In vitro testing of organic spinach seed for viable oospores of Peronospora effusa and germination of seeds to determine any possible seedling infection

The most destructive disease of spinach (Spinacia oleracea L.) in the U.S., and the most common in California and Arizona is downy mildew (Peronospora effusa (Grev.) Rabenh.). However, in the Northeast, from NY to MI downy mildew outbreaks were infrequent until the first outbreaks in commercial plantings in 2014. This research was undertaken to determine if contaminated seed could be a source of such outbreaks in Northeast areas. We examined organic spinach seed lots for presence of oospores and their viability in leftover seeds from two commercial sources supplied by growers: three remnant packets of the cultivar (‘Corvair’) from of two seed lots from 2018 and 2022 which had developed disease in grower fields. Results found that 0-4% of seeds were infested with oospores (a total of 6000 seeds examined). A total of 1,471 oospores embedded in pericarp tissue had 0 to -48% plasmolyzing, indicating a living protoplasm (viable oospores) in 4 M sodium chloride treatment. The second seed samples were purchased directly from seed companies and included seeds of 12 cultivars from 8 suppliers (totaling over 34,000 seeds examined). Eleven seed lots of six cultivars (‘Bloomsdale’ ‘Equinox’, ’Regiment’, ‘Renegade’, ‘Space’ and ‘Viroflay’) were free from visible oospores. In 17 seed lots from seven cultivars, oospores were visible on > 7% of seeds. In one sample, 57% of 96 oospores plasmolyzed in NaCl. A comparison of percent germination for infested and non-infested seed showed no significant differences.