First Report of Leaf Spot Caused by <i>Cladosporium magnoliigena</i> on Oat in China

Oat is an important fodder crop grown in northern and northwestern China for grazing livestock (Chen et al. 2023). In July 2024, typical leaf spot symptoms were discovered in Huzhu County (37.0439°N, 101.5512°E), Qinghai province, with an incidence rate of approximately 40% observed in the 1 ha oat field. The symptoms initially manifested as small brown to black lesions on the leaves, frequently accompanied by a chlorotic halo. As the disease progressed, these lesions gradually enlarged, leading to extensive leaf desiccation. Ten symptomatic leaves were collected from five infected plants, surface-sterilized with 75% ethanol for 30 s and then with 1% NaClO for 1 min, rinsed three times with sterile distilled water, and finally placed on PDA medium. After five days of incubation at 25°C, 8 morphologically similar isolates were obtained from diseased leaves by single-spore isolation (Noman et al. 2018), and three isolates (YM8, YM9, YM10) were randomly selected for further study. Each isolate colony was olivaceous-brown and grew at an average rate of 3.5 mm daily. Conidiophores 58 - 156 × 3.5 - 4.9 μm (average = 82 × 4.1 μm; n = 30), erect, stipes, slightly attenuated towards the apex, and olivaceous-brown. Conidia 3.7 - 6.3 × 2.3 - 5.5 μm (average = 5.1 × 3.7 μm; n = 50), subhyaline to olivaceous-brown, variable in shape and size (spherical, oval, fusiform, or nearly cylindrical), smooth to faintly rough-walled, conidia thin-walled. Secondary ramoconidia 7.7 - 19.1 × 2.4 - 4.7 μm (average = 13.5 × 3.7 μm; n = 30), olivaceous-brown, ellipsoid-ovoid, fusiform, subcylindrical, 0-3 septa, smooth to faintly rough-walled. Morphology was consistent to that described for Cladosporium spp. (Jayasiri et al. 2019). DNA was extracted from the three pure isolates and three nuclear protein-coding genes (ITS, TEF1-α and ACT) were amplified using primers described by Wang et al (2022). ITS (accession nos. PV752301, PV752133, PV752135), TEF1-α (PV804229, PV804230, PV804231), and ACT (PV804237, PV804238, PV804239) sequences were deposited in GenBank. Based on a multigene (ITS, TEF1-α, ACT) phylogenetic analysis, the three isolates clustered within the species C. magnoliigena with high support values (99% ML bootstrap/1.0 BI posterior probability). Pathogenicity was assessed by inoculating one-month-old oat plants with a suspension of 1 × 10⁵ conidia/mL applied as a foliar spray. Another set of plants was sprayed with sterile water served as the control. Each isolate was tested with three replicates, and each replicate included five oat plants. The inoculated plants were covered with plastic bags, which were removed 2 days post-inoculation. After 7 days, leaf spot symptoms appeared only on leaves treated with the conidial suspension, and these symptoms resembled those observed on naturally infected oat leaves. The pathogen was successfully reisolated and identified from the diseased leaves with the aim of completing Koch’s postulates. To our knowledge, this is the first report of leaf spot disease on oats caused by C. magnoliigena in China, providing a foundation for developing targeted disease management strategies.