First Report of Fusarium culmorum Causing Maize Stalk Rot in China.

Abstract

Maize stalk rot has become one of the most important diseases in maize production in China. From 2017 to 2019, a survey was conducted to determine the population diversity of Fusarium species associated with maize diseases in 18 cities across Henan Province. Maize stalk rot with an incidence of more than 20% that caused yield losses up to 30% was observed on maize variety Zhengdan958, which was grown in two continuous maize fields in Zhumadian City, Henan Province. The stem tissues from the boundary between diseased and healthy pith were chopped into small pieces (3 × 8 mm), disinfected (70% ethanol for 1 min) and then placed onto potato dextrose agar (PDA) amended with L-(+)-Lactic-acid (1 g/L) and incubated at 25°C for 4 days. Colonies on PDA produced fluffy, light yellow aerial mycelium and purple to deep brick red pigment at 25°C (Fig 1A, 1B). On carnation leaf agar (CLA), macroconidia in orange sporodochia formed abundantly, but microconidia were absent. Macroconidia were short and thick-walled, had 3 to 5 septa, a poorly developed foot cell and rounded apical cell (Fig 1C). These characteristics matched the description of Fusarium culmorum (Leslie and Summerell 2006) and isolates DMA268-1-2 and HNZMD-12-7 were selected for further identity confirmation. Species identification was confirmed by partial sequences of three phylogenic loci (EF1-α, RPB1, and RPB2) using the primer pairs EF1/EF2, CULR1F/CULR1R, and CULR2F/CULR2R, respectively (O'Donnell et al., 1998). The consensus sequences from the two isolates were deposited in GenBank (MZ265416 and MZ265417 for TEF, respectively; MZ265412 and MZ265414 for RPB1, respectively; MZ265413 and MZ265415 for RPB2). BLASTn searches indicated that the nucleotide sequences of the three loci of the two isolates revealed 99% to 100% similarity to those of F. culmorum strains deposited in the GenBank, Fusarium-ID, and MLST databases (Supplementary Table 1~3). Pathogenicity test was conducted at the flowering-stage using Zhengdan958 and Xundan20 plants according to previously described method (Zhang et al., 2016; Cao et al., 2021; Zhang et al., 2021). The second or third internodes of thirty flowering plants were drilled to make a wound approximately 8 mm in diameter using an electric drill. Approximately 0.5 mL inoculum (125 mL colonized PDA homogenized with 75 mL sterilized distilled water) was injected into the wound and sealed with Vaseline and Parafilm to maintain moisture and avoid contamination. Sterile PDA slurry was used as a control. Thirty days after inoculation, the dark-brown, soft rot of pith tissues above and below the injection sites were observed, and some plants were severely rotten and lodged (Fig 1D, 1E). These symptoms were similar to those observed in the field. No symptoms were observed on control plants. The same pathogen was re-isolated from the inoculated stalk lesions but not from the control, thereby fulfilling Koch's postulates. To our knowledge, this is the first report of F. culmorum as the causal agent of stalk rot on maize plants in China. Also, this fungus has been reported to cause maize ear rot in China (Duan et al. 2016) and produce mycotoxins such as trichothecenes, nivalenol, and zearalenone that cause toxicosis in animals (Leslie and Summerell 2006). The occurrence of maize stalk rot and ear rot caused by F. culmorum should be monitored due to the potential risk for crop loss and mycotoxin contamination.