Potato spindle tuber viroid: alternative host reservoirs and strain found in a remote subtropical irrigation area

During 2007–2012, Potato spindle tuber viroid (PSTVd) was detected in volunteer cultivated, wild and native plants during studies to determine whether Pospiviroids occur within the isolated, sub-tropical, Gascoyne Horticultural District (GHD) in central coastal Western Australia (WA). PSTVd was detected infecting volunteer crop plants of tomato, pepper and chilli; introduced weed species Solanum nigrum (blackberry nightshade), Datura leichhardtii (thornapple) and Nicandra physalodes (apple-of-Peru) (Solanaceae), and Conyza bonariensis (flaxleaf fleabane) (Asteraceae); and Australian native species Atriplex semilunaris (annual saltbush), Rhagodia eremaea (thorny saltbush) (Chenopodiaceae), and Streptoglossa sp. (Asteraceae). PSTVd was also detected infecting Physalis angulata (wild gooseberry) in the Ord River Irrigation Area (ORIA), Kimberley region in north-west WA. Comparison of sequences from the three complete and 18 partial RNA nucleotide sequences obtained from 20 GHD and one ORIA isolates with those of published sequences showed that their highest nucleotide sequence identities were to isolate AY962324 belonging to the Chittering strain from south-west WA. On phylogenetic analysis, the three completely sequenced GHD PSTVd isolates grouped within a cluster of isolates from tomato and P. peruviana. These results show that a naturally occurring PSTVd inoculum reservoir is present in the GHD. This reservoir explains the occurrence of repeated PSTVd infections in different years in field crops of tomato, pepper and chilli growing in its market gardens and small farms. These findings have implications concerning PSTVd spread in intensive solanaceous crop field production systems in other subtropical regions of the world.