First Report of <i>Alternaria alternata</i> Causing Postharvest Fruit Rot on the RallsJanet Apple in Northeast China

The RallsJanet apple is favored by consumers for its small size, crisp texture, and balanced sweet-sour flavor. Gaizhou, a city located in Liaoning, China (40°24′N, 122°21′E), is one of the primary production regions for RallsJanet apple. In March 2025, an unknown fruit rot disease was observed in a local market. Among RallsJanet 40 apples grown in Gaizhou, one apple showed symptoms of the disease, raising concerns about its potential impact on the local apple industry. Initial symptoms appeared as small brown spots on the fruit surface, which gradually expanded into dark, sunken necrotic lesions with diameters of 1–2 cm. To isolate the pathogen, diseased tissue diameter >5 mm samples were excised from symptomatic fruits. The samples were surface sterilized with 75% ethanol for 40 seconds, rinsed three times with sterile distilled water, dried on sterile filter paper, and then placed onto Potato Dextrose Agar (PDA) plates using sterile forceps. The plates were incubated at 28°C for 7 days (Liu et al. 2019). Following incubation, fungal colonies displayed dense, white, fluffy aerial mycelia, indicative of the early growth phase. Five days later, the circular colonies transitioned to gray-green, eventually forming a dark green center with white margins and a soft texture. Conidia were brown, obclavate to ovoid, and possessed 2-4 transverse and 1-3 longitudinal septa. Based on these morphological traits, the isolate was preliminarily identified as Alternaria sp. (Simmons 2007). To confirm the identification, total genomic DNA was extracted from the fungal and the Internal Transcribed Spacer (ITS) region was amplified using primers ITS1/ITS4 (White et al. 1990). The resulting sequence (GenBank accession no. PV855740.1) showed over 99% similarity to Alternaria alternata (GenBank accession no. MN337233.1). A phylogenetic tree was constructed, further supporting the identification of the isolate as A. alternata. Pathogenicity tests wee conducted using five surface-sterilized RallsJanet apples. Mycelial plugs (4 mm diameter) from PDA cultures of the isolate were placed on the fruit surface. Five apples inoculated with sterile PDA plugs served as controls. All fruits were incubated at 28 °C for seven days. Typical black-brown necrotic lesions, consistent with naturally infected samples, developed on inoculated fruits, while control fruits remained symptomless. The pathogen was successfully reisolated from the infected tissues, fulfilling Koch’s postulates. Isolated strain shares high similarity morphologically with the A. alternata strain reported by DeShields et al as the pathogen of pear rot in European pears (DeShields et al. 2021). To our knowledge, this is the first report of Alternaria alternata causing postharvest fruit rot in RallsJanet apples in northeastern China. This finding highlights the importance of monitoring emerging pathogens and provides a basis for developing targeted disease management strategies.