Several occurrences of downy mildew in commercial plantings of spinach in winter tunnels in the Northeast since 2014 were the impetus for research undertaken by Nina Shishkoff (USDA, Maryland) in collaboration with Margaret McGrath (Cornell University).
Initial work was with seed obtained from growers that was leftover from plantings where downy mildew developed. Oospores were observed on some seed. Their viability was found to be very low as determined by treating them with 4M NaCl and using ability to plasmolyze as indicator of whether each oospore examined was alive. Considering the low viability of the oospores, it is not surprising that downy mildew did not develop on seedlings grown from any of the seed.
In January 2023, organic seed was purchased of a variety that had been associated with several outbreaks. An abundance of oospores were observed on the seed. A high percentage were viable. And a seedling grown from the seed became systemically infected. DNA sequencing was done to confirm it was Peronospora effusa. Subsequently organic seed was obtained of other varieties. One had some contaminated seed in the lot, but they had fewer oospores than the first variety examined, while no oospores were found on the other three varieties. Research is on-going to obtain better understanding of how commonly this occurs.
These results add additional confirmation that the spinach downy mildew pathogen is seed-borne.
A poster about this research was presented at the 2023 Potomac Division APS meeting.
Following photographs are from Nina Shishkoff.
