First Report of Anthracnose Leaf Spot Caused by Colletotrichum americae-borealis on Oat in China.

Abstract

Oat (Avena sativa), an important source of crude protein, fiber and crude fat needed for livestock, is increasingly being cultivated in China. In August 2020, leaf spots were observed on oat cv. Bayan 7 plants in an experimental field of 3 ha with a disease incidence of 12% and a severity of 20% in Daqiao Town (26.42°N, 103.22°E), Huize County, Yunnan Province, China. Symptoms mainly consisted of reddish brown spots which were oval to irregularly circular in shape and to 3 to 4 mm in diameter (occasionally confluent). The spots were surrounded by chlorosis involving the affected leaf surfaces, due to the spread of the disease. Infected tissues (10 diseased leaves) of ten different plants were cut into small pieces (5×5 mm) at the junction of disease and healthy tissues, surface-sterilized in 75% ethanol solution for 30 s, 1% NaClO solution for 2 min, rinsed three times with sterilized distilled water, air dried, and transferred to potato dextrose agar (PDA) plates. A total of nine single-spore isolates with similar colony morphologies were obtained. After 7 days, the colonies showed white-to-gray aerial mycelia, and appeared dark brown at the center of the reverse view. Setae were brown, smooth walled, cylindrical at base, rounded at tip, containing 1 to 4 septum, and measured 50 to 190 μm long. Conidia are hyaline, aseptate, cylindrical to spindle, straight to slightly curved, rounded at both ends, and measured 14.5 to 17.5 × 3.6 to 4.1 μm in size (n = 50). Based on the morphological characteristics, the fungal pathogen was identified as being closely related to Colletotrichum americae-borealis (Damm et al. 2014). To further substantiate the identification, the ITS region of rDNA, beta-tubulin (TUB) gene, chitin synthase (CHS) gene and histone H3 (HIS3) gene were amplified by the primers described previously (Damm et al. 2014) and sequenced. Sequences of the representative isolated strain YNC-1 were deposited in GenBank (MZ676033 for ITS, MZ686211 for TUB, MZ686214 for CHS, MZ686217 for HIS3). BLAST analysis showed 99 to 100% identity to C. americae-borealis type strain CBS 136232 (GenBank Accessions: KM105224, KM105504, KM105294 and KM105364, respectively). The four locus datasets were combined by SequenceMatrix 1.8 (Vaidya et al. 2011), and the strain YNC-1 and C. americae-borealis (CBS 136232 and CBS 136855) formed a subclade with 97% bootstrap support. To test pathogenicity, five pots with each containing five 5-week-old oat seedlings (cv. Baiyan 7) were sprayed with the YNC-1 isolate conidial suspension (1 × 105 conidia/ml) while the other five pots were sprayed with sterile water as a control. All plants were kept for 48 h at 100% RH, and then moved to a nursery shelf in a greenhouse maintained at 26/18°C day/night temperatures. After 5 to 7 days post inoculation, reddish brown spots were observed on the inoculated plants and C. americae-borealis was reisolated and confirmed by morphological and molecular features. No disease was observed on control plants. To our knowledge, this is the first report of C. americae-borealis causing leaf spot on oat in China. C. americae-borealis has been reported associated with leaf spot in alfalfa (Li et al. 2021) and licorice (Lyu et al. 2020). The disease can cause a serious threat to oat crops in China and disease management strategies are needed to control its proliferation.