The Recombinant Potato virus Y (PVY) Strain, PVYNTN, Identified in Potato Fields in Victoria, Southeastern Australia.

Potato virus Y (PVY) is one of the main viruses affecting potato in Australia. However, molecular characterization of PVY isolates circulating in potato in different states of Australia has not yet been thoroughly conducted. Only nonrecombinant isolates of three biological PVY strains collected from potato were reported previously from Western Australia and one from Queensland. Here, PVY isolates collected from seed potato originating in Victoria, Australia, and printed on FTA cards, were subjected to strain typing by RT-PCR, with three isolates subjected to whole genome sequencing. All the 59 PVY isolates detected during two growing seasons were identified to be recombinants based on two RT-PCR assays. No nonrecombinant PVY isolates were identified. All the RT-PCR typed isolates belonged to the PVYNTN strain. Sequence analysis of the whole genomes of three isolates suggested a single introduction of the PVYNTN strain to Australia but provided no clues as to where this introduction originated. Given the association of the PVYNTN strain with potato tuber damage, growers in Australia should implement appropriate strategies to manage PVYNTN in potato.

Modeling Pathogen DNA Content and Visual Disease Assessment in Seed Tubers to Inform Disease in Potato Progeny Root, Stolon, and Tubers.

Measurement of pathogens on seed tubers is essential for informing likelihood of subsequent potato disease. Here we utilized quantitative PCR assessment of pathogen DNA and visual assessment of disease to measure seed tuber inoculum and used this to model development of disease in potato grown in pathogen-free soil. Analysis by recursive partitioning and modeling using receiver operating curves indicated both abundance of Rhizoctonia solani AG3 and Streptomyces scabies DNA, and disease symptoms associated with these pathogens on seed tubers could predict subsequent disease in progeny tubers and for R. solani, stolons. In contrast, abundance of Spongospora subterranea DNA and disease symptoms on seed tubers were not consistently associated with powdery scab in progeny tubers. The relationship between S. subterranea DNA and seed tuber symptoms on root galling was stronger. Symptomless seed tubers that carried high levels of S. subterranea DNA were also associated with greater root galling than those with low pathogen DNA levels. There was a modest association between root galling and powdery scab in progeny tubers. These results highlight the importance of using certified seed tubers, and demonstrate a statistical tool for measuring the impact of seed tuber-borne inoculum.

Genetic characterization of a novel Phomopsis sp., a putative biocontrol agent for Carthamus lanatus.

A large number of isolates of Phomopsis sp. have been collected from the weed Carthamus lanatus (saffron thistle) in Australia, and their potential as biological control agents for weeds of the Asteraceae has been demonstrated. An analysis of their genetic diversity and a multigene phylogenetic analysis were undertaken to ascertain whether these isolates were distinct from other species of Phomopsis that commonly attack crop species in Australia. Minimal variation was found between the Phomopsis spp. isolated from saffron thistle, except two isolates that appeared to share identity with Diaporthe helianthii and P. viticola. Analysis of the selected isolates from saffron thistle with the nucleotide sequence of the partial ITS and tefl-alpha regions demonstrated that the sequences were distinct from all other species of Phomopsis so far described from crops in Australia. These findings provide strong support for the recognition of these isolates as a separate species of Phomopsis. The implications of these findings are discussed in relation to biological control of saffron thistle.

Edaphic Soil Levels of Mineral Nutrients, pH, Organic Matter, and Cationic Exchange Capacity in the Geocaulosphere Associated with Potato Common Scab.

ABSTRACT In order to determine possible relationships between geocaulosphere soil properties and severity of common scab of potato caused by Streptomyces scabies, soils were collected from representative commercial potato fields in Canada: in Simcoe and Dufferin Counties, Ontario and across Prince Edward Island (PEI) in August 2004. Soils immediately adjacent to tubers were sampled and analyzed for select edaphic factors and for pathogen presence using polymerase chain reaction (PCR) tests with primers that amplify a region of the TxtA gene involved in regulating the biosynthesis of the thaxtomin toxin family. Individual tubers were assessed visually for scab severity. The relationships between soil chemical factors and disease severity were investigated for each region to detect the strongest relationships. Principal component analysis revealed a distinctive clustering of samples with respect to disease severity in PEI but not in Ontario soils. Total and percent saturation of K (%K) were the only factors found associated with high disease severity in soils from both provinces. In PEI soils, pH, Mg, Ca, Cu, and %K, %Mg, %Ca, and %Na were associated with high disease severity, whereas cation exchange capacity (CEC) and Al were correlated with low disease severity soils. In Ontario, high Mn content was strongly correlated with low disease severity soils, whereas %K and organic matter content were correlated with disease severity. Partitioning samples into presence or absence of the TxtA PCR product with corresponding high or low severity showed further significant relationships in the data. There was an excellent correlation between Streptomyces spp. presence as detected by PCR and disease severity in PEI soils; however, the relationship was not as clear in Ontario soils, where many PCR-positive soils had low disease incidence. Principal component and partial least square analysis indicated that disease severity was predicted by soil factors such as organic matter, CEC, pH, Al, %Ca, %Mg, and %K for PEI but not for Ontario soils. The data reveal that the relationship between scab severity and soil chemical components is complex and potentially soil specific.