First Report of Passalora Leaf Spot of Quinoa Caused by Passalora dubia in the United States
Authors: Testen A. L. , McKemy J. M. , Backman P. A. 
Published: August 2012

The Andean seed crop quinoa, Chenopodium quinoa Willd., is an important export of Bolivia, Ecuador, and Peru. Key foliar diseases of quinoa include quinoa downy mildew (caused by Peronospora variabilis Gäum) (1), Ascochyta leaf spot (caused by Ascochyta sp.) (1), and a Cercospora-like leaf spot, the latter of which has been observed on cultivated quinoa (Jose B. Ochoa, unpublished) and native Chenopodium species. Passalora dubia (Riess) U. Braun (syn. Cercospora dubia) was tested in Europe as a biological control agent for Chenopodium album (3) and has been reported on C. album in the United States (U.S. National Fungus Collections). Quinoa field plots were established in Pennsylvania during summer 2011 and Cercospora-like leaf spot symptoms were first observed on quinoa in Centre Co. and Lancaster Co. in August 2011, after an extended rainy period. Foliar symptoms were round to oval, brown to grey-black lesions, less than 1 cm in diameter, with darker brown, reddish margins. Similar symptoms were observed on C. album weeds within both fields. Using a hand lens, conidia were observed within sporulating lesions. Conidia were hyaline and septate, 25 to 98 μm × 5 to 10 μm, and had an average of six cells per conidium. The fungus was isolated by picking single conidia from sporulating lesions (under a dissecting scope) and incubated on V8 agar in the dark at 20°C to induce sporulation. For DNA extraction, cultures were grown in potato dextrose broth amended with yeast extract. The internal transcribed spacer (ITS) region was amplified using primers ITS4 and ITS5 (2), and the resulting sequence shared 99% maximum identity with a vouchered isolate of P. dubia (GenBank EF535655). To test the pathogenicity of our P. dubia isolate, 5.9 × 103 conidia/ml (suspended in sterile water with 0.1% Tween 20) or the control solution with no conidia were sprayed, using an atomizer, onto 2-month-old quinoa plants, with 18 replications per treatment. Plants were covered with a humidity dome and maintained at >99% RH for 48 h. Plants were grown in the greenhouse at approximately 65% RH. After 1 month, circular to oval light brown lesions (<1 cm diameter) with darker margins were observed on approximately 10% of the leaves of inoculated plants, whereas no symptoms were observed on the control plants. Infected leaves were collected, incubated in a humidity chamber, and conidia were picked from sporulating lesions and inoculated onto V8 agar amended with 3% (w/v) fresh, ground quinoa plant tissue (4). Cultures were maintained at 20°C with 16-h photoperiod to induce sporulation. The identity of the reisolated fungus was confirmed morphologically and by DNA sequencing to be identical to the isolate used to test Koch's postulates. P. dubia was also isolated from C. album lesions and infected C. album may have served as a source of inoculum for quinoa. To our knowledge, this is the first report of Passalora leaf spot of quinoa in the United States.

References: (1) S. Danielsen. Food Rev. Int. 19:43, 2003. (2) S. Goodwin et al. Phytopathology 91:648, 2001. (3) P. Scheepens et al. Integ. Pest. Man. Rev. 2:71, 1997. (4) M. Vathakos. Phytopathology 69:832, 1979.