefore do not meet the corresponding criterion to qualify as a potential Union quarantine pest. The Panel is unable to conclude on the potential impact of non-EU PVM-D isolates in the EU territory, but PVM-D isolates meet all the other criteria to qualify as a potential Union quarantine pest.

Genomic variability in potato virus M and the development of RT-PCR and RFLP procedures for the detection of this virus in seed potatoes

Potato virus M (PVM, Carlavirus) is considered to be one of the most common potato viruses distributed worldwide. Sequences of the coat protein (CP) gene of several Canadian PVM isolates were determined. Phylogenetic analysis indicated that all known PVM isolates fell into two distinct groups and the isolates from Canada and the US clustered in the same group. The Canadian PVM isolates could be further divided into two sub-groups. Two molecular procedures, reverse transcription - polymerase chain reaction (RT-PCR) and restriction fragment length polymorphism (RFLP) were developed in this study for the detection and identification of PVM in potato tubers. RT-PCR was highly specific and only amplified PVM RNA from potato samples. PVM RNAs were easily detected in composite samples of 400 to 800 potato leaves or 200 to 400 dormant tubers. Restriction analysis of PCR amplicons with MscI was a simple method for the confirmation of PCR tests. Thus, RT-PCR followed by RFLP analysis may be a useful approach for screening potato samples on a large scale for the presence of PVM.

Macroarray Detection of Eleven Potato-Infecting Viruses and Potato spindle tuber viroid

A macroarray was developed for the detection of 11 potato viruses and Potato spindle tuber viroid. The 11 viruses detected included those commonly found or tested for in North American potato seed certification programs: Alfalfa mosaic virus, Cucumber mosaic virus, Potato mop top virus, Potato leafroll virus, Potato latent virus, Potato virus A, Potato virus M, Potato virus S, Potato virus X, Potato virus Y, and Tobacco rattle virus. These viruses were detected using oligonucleotide 70-mer probes and labeled targets prepared by a random primed amplification procedure. Potato plants analyzed included those infected with 12 reference virus stocks and 36 field isolates. Results from the macroarray were entirely consistent with those obtained using a standard serological assay (enzyme-linked immunosorbent assay). Four isolates of Potato spindle tuber viroid, in mixed infection with one or more viruses, also were detected in the array, although strong hybridization signals required amplification with viroid-specific primers in combination with anchored-random primers. In individual plants, up to four viruses, or a viroid plus two viruses, were detected, with no apparent competition or inhibition. Macroarrays are a cost-effective approach to the simultaneous diagnostic detection of multiple pathogens from infected plants.

Development of universal primers for detection of potato carlaviruses by RT-PCR

To facilitate efficient and accurate detection of potato-infecting carlaviruses, degenerated universal primers were designed based on conserved amino acid and nucleotide sequences. Two sense primers, Car-F1 and Car-F2, were based on the amino acid sequences "SNNMA" and "GLGVPTE", respectively, in the coat protein. The reverse primer, Car-R, which was located at the border of the nucleic acid binding protein gene and the 3' untranslated region, and dT-B, which was derived from the oligo-dT targeting the poly(A) tail, were selected. Successful application of fragments within the predicted size range of carlaviruses was obtained using Car-F1 paired with either Car-R or dT-B from tested carlaviruses (Potato virus S, M and latent) by RT-PCR. The Car-F2 failed to yield clear-cut fragments within the predicted size range when paired with either Car-R or dT-B in RT-PCR. However, a less degenerated version of the primer, Car-F2b, resulted in amplicons within the predicted size range when paired with either Car-R or dT-B. Sequencing of the tentative carlavirus-fragments resulting from Car-F1/Car-R and Car-F2b/dT-B proved their carlavirus-origin, thus indicating the high specificity of these primers. The sensitivity of Car-F1/Car-R or Car-F2b/Car-R mediated RT-PCR for the detection of carlavirus-infected potato tubers were assessed using composite samples containing one carlavirus-infected-potato-tuber RNA sample with up to 49 virus-free-potato-tuber RNA samples under the optimal annealing temperature. The target carlaviruses were detected readily from all composites, demonstrating a high sensitivity. The method was further evaluated using presumed virus-free or carlavirus-infected potatoes of several cultivars, and reliable results were obtained.