Corn is the most important cereal crop in China. Over 34.94 million ha of corn is cultivated in the country annually. However, fungal diseases are a major limiting factor in corn production. In August 2008, 50 ha in several corn fields in Liaoning, Jilin, and Heilongjiang provinces were observed to be severely affected by a disease causing a yield loss of 30%. Results from field surveys suggested an epidemic during late corn growth stages that affected corn sheaths, causing irregularly circular spots with grayish brown to dark brown lesions. Lesions ranged from 2.5 to 3 × 3 to 5 cm. To isolate the causal agent, tissue was removed from the border of lesions and surface sterilized in 75% ethanol for 30 sec and 0.1% HgCl2 for 1 min. The sample was then triple rinsed in sterile distilled water. The isolate was purified and subcultured on potato dextrose agar (PDA) at 25 ± 2°C. The initial color of the mycelium was white, turning brown after being cultured for 7 days. A pale brown to dark brown pigment developed in the agar beneath the colony. Chlamydospores, solitary but also in short chains, measuring 7.2 to 15.3 μm, were produced on carnation leaf agar (CLA) after 10 days and became verrucose 20 days later. Macroconidia were produced on CLA in orange sporodochia from monophialides on branched conidiophores, usually 5- to 7-septate, and apical cells were tapered and elongate. Basal cells were prominent, foot-shaped, and elongated in appearance. Microconidia were not observed (1). These morphological characteristics matched the description of Fusarium equiseti reported by Leslie and Summerell (1). A pathogenicity test was conducted with an isolate from each of the 36 corn plants by spraying 2 ml of spore suspension (106 conidia/ml) on 45-day-old corn sheaths (cv. Huang Zao). For the control treatment, 36 corn plants were sprayed with an equal volume of sterilized water. Inoculated plants were placed in a greenhouse at 32 to 34°C and 95% relative humidity. Typical irregularly circular lesions were observed 7 days after inoculation, except in the control samples. Each treatment was replicated three times. The suspected pathogen was consistently re-isolated from diseased tissue according to Koch's postulates, and was found to be morphologically similar to F. equiseti. Preliminary morphological identification of the fungus was confirmed by a PCR assay using genomic DNA extracted from the mycelia of a 7-day-old culture on PDA at 25 ± 2°C. A 750-bp amplified region of the transcription elongation factor (TEF) of rDNA was generated using TEF1 (5′-ATGGGTAAGGAGGACAAGAC-3′) and TEF2 (5′-GGAAGTACCAGTGATCATGTT-3′) primers. The TEF region (GenBank Accession No. KF754798) was sequenced by Sangon Biotech Co., Ltd. (Shanghai, China) and displayed 99% nucleotide similarity with the rDNA-TEF of F. equiseti (JN127347.1) separately after a BLASTn search in GenBank. Based on the symptoms, fungal morphology, TEF sequence, and pathogenicity testing, this fungus was identified as F. equiseti. To our knowledge, this is the first report of F. equiseti on corn sheaths in China. This report will establish a foundation for further study of F. equiseti to address the disease effectively and to determine the severity of damage caused by F. equiseti.
Reference: (1) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell, Ames, IA, 2006.
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