Potato spindle tuber viroid eradicated in Western Australia
Potato spindle tuber viroid (PSTVd) was detected for the f irst time in an isolated hydroponic greenhouse tomato (Lycopersicon esculenturn) erop in the great southern region of Western Australia (WA). An urgent eradication plan was activated because of the highly contagious nature of the pathogen. The tomato erop on the infected property was destroyed and the greenhouses and surrounding area were decontaminated. Surveys of 15 other hydroponic tomato crops did not detect any other PSTVd infected tomato plants in WA.
Assessment of pospiviroid transmission by myzus persicae, macrolophus pygmaeus and bombus terrestris
During the past decade, pospiviroid outbreaks in tomato and sweet pepper crops have been regularly reported worldwide. Although seed transmission was often suspected, transmission from asymptomatic ornamentals and weeds through pruning tools, or via insect vectors could not be excluded. To evaluate the risk of pospiviroid transmission via insects, three insect spp. commonly observed in these crops were used in multiple transmission experiments with Potato spindle tuber viroid (PSTVd), Tomato apical stunt viroid (TASVd), Pepper chat fruit viroid (PCFVd) and Tomato chlorotic dwarf viroid (TCDVd)-infected source plants, such as tomatoes, petunias and peppers. The insect species belong to three different functional groups: a phloem-feeding pest (Myzus persicae Sulzer), a pollinator (Bombus terrestris L.) that feeds on pollen and nectar, and a generalist predator (Macrolophus pygmaeus Rambur), that consumes several species of insect pests and feeds on plant saps and pollen. For each insect, four transmission experiments were conducted in which different receptor host plants were challenged. During the experiments, insects that were seen probing or feeding on were captured and tested for viroids using PCR. The percentage of each insect species that was pospiviroid positive was 100 % for M. persicae, 4 % B. terrestris, and 0 % for M. pygmaeus. Although these results indicate that M. persicae may be a vector, none of the receiving host plants in four separate experiments tested positive. However, transmission by B. terrestris is possible since one of 18 receiving tomato plants tested positive for TCDVd in one of four transmission experiments, resulting in an overall transmission rate of 2.6 %. Based on these results, we believe that the use of pollinating insects and biological control agents in greenhouses does not imply a major phytosanitary risk for viroid dispersal.
Eleven new sequence variants of citrus exocortis viroid and the correlation of sequence with pathogenicity.
Full-length double-stranded cDNA was prepared from purified circular RNA of two new Australian field isolates of citrus exocortis viroid (CEV) using two synthetic oligodeoxynucleotide primers. The cDNA was then cloned into the phage vector M13mp9 for sequence analysis. Sequencing of nine cDNA clones of isolate CEV-DE30 and eleven cDNA clones of isolate CEV-J indicated that both isolates consisted of a mixture of viroid species and led to the discovery of eleven new sequence variants of CEV. These new variants, together with the six reported previously, form two classes of sequence which differ by a minimum of 26 nucleotides in a total of 370 to 375 residues. These two classes correlate with two biologically distinct groups when propagated on tomato plants where one produces severe symptoms and the other gives rise to mild symptoms. Two regions of the native structure of CEV, comprising 18% of the total residues, differ between the sequence variants of mild and severe isolates. Whether or not both of these regions are essential for the variation in pathogenicity has yet to be determined.