Nitrogen dioxide treatment reduces the viability of seedborne Verticillium dahliae and the transmission of Peronospora effusa in spinach.

The global spinach (Spinacia oleracea) seed trade has contributed to the spread of plant pathogens such as Verticillium dahliae and Peronospora effusa. Verticillium dahliae causes Verticillium wilt in a broad range of crops, including lettuce (Lactuca sativa), strawberry (Fragaria x ananassa), and others. Verticillium wilt occurs only in the spinach seed production fields and is not observed in the spinach production fields as the crop is harvested well before symptom manifestation. In contrast, P. effusa exclusively infects spinach in production fields causing destructive downy mildew. Managing these pathogens is challenging. Both pathogens survive in seeds, so efforts to minimize seed infestations are an important component of management. In this study, we examined nitrogen dioxide (NO2) and sulfur dioxide (SO2) treatments against V. dahliae and P. effusa in spinach seeds. Compared with untreated infested control seeds, treatments with 3% NO2 for three days significantly reduced viable seedborne V. dahliae infestation by 60.7% as observed in grow-outs from whole seeds and reduced viable V. dahliae colonies by 91.0% when seeds were ground and plated on the semi-selective NP-10 medium following seed treatment. The 3% NO2 treatment eliminated the transmission of downy mildew from oospore-infested spinach seeds. Notably, NO2 treatment also significantly enhanced seed germination. In contrast, SO2 treatments (0.25-0.5% for one hour) only reduced the viability of seedborne V. dahliae by 19.5%-24.7% but reduced seed germination by 7.0%-16.4%. These findings suggest that NO2 treatment has potential as an effective treatment for controlling V. dahliae and P. effusa in spinach seeds and for managing seedborne pathogens in other crops.