First Report of Ascochyta Leaf Spot of Quinoa Caused by Ascochyta sp. in the United States
Authors: Testen A. L. , McKemy J. M. , Backman P. A. 
Published: January 2013

The Andean crop quinoa (Chenopodium quinoa Willd.), an amaranthaceous pseudograin, is an important food and export crop for this region. Quinoa is susceptible to Ascochyta leaf spot reportedly caused by Ascochyta hyalospora and/or A. caulina (1,2), and quinoa seeds can be infested by A. hyalospora (3). Quinoa fields were established in Pennsylvania during summer 2011. Widespread leafspot symptoms were observed on quinoa in mid-August 2011 in Centre County, PA. Tan to reddish-brown, irregularly shaped lesions were observed with numerous black pycnidia randomly distributed within each lesion. Crushed pycnidia revealed sub-hyaline to light brown, 1 to 2, or less often 3 septate, cylindrical to ovoid spores, 13 to 25 μm long by 5 to 10 μm wide. Pure cultures of Ascochyta were obtained by plating pycnidia from surface disinfested leaves onto half strength acidified potato dextrose agar (APDA). To obtain conidia for pathogenicity trials, cultures were transferred to oatmeal agar and placed in a 20°C incubator with a 12-h photoperiod. Conidia were harvested by scraping 2-week-old cultures. The conidial suspension was filtered through cheesecloth and adjusted to 1.8 × 105 conidia/mL. Tween 20 (0.1%) was added to the final inoculum and sprayed (with a Crown Spra-tool) onto ten 1-month old quinoa plants. Six plants sprayed with sterile water with 0.1% Tween 20 served as controls. Plants were placed in a growth chamber and bagged for 48 h to maintain >95% humidity. After 48 h, tan, irregularly shaped lesions were observed on inoculated plants, but no symptoms were observed on control plants. Plants were grown for 2 more weeks to observe symptom development, and then leaves with characteristic lesions were collected for isolation. Symptomatic leaves were surface disinfested in 10% bleach for 1 min and tissue from the lesion periphery was plated onto APDA. Obtained cultures were morphologically and molecularly identical to those obtained from quinoa fields. For molecular identification of the pathogen, DNA was extracted from cultures of Ascochyta and amplified using ITS4 (TCCTCCGCTTATTGATATGC) and ITS5 (GGAAGTAAAAGTCGTAACAAGG) primers. Sequences obtained shared 99% maximum identity with a GenBank accession of A. obiones (GU230752.1), a species closely related to A. hyalospora and A. caulina (4). However, the obtained pathogen is morphologically more similar to A. hyalospora and A. chenopodii, but not to A. caulina or A. obiones. At this time, final species identification is impossible because no GenBank sequence data is available for A. hyalospora or A. chenopodii. To our knowledge, this is the first report of Ascochyta leaf spot of quinoa in the United States. The impact of Ascochyta leaf spot on domestic and global quinoa production is unknown, but management of foliar diseases of quinoa, including Ascochyta leaf spot, is a critical component of any disease management program for quinoa.

References: (1) S. Danielsen. Food Rev. Int. 19:43, 2003. (2) M. Drimalkova. Plant Protect. Sci. 39:146, 2003. (3) G. Boerema. Neth. J. Plant. Pathol. 83:153, 1977. (4) J. de Gruyter. Stud. Mycol. 75:1, 2012.