Plasmolysis and Vital Staining Reveal Viable Oospores of <i>Peronospora effusa</i> in Spinach Seed Lots

Production of oospores by Peronospora effusa, the causal agent of downy mildew on spinach (Spinacia oleracea), was reported on spinach seed over three decades ago. In view of the rapid proliferation of new races of P. effusa worldwide, seedborne transmission of this pathogen has been suspected but methods to test the viability of seedborne oospores have not been available. Eighty-two seed lots of contemporary spinach cultivars were evaluated for the presence of P. effusa using a seed-wash method and the sediment was examined by microscopy. Of the analyzed seed lots, 16% were positive for oospores and an additional 6% for sporangiophores characteristic of P. effusa. Application of a P. effusa-specific quantitative polymerase chain reaction assay showed that 95% of the 59 tested seed lots were positive for P. effusa. The viability of oospores from five seed lots that were proven to carry the pathogen from the above tests was tested using two independent methods, one involving plasmolysis and the other trypan blue staining. The oospores plasmolyzed in 4 M sodium chloride and were deplasmolyzed in water, demonstrating an active and viable cell membrane. Similarly, viable oospores failed to take up the trypan blue stain. Overall, 59% of the oospores were viable in the plasmolysis test and 45% with the trypan blue test. These results indicate the presence of P. effusa oospores in contemporary spinach seed lots, and suggest that the transmission of viable oospores of P. effusa in spinach seed does occur. Elimination of the pathogen on seed, in addition to other management approaches, will be useful in reducing the extent and severity of downy mildew on spinach crops and diminishing pathogen spread through seed.