Occurrence of Maize Ear Rot Caused by Fusarium fujikuroi in China

Abstract

Maize (Zea mays L.) is one of the most important crops in China. Ear rot is a serious disease that often leads to a decline in maize yield and quality and is a hazard to human and livestock health owing to potential mycotoxin contamination of grain. The incidence of maize ear rot usually ranges from 20 to 40% at different fields, even reaching as high as 70% at certain plots in Chongqing and surrounding areas (Zhou et al. 2018). In August 2016, a survey to determine population composition and distribution of Fusarium spp. causing maize ear rot was conducted in 22 counties in Chongqing. Maize ears with white, pink, or salmon-colored mold were collected from fields for further analysis. Symptomatic kernels were soaked in 2% sodium hypochlorite solution for 3 min, rinsed with sterile water three times, placed on acidified potato dextrose agar (PDA), and incubated at 26 ± 1°C for 3 to 5 days. The newly grown-out fungal colonies displaying morphological characteristics of Fusarium spp. were transferred onto fresh PDA and Spezieller Nahrstoffarmer agar and purified by the single-spore isolation method (Yang et al. 2008). Fusarium spp. were identified based on morphological characteristics (Leslie and Summerell 2006) and sequence analysis of the translation elongation factor-1α (TEF-1α), β-tubulin, and mitochondrial small subunit (mtSSU) genes (Jiang et al. 2018). The predominant Fusarium spp. were Fusarium verticillioides, F. proliferatum, and F. meridionale, which were the main causal agents of maize ear rot in these areas. In addition, morphological characteristics of several isolates from Beibei, Jiangjin, Xiushan, Rongchang, Kaixian, and Wanzhou Districts were found to be identical to those of Fusarium fujikuroi Nirenberg, which grouped into the Gibberella fujikuroi species complex. Colonies on PDA produced abundant white aerial mycelia initially, but the mycelia became pale pinkish and dark violet with age. Microconidia were abundant and small in size (4.9 to 11.3 × 2.2 to 3.2 µm), colorless, oval to club shaped, and 0- to 1-septate. Macroconidia were slender, almost straight, sickle shaped, medium length, with three to five septa (17.1 to 39.2 × 2.6 to 3.3 µm). No chlamydospores were observed. Identity of the fungus was also confirmed by sequence comparison of the partial TEF-1α, β-tubulin, and mtSSU genes of two representative isolates. The resulting sequences of the TEF-1α, β-tubulin, and mtSSU genes showed 100, 100, and 99% sequence identity to those of F. fujikuroi (GenBank accession nos. MH263736.1, MH263737.1, and MF984420.1, respectively). DNA sequences of partial TEF-1α, β-tubulin, and mtSSU genes from two representative isolates (FCQD90 and FCQD97) were deposited in GenBank under accession numbers MK328875 and MK328876, MK385626 and MK385627, and MK385628 and MK385629, respectively. A pathogenicity test was performed on maize inbred lines B73 and Ye 107. Four days after silk emergence, 2 ml of conidial suspension (1 × 10⁶ conidia /ml) of each isolate was injected into each of 10 maize ears through the silk channel. Control plants were inoculated with sterile distilled water. The typical Fusarium ear rot symptom (white or white-pink mold) was observed on inoculated ears and the infected kernels. No symptom developed on the water-inoculated controls. F. fujikuroi was reisolated from the symptomatic ears but not from the control. F. fujikuroi was previously reported associated with seedling damping-off on soybean, stem wilt of Canna edulis, and root rot of Reineckia carnea (Jiang et al. 2018; Pedrozo et al. 2015; Sun et al. 2018). To our knowledge, this is the first report of F. fujikuroi causing maize ear rot in China, and this disease should be paid sufficient attention owing to a serious risk of mycotoxin contamination in maize.