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First Report of Downy Mildew of Quinoa Caused by Peronospora farinosa f. sp. chenopodii in Denmark

Downy mildew, caused by Peronospora farinosa (Fr.) Fr. f. sp. chenopodii Byford, is the most important disease of quinoa (Chenopodium quinoa Willd.) in the high mountainous areas of the Andes in South America (1), where the crop originated. While quinoa is not well known outside its traditional growing area, interest in this crop is increasing rapidly due to its high nutrient quality, many uses for human consumption, and agronomic characteristics of the plant; it is one of the most drought, salt, and frost tolerant crops known (2). From 1990 to 2001, several field trials were conducted in Denmark to study the performance of quinoa germ plasm of different origin under Danish conditions. Natural infection by downy mildew was observed each year on a range of cultivars, as well as on the common weed species, C. album L., which was observed growing near the quinoa. In 2001, 25 isolates of P. farinosa were collected from a field trial at the experimental station of the Royal Veterinary and Agricultural University in Taastrup, Denmark, from two Dutch quinoa cultivars (Carmen and Atlas) and one Danish breeding line (G205). Plants showed typical downy mildew symptoms: chlorotic lesions on the upper leaf surface and grayish spore masses on the lower leaf surface. Microscopic examination of the material showed the presence of dichotomously branched sporangiophores (200 to 400 μm long), which tapered to a blunt point and produced ellipsoidal, light brown sporangia with mean dimensions of 22 × 18 μm. The isolates were propagated and maintained by inoculation of detached quinoa leaves of two highly susceptible Peruvian cultivars (Toledo and Pandela) with a sporangium suspension (10⁵ sporangia per ml), followed by incubation on water agar plates (10 to 15 leaves per plate) at 15°C and 12 h light/dark for 9 to 11 days. All isolates grew readily on these cultivars, and microscopic examination of the pathogen showed the same morphology as the original isolate. When the isolates were tested for their virulence on a range of quinoa cultivars with different geographic origin, specific interactions between host genotype and pathogen isolate were observed, measured as the degree of sporulation in the detached leaf assay, suggesting the presence of several pathotypes. To our knowledge, this is the first report of P. farinosa on quinoa in Denmark. Downy mildew should be considered as a potential problem for future large-scale quinoa production in Europe.

References: (1) S. Danielsen. (Abstr.) Phytopathology 90(suppl):S17, 2000. (2) N. W. Galwey. Biologist 36:267, 1989.