First report of Stenocarpella macrospora causing Diplodia leaf streak disease of maize in Yunnan Province, China

Maize (Zea mays. L) is cultivated globally as a staple food crop, animal feed, and biofuel. However, persistent diseases in maize have led significant yield losses and a decline in grain quality (Yang et al., 2017). In August 2024, long and narrow lesions were observed on the leaves of maize growing in a field in Luoping County, Qujing City, Yunnan Province, China (24° 94' 87" N, 101° 32' 24" E). The incidence of the disease was investigated to be about 15-30% in different varieties of maize planting areas in seven townships. Initial symptoms appeared as yellow to tan stripes, with the lesions measuring 3 to 48 mm in width and 45 to 560 mm in length. In severe cases, the lesions expanded, leading to complete leaf necrosis, which caused significant damage to maize production. Dark brown sub-epidermal pycnidia and conidia were observed on the necrotic leaf tissue. The pycnidia were globose or sac-like structures, with short and narrow conidiophores. The newly formed conidia are narrow, non-septate, and have transparent cytoplasm. The conidia are subcylindrical to narrowly ellipsoid, straight, curved, or occasionally irregular, measuring 38 to 86 × 7.2 to 12.3 µm. They are 0-3-septate, smooth-walled, pale brown, apex obtuse, base truncate. Therefore, 80 individual conidia were picked under a dissecting microscope, transferred to PDA plates containing antibiotics (10 mg/L rifampicin and ampicillin) followed by incubation at 28°C for 7 days (Fei et al., 2019). After seven days, the colonies of all isolates were turned grey and downy. Based on the maize leaf symptoms and the morphological characteristics of the pycnidia and conidia, the fungus was tentatively identified as Stenocarpella spp. (Luna et al., 2016). To confirm the identity of all isolates, DNA was extracted from 10 randomly selected isolates using the cetyl trimethyl ammonium bromide (CTAB) method. The internal transcribed spacer (ITS) region and the translation elongation factor 1α (TEF-1α) gene were amplified using the primer pairs ITS-1/ITS-4 and EF-1/EF-2, respectively, and then sequenced (Lamprecht et al., 2011). After comparison, the two target sequences of 10 isolates were consistent. BLASTn analysis revealed 99.27% and 100.00% homology with the two target sequences of Stenocarpella macrospora (syn. = Diplodia macrospora) (GQ259128.1, MG934504.1). Based on phylogenetic analysis of two targets, the isolate MS01 was clustered on the same evolutionary branch with S. macrospora. The two target sequences of isolate MS01 was submitted to GenBank under the accession numbers PQ241617.1 and PQ243292.1. To confirm the pathogenicity, a suspension (1 × 106 spores /mL) of isolate MS01 conidial was sprayed onto nine healthy leaves of maize plants in vitro and three pots cultivated with the plants (five plants per pot) in vivo. Plant treated with sterilized water served as control group. The treated plants were incubated at 28°C, 90% relative humidity with alternating light-dark cycles (i.e. 12 h shifts) for 3-10 days. At the same time, the conidial suspension was injected and inoculated onto maize ears, with water used as a blank control. Each treatment contained 3ears, 10 µL of the suspension was applied to each seed treatment. Inoculated leaves developed brown spots that gradually expanded into streak lesions extending toward the ends of the leaves, similar to those observed in the field. Additionally, the maize seeds become also moldy, whereas the control leaves and ears remained symptomless. The same pathogen was re-isolated from the infected leaves and ears, confirming Koch's postulates. Previously, S. macrospora and S. maydis have been reported in many countries across North, Central and South America, Africa, Asia, and Oceania, where maize is grown commercially. These pathogens are known to infect maize ears, stalks, and leaves of maize and also can produce mycotoxins (Romero et al., 2015; Snyman et al., 2011). To our knowledge, S. macrospora causing dry rot and ear rot of maize has been reported in China, but it is mostly found in quarantine at the time of importation (Tai et al., 1979; Shan et al., 2024). This is the first report of S. macrospora causing Diplodia leaf streak disease of maize in Yunnan Province, China. Since Stenocarpella spp. is one of the most destructive diseases of maize globally, the results of this study provide a foundation for developing effective prevention, control, and quarantine strategies for managing this disease.