John Wesley Randles

A biotic hypothesis for the eucalypt disease of unknown aetiolgy, Mundulla Yellows (MY), was tested by seeking non-host nucleic acids in affected Eucalyptus camaldulensis trees using high throughput sequencing (HTS). Ten contigs were selected from a pool of nucleic acids unique to diseased trees, and oligonucleotide probes were prepared for specific dot-blot hybridisation assay of these contigs. One of the contigs was found to be associated with a few MY-affected trees collected in South Australia and Western Australia, as well as with similarly affected E. camaldulensis trees growing in Syria. The probes for this contig represented opposite polarities of the contig, and both hybridised with the same positive samples even after alkali treatment confirming that the contig that they represented was dsDNA. This contig, comprising 766 nt, showed 95% similarity to whole genome shotgun sequence of Leifsonia xyli . This preliminary microbiome analysis of MY affected trees provides the first evidence that a Leifsonia species may infect eucalypts and that it should be further studied as a potential agent of MY.

Studies on the ways in which viroids are transmitted are important for understanding their epidemiology and for developing effective control measures for viroid diseases. Viroids may be spread via vegetative propagules, mechanical damage, seed, pollen, or biological vectors. Vegetative propagation is the most prevalent mode of spread at the global, national and local level while further dissemination can readily occur by mechanical transmission through crop handling with viroid-contaminated hands or pruning and harvesting tools. The current knowledge of seed and pollen transmission of viroids in different crops is described. Biological vectors shown to transmit viroids include certain insects, parasitic plants, and goats. Under laboratory conditions, viroids were also shown to replicate in and be transmitted by phytopathogenic ascomycete fungi; therefore, fungi possibly serve as biological vectors of viroids in nature. The term “mycoviroids or fungal viroids” has been introduced in order to denote these viroids. Experimentally, known sequence variants of viroids can be transmitted as recombinant infectious cDNA clones or transcripts. In this review, we endeavor to provide a comprehensive overview of the modes of viroid transmission under both natural and experimental situations. A special focus is the key findings which can be applied to the control of viroid diseases.

Viroids were discovered by Diener in 1971

In situ hybridization and promoter analysis reveal that cotton leaf curl Multan betasatellite localizes in the phloem