First report of quinoa anthracnose caused by Colletotrichum nigrum and C. truncatum in the United States

Authors: Pal, Narinder; Testen, Anna

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2020
Publication Date: 2/8/2021
Citation: Pal, N., Testen, A.L. 2021. First report of quinoa anthracnose caused by Colletotrichum nigrum and C. truncatum in the United States. Plant Disease. 105(3):705. https://doi.org/10.1094/PDIS-07-20-1568-PDN.
DOI: https://doi.org/10.1094/PDIS-07-20-1568-PDN

Interpretive Summary: Quinoa is an important, highly nutritious crop native to the Andean Region and is quickly being grown in new regions. A new quinoa disease, quinoa anthracnose, was identified at the North Central Regional Plant Introduction Station, part of the National Plant Germplasm System. This disease is caused by two pathogens, Colletotrichum truncatum and Colletotrichum nigrum, and could limit quinoa production. Identifying the cause of this disease is the first step to identifying disease management tools to reduce damage from this pathogen.

Technical Abstract: Quinoa (Chenopodium quinoa Willd.) is increasingly produced outside its native Andean range. In September 2019, stem lesions were observed on six plants of PI 510547 (25% severity) and PI 596293 (75% severity) in a demonstration planting at the North Central Regional Plant Introduction Station in Ames, IA. Stem lesions were bleached, silvery-white to gray, slightly sunken, oval to linear with slightly tapered tips and contained setose acervuli. Fungi were isolated from stems following surface disinfestation with 70% ethanol and plating onto ½ acidified potato dextrose agar (APDA) and V8 medium. On V8 medium, CQ1 produced sparse, flat, gray mycelia with profuse sclerotia and hyaline, aseptate, cylindrical conidia (21.0 (19.2-24) by 4.3 (2.4-4.8) µm). CQ2 produced fluffy, gray to dark gray mycelia with profuse sclerotia and acervuli and hyaline, aseptate, falcate conidia (26.8 (24-31.2) by 2.4 µm). Direct hyphal PCR was used to amplify ITS (ITS1/ITS4), ACT(ACT-512F/ACT-783R), GAPDH (GDF1/GDR1), CHS-1 (CHS-79F/CHS-234R), and TUB2 (T1/Bt-2b, CQ1 only) (Fu et al. 2019, Liu et al. 2013), and products were sequenced bidirectionally (MT772082-3, MT786524-30). A maximum likelihood tree was generated in MEGA X (Kumar et al. 2018) from a multiple sequence alignment of vouchered CBS isolates (Liu et al. 2013) and CQ1 was identified as Colletotrichum nigrum. CQ2 sequences showed 99-100% similarity to Colletotrichum truncatum sequences in Genbank (MN581860, MK675238, MF682518, MK118057). Two isolates each of colony morphology were grown on V8 medium under 12 hours of near UV light for two weeks. Approximately 5 mL of agar medium with hyphae was sterilely removed and macerated in 6 mL of sterile distilled water to create a slurry. Non-inoculated medium was macerated in sterile water as a control. Forty-day old PI 634920 quinoa seedlings were inoculated by making three, 2-3 mm incisions between the cotyledons and first true leaves with a sterile razor blade. Following wounding, 500 µL of slurry was placed on a 2.54 cm2 piece of sterile cheesecloth and placed against the wound and wrapped with Parafilm. Six plants were inoculated per isolate and control. After two weeks, sunken, bleached to tan areas extended past the initial wound sites on stems of inoculated plants. No discoloration or sunken tissue was observed on control plants. Plants were tented with plastic film for one week. Acervuli were observed on C. truncatum- and C. nigrum-inoculated stems, and sclerotia were observed on C. nigrum-inoculated stems. Stems were surface disinfested with 10% bleach for 15 seconds and 3-5 mm pieces were plated onto ½ APDA. Colony morphologies matched those of the original inoculum for every inoculated plant, except for control plants from which Colletotrichum was never isolated. At the same time stems were inoculated, approximately 100 µL of slurry was placed on 3-5 detached quinoa leaves in Petri dishes with moistened blotter paper and incubated for 48 hours at 25° C. Brown, circular lesions developed on leaves inoculated with Colletotrichum spp., but no lesions developed on control-slurry leaves. Colletotrichum spp. cause diseases in quinoa relatives including spinach (Kurt 2015), beets (Gourley 1966) and amaranth (Wu 2001). To our knowledge, this is the first description of Colletotrichum spp. causing stem lesions on quinoa. This disease may emerge in new quinoa production regions with conducive climates, but its current impact on quinoa production is unknown.