Occurrence of Crown Rot Caused by Fusarium pseudograminearum in Oat (Avena sativa) in China.

Oats (Avena sativa) are an important fodder crop for grazing livestock in northern and northwestern China (Chen et al. 2021; Yang et al. 2010). In May 2019, a crown rot disease was observed in a field where oat has been grown continuously for 5 years in Yongchang County (37.52° N, 101.16° E), Gansu Province, with an average disease incidence of 3%. Affected plants were stunted and displayed crown and basal stem rot. The basal stem showed chocolate brown discoloration and several basal stems appeared slightly constricted. Three disease plots were surveyed and at least 10 plants were collected from each plot. Infected basal stems were disinfested by immersion in 75% ethanol for 30 s and 1% NaClO for 2 min, rinsed three times in sterilized water. Then they were placed on potato dextrose agar (PDA) medium, incubated at 20°C in the dark. Isolates were purified by single spore cultures (Leslie and Summerell, 2006). Ten monosporic cultures were consistently isolated with similar phenotypes. Then those isolates were transferred onto carnation leaf agar (CLA) and incubated at 20°C under blacklight blue lamps. On PDA, isolates produced abundant aerial mycelium, densely floccose, reddish-white to white with deep-red to reddish-white reverse pigmentation. On CLA, macroconidia of the strains were formed in sporodochia, but no microconidia were found. Macroconidia (n=50) were relatively slender, curved to almost straight, commonly 3-7 septate, 22.2 to 43.7 × 3.0 to 4.8 μm (averag 28.5 × 3.9 μm). The morphological characteristics of this fungus fully fit the description of Fusarium species (Aoki and O'Donnell, 1999). For molecular identification, total genomic DNA of representative strain Y-Y-L was extracted by using an HP Fungal DNA Kit (D3195), and the elongation factor 1 alpha (EF1-α) gene and RNA polymerase II second largest subunit (RPB2) gene were amplified using primers EF1 and EF2 (O'Donnell et al. 1998) and RPB2-5f2 and RPB2-7cr (O'Donnell et al. 2010), respectively. The sequences were deposited in GenBank (Accession numbers: OP113831 for EF1-α and OP113828 for RPB2). A nucleotide BLAST search revealed RPB2 and EF1-α sequences to be 99.78% and 100% similar to the corresponding sequences of the ex-type strain NRRL 28062 Fusarium pseudograminearum accessions numbers MW233433 and MW233090, respectively. In the maximum-likelihood phylogenetic tree, three Chinese strains (Y-Y-L, C-F-2 and Y-F-3) were grouped with the reference sequences of F. pseudograminearum with a high bootstrap supporting values of 98%. For pathogenicity tests, millet seed-based inoculum of F. pseudograminearum was prepared using a modified procedure (Chen et al. 2021). Healthy oat seedlings (4 weeks old) were transplanted into plastic pots containing pasteurized potting mix infested with millet seed-based inoculum of strain Y-Y-L F. pseudograminearum at a rate of 2% by mass fraction. Control seedlings for comparison were transplanted into pots containing potting mix without inoculum. Each treatment was inoculated with 5 pots each, with 3 plants per pot. Plants were maintained for 20 days under greenhouse conditions at 17 to 25℃ with all the inoculated plants developing symptoms similar to those observed in the field, while control plants remained healthy. The fungus was re-isolated from the basal stems of inoculated plants, and confirmed phenotypically and molecularly as F. pseudograminearum. F. pseudograminearum has been reported associated with crown rot in oat in Tunisia (Chekali et al. 2019). To our knowledge, this is the first report of F. pseudograminearum causing crown rot in oat in China. This study provides a basis for identifying pathogens causing oat root rot and managing the disease.