First Report of Alternaria alternata Causing Leaf Spot on Kerria japonica in China

Kerria japonica (Rosaceae) is a deciduous ornamental shrub native to East Asia and widely used in urban landscaping in China. In October 2025, a leaf spot disease was observed on K. japonica in Nanyang City (32°58′46″N, 112°30′26″E). Approximately 0.67 ha of landscape plantings was surveyed, and 100 plants were randomly examined. The disease incidence was 36%, with a disease index of 21.4. Initial symptoms were small brown lesions emerging on leaf margins and tips that expanded along veins into irregular dark brown spots surrounded by yellow chlorotic halos. Twenty symptomatic leaves were randomly collected. Small tissue pieces were excised from the margins of lesions, surface sterilized with 75% ethanol for 30 s followed by 1% NaClO for 1 min, rinsed twice with sterile distilled water, and placed on potato dextrose agar (PDA). In total, 26 purified fungal strains were obtained and three isolates (DT09, DT11, and DT25) from different sampling sites were selected for further analysis. After 7 days on PDA at 25°C, colonies were grayish brown and floccose with entire margins; the reverse was dark brown to black. Conidia were dark brown, obclavate or spheroidal, produced in chains of 4 to 7, and had 2 to 4 transverse septa and 0 to 2 longitudinal septa. Conidia measured 13.2-15.5 × 4.1-5.8 μm (n = 100). Genomic DNA was extracted from fresh mycelia using the CTAB method. The internal transcribed spacer (ITS) region, translation elongation factor 1-alpha gene (tef1), glyceraldehyde 3-phosphate dehydrogenase gene (gapdh), the second largest subunit of RNA polymerase II (rpb2), the major allergen gene (Alt a 1), endopolygalacturonase gene (endoPG), and an anonymous gene region (OPA10-2) were amplified by PCR using primer pairs ITS1/ITS4, EF1-728F/EF1-986R (Carbone and Kohn 1999), gpd1/gpd2 (Berbee et al. 1999), 5F/7cR, Alt-for/Alt-rev, PG3/PG2b and OPA10-2L/OPA10-2R, respectively. The sequences were deposited in GenBank under accession numbers PZ596530–PZ596532 (ITS), PZ631781–PZ631783 (tef1), PZ631787–PZ631789 (gapdh), PZ631790–PZ631792 (rpb2), PZ631778–PZ631780 (Alt a 1), PZ631784–PZ631786 (endoPG), and PZ760892–PZ760894 (OPA10-2). Maximum-likelihood analysis of the concatenated seven-locus dataset was conducted using MEGA12 under the Kimura 2-parameter model with a discrete gamma distribution, with 1,000 bootstrap replicates. The three isolates clustered within the A. alternata clade with 95% bootstrap support, consistent with the taxonomic framework of Woudenberg et al. (2015). Pathogenicity was tested on healthy potted K. japonica seedlings using each isolate, with three plants per treatment. Four leaves on each plant were inoculated without wounding with 50 μL of a conidial suspension (1 × 10⁶ conidia mL⁻¹) to each leaf. Control leaves received 50 μL of sterile distilled water per leaf. Plants were maintained in controlled-environment growth chambers at 28°C and 90% relative humidity under a 12-h light/12-h dark photoperiod. At 7 days postinoculation, inoculated leaves developed lesions resembling those observed in the field, whereas controls remained asymptomatic. The experiment was conducted twice with similar results. The fungus was reisolated from the symptomatic leaves and identified as A. alternata based on morphological and molecular characteristics, fulfilling Koch’s postulates. Leaf spots caused by Corynespora cassiicola and Diaporthe pescicola on K. japonica have previously been reported in Korea and China (Lee et al. 2015; Zhang et al. 2022). To our knowledge, this is the first report of A. alternata causing leaf spot on K. japonica in China. This finding expands the known host range of A. alternata and provides useful information for the diagnosis, monitoring, and management of leaf spot disease on K. japonica.