The 2022 potato crop in both PEI and New Brunswick contained higher levels of PVY compared to previous years, Dr. Mathuresh Singh told growers at the recent PEI Potato Conference in Summerside.
"Over the years, pass-rates of seed lot samples in Post-harvest testing for PVY (PHT) have been generally trending higher," said the researcher with Agricultural Certification Services Inc. in Fredericton.
He explained PVY levels that qualify are different in NB and PEI, with this province having a passing cap of three per cent in post harvest testing. He explained PVY levels were far higher than experienced for at least a decade, causing a substantially lower pass-rate in both provinces.
Dr. Singh told the industry meeting similar PVY problems have also occurred in Maine and other Canadian provinces over the last several years. He suggested a combination of several factors may have caused the higher numbers in the Maritimes including what was planted in the field, aphids acting as vectors for PVY transmission, or changes either in the virus or the management techniques of growers.
"Over the last decade, PVY inoculum planted in the field has steadily declined," he went on to say. "During this time, tighter limits on maximum PVY in marketed seed, price premium of cleaner seed, and value recognition by growers reduced PVY."
Even though mean PVY in planted crop was similar to the preceding year, the guest speaker noted "that hides a large 2022 increase in seed at elevated PVY levels in larger fields . He explained that in fields greater than 28 hectares, there were much higher PVY levels than 2021.
"Weighted for field size (proportional to harvested tubers) high PVY in large fields is far more dangerous," Dr. Singh said.
As well, he reminded growers that published reports show the retail market for private gardens can be a substantial source of PVY, even though the scale is small. PVY was discovered in a private garden in NB in 2021, leading to a small survey of retail seed potatoes The 858 seed potatoes tested over five lots were sourced from retail stores that supplied private garden with all of the seed coming from PEI. In three of the five lots, there was greater than four percent , ranging from 0% to 19% He noted 10.7% of all tubers tested had PVY.
He noted research in New Brunswick last year showed disproportionately high PVY levels in Caribou Russet and Russet Norkotah slightly higher levels in Goldrush and low PVY in Chieftain, Innovator, Shepody, Snowden and Superior.
"The largest component of PVY is Russet Burbank because it is the greatest acreage," he said.
He said planted PVY and the variability of the different varieties was not substantially different last year compared to previous years. Aphids transmit PVY from one potato plant to another, and their abundance and species composition varies from year to year.
He noted aphids peak in two periods, early at the end at the end of June and in mid-August , He noted colonizing species that are most efficient at PVY transmission arrive later in the year. He noted 2022 was an exceptional year as 2022 with the late peak (early August) higher than ever observed before.
Dr Singh explained weekly aphid captures in NB and PEI are very different with New Brunswick showing 10 times the aphids captured per week compared to PEI. However he said there was a strong shared rise in aphids late in season only in 2022, and only due to Green Peach Aphid.
He explained this year does not look promising with a warmer winter than usual likely meaning higher survival of aphids, Dr. Singh said many aphids also developed a resistance to insecticides. Dr. Singh said the PVY virus is also changing.
At least four strains of PVY occur in potato fields that show different rates of spread, have distinct visual symptoms and effects on tuber yield and quality: • PVYO – the “ordinary” traditional strain • PVYN:O and PVYNTN – newer recombinant strains that became dominant last decade and PVYN-Wi – another recombinant genetically similar to N:O only recently discovered in New Brunswick.
"Different species of aphids spread strains of PVY with different efficiency," he explained. "Depending on the virus strain (or even different isolates of the same strain [grey bars]) and the species of aphid, efficiencies of PVY infection can vary by >10 times."
The guest speaker also noted management intensity varied between growers and correlated with PVY. Growers that focus mostly or exclusively on seed production did well, as did producers that used higher oil concentrations, more frequently .
"Growers using diverse insecticides together with oil did well," he concluded. "Growers with shorter season crops, and top kill early did well • Everyone dealt with unusually high aphids in 2022, some kept PVY low."
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