Quinoa (Chenopodium quinoa Willd.) is a traditional food originally from the Andes Mountains in South America. It was first planted in China in 1987 and is grown in Tibet, Gansu, and Qinghai provinces. In May 2021, 40% of 2-month-old quinoa plants in the 3.4 hm² experimental base of Qinghai University (36.7262° N, 101.7487° E) were found to have leaves with grey-brown subcircular spots (about 0.4 to 0.7 cm) with black dots (acervuli). Severely infected plants exhibited symptoms such as withered and stunted growth. The diseased-healthy junctions of infected leaves (0.5 cm) were cut out, disinfected with 3% NaClO for 1.5 min, washed three times with sterile water, dried, placed on water agar, and incubated at 25°C for 48 h. After sporulation was seen on the leaf surface, spore suspensions were prepared by placing conidia in sterile water using a pipette. Next, 200 μl of each spore suspension was spread on the surface of water agar and incubated at 25°C for 12 h. Single spores were selected under a stereomicroscope and cultured on potato dextrose agar (PDA) (Qi et al. 2022). The mycelium of two representative isolates (20DLMF-5-4-1 and 20DLMF-7-4-1) was grey-black with white edges and included a fluffy aerial mycelium. Conidia were unicellular, colorless, long ellipsoid or curved moon shaped, averaging 14.3 × 1.8 to 20.2 × 2.2 μm (n=100). The light brown appressoria were ovoid, averaging 8.5 × 5.2 to 7.7 × 4.1 μm (n=20). Spherical, dark brown acervuli were observed on the leaves, averaging 160 to 200 μm (n=20), and there were dark brown spiny bristles. The ITS, partial ACT, CHS, GAPDH and TUB2 genes were amplified from genomic DNA of the two isolates (Weir et al. 2012). Sequences were deposited in GenBank (accession no. OQ871595 to OQ871602 for ACT, CHS, GAPDH, and TUB2, and OQ860235 to OQ860236 for ITS) and showed over 99% identities with the corresponding sequences of C. spinaciae CBS125347 and CBS128.57 (Vu et al. 2019; Damm et al. 2009). Both isolates clustered with the type culture of C. spinaciae (CBS125347, CBS128.57), with 100% bootstrap support in the phylogenetic tree. Thus, according to the morphological and molecular characteristics, the two isolates were identified as C. spinaciae. Pathogenicity tests were conducted on 24 healthy, tender leaves of six 1-month-old quinoa plants, with three replicates (Yang et al. 2021). The leaves were gently scratched in 3-4 areas with a sterile needle. A conidial suspension (105 conidia/ml) of the two isolates was sprayed on these wounds. The control group was unscratched and sprayed with sterile water. The plants were incubated in a greenhouse at 25°C for 24 h in the dark and 7 days in the light. Tiny grey-brown spots appeared on day 3 (about 0.4 to 0.6 cm) and continued to enlarge until perforations and ruptures developed on day 7. Subsequently, acervuli were observed on the surface of the leaves. The control leaves remained healthy. Isolates were reisolated from the symptomatic leaves and they had the same morphological and molecular characteristics as the original isolates, confirming Koch's postulates. To our knowledge, this is the first report of C. spinaciae causing quinoa leaf anthracnose in China. C. spinaciae seriously affects the yield and quality of quinoa and has been previously reported to cause anthracnose of Vicia sativa in China (Wang et al. 2019). The results provide a basis for the study and control of quinoa leaf anthracnose.
Keywords: Colletotrichum spinaciae; anthracnose; quinoa.