Occurrence of rust disease Caused by Gymnosporangium pleoporum on Cotoneaster acutifolius in China

Species of Gymnosporangium are major pathogenic rust fungi causing diseases and significant economic losses of plants in the Cupressaceae and Rosaceae family (Kern 1973). During our investigation of rust fungi in Qinghai Province, northwestern China, we found the spermogonial and aecial stages of Gymnosporangium species on Cotoneaster acutifolius. C. acutifolius is the woody plant that range in habit from ground-covers to airy shrubs and medium-sized trees(Rothleutner et al. 2016). Upon investigation in the field, the incidence of rust on C. acutifolius was 80% and 60% (n = 100) for 2020 and 2022, respectively. C. acutifolius leaves showing abundant aecia were collected from Batang forest area of Yushu (32.45°N, 97.19°E, alt. 3835 m), Qinghai, China, from August to October of both years. The rust makes its symptom first on the upper surface appearing yellow then dark brow, aggregated spermogonia in yellow-orange leaf spots. These spots enlarge gradually and displaying an orange-yellow and are often bordered by red concentric rings. In the later stage, many pale yellow, roestelioid aecia developed on the abaxial surfaces of the leaves or fruits. The morphology of this fungus was studied using light microscopy and scanning electron microscopy (JEOL, JSM-6360LV). Microscopic examination show that aecia are foliicolous, hypophyllous, roestelioid, and producing cylindrical peridia that are acuminate, splitting above and becoming some-what lacerate nearly to base, somewhat erect after dehiscence. Peridial cells are rhomboid, and 42 to 118 × 11-27μm(n=30). They have smooth outer walls and rugose inner and side walls with long obliquely arranged ridges. Aeciospores are ellipsoid, chestnut brown, 20 to 38 × 15 to 35 μm(n=30), wall densely and minutely verrucose, 1 to 3 μm thick, with 4 to 10 pores. Whole genomic DNA was extracted (Tian et al. 2004), and the internal transcribed spacer 2 (ITS2) region was amplified with the primer pair ITS3 (Gardes and Bruns 1993) and ITS4 (Vogler and Bruns 1998). The sequence of the amplified fragment was deposited in the GenBank database (GenBank Accession No. MW714871). A BLAST search of GenBank showed a high identity (> 99%) with the reference sequences of Gymnosporangium pleoporumGenBank Accession No. MH178659 and MH178658). G. pleoporum was first described from specimens in the telial stage found on Juniperus przewalskii, from Menyuan of Qinghai in China (Tao et al. 2020). In this study, the fungus G.pleoporum of spermogonial and aecial stages were collected from the C. acutifolius, we confirmed the alternate host of G. pleoporum based on DNA extraction results. To our knowledge, this is the first record of G. pleoporum causing rust disease on C. acutifolius. Because the alternate host can be infected by several different species of Gymnosporangium (Tao et al. 2020), further investigations are needed to verify heteroecious of the rust fungus.