Outbreak of Choanephora Stem Rot Caused by Choanephora cucurbitarum on Quinoa (Chenopodium quinoa) in China.

Abstract

Quinoa (Chenopodium quinoa Willd.) has been cultivated for centuries as an important subsidiary grain crop. In early August 2015 and 2016, Choanephora stem rot symptoms were observed on plants of Quinoa cultivar Longli 1 in a field with 0.3 ha planting area in Yanqing district, Beijing, China. This disease caused about 70% yield loss of quinoa. Disease symptoms initially appeared as water-soaked spots on the stem that turned into brown to dark brown lesions covering the whole stem, often resulting in the infected part breaking off the rest of the plant. Serious infections often caused the plant to wither and die under high humidity. The infected tissues produced an abundance of sporangiophores bearing brown to black sporangiola. To isolate the causal agent, sections of diseased stem were surface disinfected with sodium hypochlorite, placed on acidified potato dextrose agar (APDA), and incubated at 25°C with a 12-h photoperiod. Two isolates were obtained (ZD61 and ZD62). The colonies of both isolates grew quickly on PDA and were initially white with abundant aerial mycelia. The mycelia later turned pale yellow and produced abundant sporangiophores bearing sporangiola. Sporangiophores were smooth, hyaline, and aseptate, erect, unbranched, and apically dilated to form a clavate vesicle, which produced dichotomously branched and distally clavate secondary vesicles bearing monosporous sporangiola. Monosporous sporangiola were broadly ellipsoid, indehiscent, pediculate, brown to dark brown, with distinct longitudinal striations, and were 7 to 15 μm wide and 11 to 24 μm long by measuring 30 sporangiola. Sporangia contained few or multiple spores. They were also spherical, initially white to yellow, and pale brown to dark brown at maturity and measured 63 to 160 μm. Single-celled sporangiospores were aseptate, ellipsoid, and brown to pale brown, with multiple hyaline polar appendages and striations on the wall. They were 6 to 11 μm wide and 9 to 22 μm long by measuring 50 spores. The morphological and cultural characteristics of both isolates were identical to those of Choanephora cucurbitarum (Berk. & Ravenel) Thaxt. (Kirk 1984). Genomic DNA was extracted from the isolates with the DNA Isolation Kit (Sangon Biotech, Shanghai, China). The internal transcribed spacer (ITS) regions of the two isolates were sequenced using primers ITS1 and ITS4 (White et al. 1990). The two resulting sequences were submitted into NCBI (GenBank MG650193 and MG650194), which were 99% identical to known C. cucurbitarum strains (GenBank KY080447, KY080446, KU877802, and KR873353) according to a BLAST analysis. Pathogenicity tests were conducted on hypocotyls of 4-week-old quinoa seedlings inoculated with 2-day-old mycelia plugs. Control plants were inoculated with sterile APDA plugs. Plants were incubated under humid conditions for 3 days and then transferred to a greenhouse (28°C). After 4 to 5 days, lesions were evident on the stems of inoculated plants but not on control plants. After 7 days, all inoculated plants had developed brown stem lesions and were wilted. Additionally, C. cucurbitarum was reisolated from the inoculated plants, fulfilling Koch’s postulates. The fungus C. cucurbitarum reportedly has a wide host range. In China, C. cucurbitarum was recently observed on Moringa oleifera causing seed pod rot (He et al. 2017). However, to our knowledge, this is the first report of C. cucurbitarum causing stem rot on C. quinoa in China or globally.