Cause Peronospora variabilis (formerly Peronospora farinosa f. sp. chenopodii), a fungus-like microorganism that is seedborne. Spores produced on affected leaves are disseminated by splashing water. Cool, wet weather favors disease development.
Symptoms Yellow-pink areas develop on the upper leaf surface and grayish patches of sporulation develop on both surfaces of affected leaves. Affected parts turn brown, and leaves die. Atypical symptoms include lighter-green patches on the upper leaf surface, leaf cupping, and plant stunting.
Cultural control
- Plant seed that is free of the pathogen.
- Avoid overhead irrigation or apply such that plants are not wet for extended periods of time.
References Testen, A.L., McKemy, J.M., and Backman, P.A. 2012. First Report of Quinoa Downy Mildew Caused by Peronospora variabilis in the United States. Plant Disease 96:146.
Testen, A.L., Jiménez-Gasco, M.N., Ochoa, J.B., and Backman, P.A. 2014. Molecular detection of Peronospora variabilis in quinoa seed and phylogeny of the quinoa downy mildew pathogen in South America and the United States. Phytopathology 104:379-386.