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.
Molecular cloning of potato spindle tuber viroid (PSTV) cDNA synthesized by enzymatic elongation of PSTV-specific DNA primers: A general strategy for viroid cloning
Different cDNAs were synthesized by primer extension from the RNA of the severe strain KF 440 of potato spindle tuber viroid (PSTV) with the aid of reverse transcriptase using three PSTV-specific DNA molecules as primers. The cDNAs were made double-stranded and cloned into plasmid pBR 322. Various overlapping subgenomic DNA fragments were prepared from these clones and recombined in two different ways. In both cases a PSTV DNA copy was obtained which represented the entire PSTV RNA genome. The sequence of the DNA of one of the resulting full-length clones was identical with the original PSTV isolate, whereas the other clone showed one nucleotide change. On the basis of these results the advantages and problems of different strategies for the molecular cloning of the circular viroid RNA genome are discussed.