First Report of Maize Ear Rot Caused by Penicillium oxalicum in Yunnan, China

In August 2024, Penicillium ear rot disease was identified in the corn variety Xingyu No. 5 in Luzhi Town, Yimen County, Yuxi City, Yunnan Province (24°40'54'N, 101°57'47'E), China, with a disease incidence of 100%. The primary symptom observed involved withered and grass-colored bract leaves of the spike, accompanied by a green mold layer during symptom development. Furthermore, the surface of the spike grains were covered with light green to green mold layers, and the grains demonstrated cracking, fragility, and powdery features. For pathogen isolation, 1 g of corn kernels were immersed in a 75% alcohol solution for 30 s, followed by three rinses with sterile water. The kernels were positioned on sterilized paper, dried, and subsequently ground in a mortar with 9 mL of sterilized water. An aliquot of 100 μL of the suspension was uniformly distributed onto potato dextrose agar (PDA) plates supplemented with antibiotics (10 mg/L rifampicin and ampicillin). The conidia attachment on the diseased tissue was examined under a stereomicroscope. A capillary was used to dip the conidia on the diseased tissue and spread evenly on a water AGAR (WA) plate. After an overnight culture at 28 °C, the germinated individual spores were picked up under a stereomicroscope using a sterile inoculation needle. Next, it was transferred to a PDA plate containing antibiotics (10 mg/L rifampicin and ampicillin) and incubated in the dark at 28°C for a period ranging from 3 to 7 days. Mycelial tips from the edges of the colonies were selected and transferred to fresh PDA plates. Using this method, five isolated strains displayed a white colony appearance, which then expanded to reveal bluish-green centers that protruded upward, accompanied by dense white mycelium at the edges. The colony exhibited a velvety texture with granules susceptible to detachment and dispersal. The underside of the colonies were light yellow, with no notable pigmentation surrounding them. Small, oval-to-round spores (5.5–7.0 μm) formed chains or clusters. Based on colony morphology and growth characteristics, the isolates were initially identified as belonging to the genus Penicillium sp. (Visagie et al., 2014). Molecular analysis involved extracting DNA using the cetyltrimethylammonium bromide (CTAB) method. Amplification and sequencing of the ITS, β-tubulin (TUB2), and calmodulin (CaM) genes were conducted using the primers ITS1/ITS4 (White et al., 1990), BT2a/BT2b (Glass et al., 1995), and CL1C/CL2C (Hong et al., 2005), respectively. Sequence data for the ITS, TUB2, and CaM regions from strain YXML-5 were submitted to GenBank under accession numbers PV731424.1, PV791140.1, and PV791141.1, respectively. A phylogenetic analysis revealed that the strain YXML-5 clustered together with the reference strain of P. oxalicum AG559. Pathogenicity was confirmed by inoculating corn cobs at the grain-filling stage with a conidial suspension of YXML-5 at 1×105 spores/mL, applying 10 μL per seed (30 seeds per treatment). Cobs were treated with sterile water as the negative control. All samples were cultured at 28°C with 90% relative humidity under alternating 12-hour light and dark cycles for five days. A light green mold layer developed at the inoculation sites, replicating the field symptoms, while control cobs remained symptom-free. The re-isolation of the pathogen from infected tissues substantiates its pathogenic potential. Penicillium oxalicum has been documented to induce corn leaf blight, infect kernels, and produce mycotoxins, thereby posing a risk to the safety of food and feed supplies for humans and animals (Han et al., 2023; Yang et al., 2020). In conclusion, this study highlights that precise identification of this pathogen is crucial for the development of future prevention and control measures against corn ear rot caused by P. oxalicum.