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Potato virus Y (PVY) is a significant threat to potato (Solanum tuberosum) production in Mexico. The presence of recombinant strains of PVY circulating in potato has been reported in the country, but no systematic study on the genetic diversity of PVY in potato and prevalence of PVY strains has been conducted yet. We report on a series of surveys in seed potato production areas in two states in Mexico, namely, Chihuahua and Jalisco, between 2011 and 2019. PVY was detected through the period of nine years in multiple potato cultivars in both states, often remaining asymptomatic in the most popular cultivars, such as ‘Fianna’ and ‘Agata’. When typed to strain, all PVY samples studied were found to have N-serotype, and were all identified molecularly as isolates of the same recombinant strain, PVYNTN. Five of these PVY isolates were tested on tobacco (Nicotiana tabacum), where they induced vein necrosis supporting the molecular typing. This identification was also confirmed biologically on differential potato cultivars, where one PVYNTN isolate from the 2013 survey triggered the hypersensitive resistance conferred by the Nztbr gene in the cv. Maris Bard. Seven of these Mexican PVYNTN isolates, collected between 2013 and 2019, including two PVY isolates from potato tubers exhibiting potato tuber necrotic ringspot disease, were subjected to whole genome sequencing and found to show a typical PVYNTNa recombinant structure. When subjected to phylogenetic analysis, Mexican PVYNTN sequences clustered in more than three separate clades, suggesting multiple introductions of PVYNTN in the country. The wide circulation of the PVYNTN strain in Mexican potato should be considered by potato producers, to develop mitigation strategies for this PVY strain associated with tuber necrotic symptoms.

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.

Potato is an important source of food in South Korea, and viruses represent a significant threat to sustainable and profitable potato production. However, information about viruses affecting the potato crop in South Korea is limited. In 2017, potato plants of five cultivars exhibiting foliar mosaic, crinkling, and mottle were collected in two seed potato production areas, in Gangwon-do and Jeollabuk-do Provinces, and subjected to virus testing and characterization. Potato virus Y (PVY) was found associated with mosaic symptoms, and samples were characterized using reverse transcription polymerase chain reaction (RT-PCR) and whole genome sequencing. All analyzed PVY-positive samples were found to represent the same recombinant PVY strain: PVYNTN. Three PVY isolates were subjected to whole genome sequencing using overlapping RT-PCR fragments and Sanger methodology, and all three were confirmed to represent strain PVYNTNa after a recombination analysis of the complete genomes. In phylogenetic analysis, the three South Korean isolates were placed most closely to several PVYNTNa isolates reported from Japan and Vietnam, suggesting a common source of infection. This is the first report and complete molecular characterization of a PVYNTN strain present in the country, and because this strain induces tuber necrotic ringspot disease in susceptible cultivars of potato, appropriate management tools need to be implemented to mitigate potential tuber quality losses.

In September 2014, a high rate of bulb rot (5-15% depending on producer) was reported across all cultivars developing early in the storage season in the onion producing region of southwestern Idaho. Spanish yellow onion bulbs cv. Vaquero displaying tan to light brown necrotic rot were obtained. The bulb rot originated in the neck and spread to successive scales (Figure 1). In August 2015, onion cv. Redwing and Vaquero were observed to have wet necrotic lesions developing on leaves in the field (Figure 2). Margins of necrotic tissue, 1-2 cm3, were excised, surface sterilized, plated on water agar medium and incubated at 24°C. Hyphal growth was sub-cultured from eight strains (A- D in 2014; E-H in 2015) to fresh potato dextrose agar to obtain pure cultures. Cultures were characteristic of Fusarium species as described by Nelson et al. (1983) with the presence of microconidia formed on polyphialides with macroconidia present. Primers ITS4-A1 and ITS5 primers (White et al. 1990); EF-1 and EF-2 (O'Donnell et al. 1998); and fRPB2-5F and fRPB2-7cR (Liu et al. 1999) were used to amplify regions of the ITS, elongation factor 1-α and the second largest subunit of DNA-directed RNA polymerase II. Amplicons were sequenced and analyzed using BLAST (https://www.ncbi.nlm.nih.gov/) and in combination using Pairwise DNA Alignment and Polyphasic Identification (http://www.westerdijkinstitute.nl/Fusarium/DefaultInfo.aspx?Page=Home) as described by O'Donnell et al. 2015. Analysis indicated that these strains are Fusarium proliferatum, which is part of the F. fujikuroi species complex (O'Donnell et al. 1998). Similarity (99.5%) was observed in pairwise analyses and the polyphasic identification clustering to representative F. proliferatum strain NRRL 22944 and others. Sequences were submitted to Genbank and registered accession numbers are found in Table 1. To complete Koch's postulates, cv. Vaquero onion bulbs were surface sterilized and injected with 3 × 105 microconidia into the shoulder of each bulb. Five bulbs were inoculated for each isolate, placed in a mesh bag, and incubated at 30°C in the dark. Five bulbs injected with sterile water and five non-inoculated bulbs served as controls. After 14 days, each bulb was sliced vertically down the center and inspected for rot. All eight strains induced tan to light brown necrotic rot symptoms in each inoculated bulb. No symptoms were observed for the water inoculated and the non-inoculated onion bulbs. A fungus was isolated from the necrotic tissue and confirmed to be F. proliferatum as described above. Ten µl aliquots containing 1 × 105 microconidia of F. proliferatum strains (C, E-H) were applied to leaves in triplicate of 12-week-old onion plants (cv. Vaquero) wounded with a 21-gauge needle. Water controls were included. Within three days lesions, with light chlorosis, began to form and quickly spread on the leaves. A fungus was isolated and confirmed to be F. proliferatum as described above. This is the first extensive description and identification of F. proliferatum causing bulb rot in storage in Idaho (Mohan et al. 1997). In addition, this is the first report of the fungus causing leaf infection in the field. These findings confirm F. proliferatum as the causal agent of the high incidence of bulb rot observed in 2014 and 2015. This bulb rot continues to occur in southwestern Idaho and since the pathogen can cause leaf infections growers are encouraged to be vigilant for both leaf lesions during the growing season and bulb rot in storage.