First report of Corn Ear Rot Caused by Fusarium asiaticum in China.

Corn (Zea mays L.) ear rot, caused by various pathogens, is one of the most significant diseases of corn worldwide. In September 2020, a survey was undertaken to identify pathogenic fungi associated with corn ear rot in Suihua city (46.63°N 126.98°E), Heilongjiang Province, China. The average disease incidence was 14.2% and 15.6% in each of two fields sampled (~5 ha) using a five-point method (100 plants/each point). Twenty tissue samples from 20 diseased ears, showing white or pink mold on the surface of corn ears, were surface disinfected in 0.5% NaOCl for 5 min, rinsed 3 times in autoclaved distilled water. After drying, four treated corn kernels (one kernel/each ear) were placed onto potato dextrose agar (PDA) amended with 50 µg/mL streptomycin. The plates were sealed with parafilm sealing film and cultured in the dark at 26℃ with 80% RH for 3 days in an incubator. A total of 12 morphologically similar fungal isolates were obtained and subcultured by transferring hyphal tips for 3-5 days. Single-conidium isolates were generated with methods reported previously (Leslie and Summerell 2006). Colonies on PDA, reaching 20.3-20.9 mm·d-1 at 26℃, consisted of white to pale yellow, locculent, lush and dense aerial mycelium with red to apricot color. Macroconidia of six isolates randomly selected on carnation leaf agar (CLA) were falciform with a foot cell, three- to six-septate, measuring 12.6-67.2 × 2.6-5.4 µm (n = 100) in the dark at 26℃ with 80% RH for 5 days. No microconidia were observed. Based on these characteristics, the isolates were preliminarily identified as Fusarium asiaticum (Chang et al. 2020; Leslie and Summerell 2006). Genomic DNA of three representative isolates YSF2, YSF4 and YSF7 were extracted and the translation elongation factor 1-α (TEF-1ɑ) gene was amplified and sequenced using the primers EF1-728F/EF1-986R (Carbone and Kohn 1999). The DNA sequences of YSF2, YSF4 and YSF7 were deposited in GenBank (OL631287.1, OP272129 and OP272130). Analysis of TEF-1ɑ sequences of YSF2, YSF4 and YSF7 showed that they were 100% identical to F. asiaticum isolates NRRL 26156 (AF212452.1) in NCBI and NRRL 13818 (AF212451.1) in Fusarium MLST. A pathogenicity test was performed on corn cv. Xinxin 1. Four days after silk emergence, 3 mL conidial suspension (106 macroconidia/ml) of each 12 isolates was individually injected into the center of the ear through the husk sideways, penetrating the kernels to a depth of about 5 mm (6 ears/each isolate) in the field (Guo et al. 2020). Six corn ears treated with sterile distilled water were used as the control. After inoculation, normal field management was carried out. All inoculated ears showed symptoms similar to those observed in the field 50 days after inoculation, while no symptoms were observed on the blank control ears. Five fungal isolates with the same colony morphology as the naturally occurring ear rot in the field were re-isolated from the inoculated corn kernels and confirmed to be F. asiaticum according to morphological characteristics and sequence analysis of five fungal isolates. F. asiaticum has previously been reported to cause corn ear rots in Japan (Kawakami et al. 2015). To our knowledge, this is the first report of F. asiaticum causing corn ear rot in Northeast China. Corn ear rot poses a threat to significantly reduce the quality of corn in a major production zone of maize in China. Therefore, its distribution needs to be investigated and effective disease management strategies developed.