Seed transmission of downy mildew (Peronospora farinosa f.sp. chenopodii) in quinoa and effect of relative humidity on seedling infection

Abstract

Peronospora farinosa f.sp. chenopodii causes downy mildew in quinoa, an ancient, nutritious seed crop from the Andean highlands. Downy mildew is the most important disease of quinoa in the Andes. Hundred and twenty-eight seed samples of quinoa from different provinces in Peru were tested for the presence of oospores. Oospores were found in 19 seed samples and ultramicrotome cuts of fixed seeds showed that the oospores are located in the pericarp. Seeds of the oospore-infected samples were sown in the green house at two levels of relative humidity. There was a significant effect of humidity on the degree of visible seedling infection after 21 days. At the lower humidity, heavy sporulation in cotyledons of 12 samples was observed (0.20-0.85% seedling infection). At the higher humidity, sporulation was found in cotyledons of 18 samples (0.21-8.83% seedling infection). There was a significant difference in visible seedling infection among samples at high but not at low humidity. No correlation was found between degree of seed and seedling infection, nor between oospore density and seedling infection, which may be due to varying degrees of latent infection and differences in viability and/or physiological maturity of the oospores. It is concluded that downy mildew of quinoa can be seed transmitted.