Plum (Prunus salicina Lindl.) is an important economic fruit crop and widely planted in China. In April 2025, brown spots were observed on plum fruits (cv. Sanhua) in a plantation of Gongcheng County, Guilin City, Guangxi Province, China (24°54′41.76″N, 110°47′12.84″E). The affected area spanned approximately 3.0 ha, with a disease incidence of around 20%. The early spots were light brown and appeared as circular or irregular lesions. Over time, these spots developed into brown to brown-black large necrotic regions covering the entire fruit surface. Eventually, the diseased fruits fell off the tree. To isolate the pathogen, nine diseased fruits were collected and typical lesions were cut into small pieces (5×5 mm), disinfected with 75% ethanol for 15 seconds, followed by 1% NaClO for 2 min, rinsed three times with sterile water, and plated onto potato dextrose agar (PDA) plates. The plates were then incubated at 25°C in darkness for 5 days. Pure cultures were obtained using the single-spore isolation. The colonies were brown to black with abundant whitish margins. Conidia were pale brown, ellipsoid to obclavate, with ornamented walls, 1 to 6 transverse and 0 to 3 vertical septa, and length × width of 14.9 to 51.2 × 9.1 to 16.7 μm (n = 100). Their morphological characteristics were consistent with Alternaria spp. (Simmons et al. 2007). Two representative isolates, GXAL1 and GXAL2, were selected for further molecular analyses. The internal transcribed spacer (ITS) region, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), translation elongation factor 1-alpha (TEF1-α), and Alternaria major allergen (Alt a 1) were amplified with the primers ITS1/ITS4 (White et al. 1990), gpd1/gpd2, EF1-728F/EF1-986R, and Alt-for/Alt-rev (Woudenberg et al. 2015). The resulting sequences were deposited in GenBank (ITS, PV687672 and PV687673; GAPDH, PV695565 and PV695564; TEF1, PV695562 and PV695563; Alta1, PV695566 and PV695567). The reconstructed maximum likelihood tree showed that strains GXAL1 and GXAL2 clustered with the Alternaria alternata type strains CBS 102596 and CBS 121348. Pathogenicity tests were conducted on healthy plants with nonwounded plum fruits (cv. Sanhua). Six healthy and surface-sterilized fruits on healthy plants were inoculated with mycelial plugs (6 mm) from ten-day-old colonies (Ze et al. 2024). For the control, six fruits were inoculated using PDA agar plugs. All tested fruits were enclosed in plastic bags and plants were maintained at 25°C and 80% humidity in a greenhouse. The pathogenicity test was performed three times. After 14 days, all inoculated plum fruits showed brown spot symptoms resembling those observed in the plantation, whereas control fruits remained symptomless. The inoculated fungus was reisolated and confirmed as Alternaria alternata by morphological characteristics. A. alternata has been reported causing black spot of persimmon fruit in China (Cheng et al. 2023). To our knowledge, this is the first report of Alternaria alternata causing brown spot on plum fruit in China, while Alternaria alternata is reported to cause a post harvest rot of plum fruit, this report of brown spot on actively growing fruit is unique, which will aid future management of this disease.
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