First Report of Alternaria alternata and Stemphylium vesicarium Causing Postharvest Rot of European Pear ( Pyrus communis ) in Chile

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During the 2021-2022 growing seasons, mature fruit of ‘Abate Fétel’ and ‘Packham's Triumph’ were collected from a packinghouse in the Maule region (Central Chile; 35°02' S, 71°16' W) exhibiting lateral Alternaria rot and brown spot lesions, with an incidence between 0.5 to 4.2% during cold storage. Symptoms include round, brown-black, dry firm lesions that, after 60 days at 0°C, began to soften pulp, showing rot. Symptomatic pears (n = 80) were surface-disinfected (75% ethanol, 35 s), and tissue fragments from lesion margins were plated on potato dextrose agar amended with 92% lactic acid (0.2 mL/liter; APDA) and incubated at 20°C for 7 days. Isolates were identified as Alternaria (n=21), and Stemphylium (n=6) spp. Alternaria-like isolates (Simmons, 2007) were separated by colony and sporulation patterns. In group 1 (n = 8), colonies were dark olive to gray with whitish margins. Conidiophores were pale to medium olive, forming chains of 6 to 8 conidia, with secondary chains of 2 to 5 conidia. Conidia were light brown to brown, ellipsoid to obclavate, averaging 25.8 ± 4.2 × 12.1 ± 1.9 μm (n = 40), with 2 to 5 transverse and 0 to 3 longitudinal septa. Group 2 (n = 16) produced similar colonies, with conidia averaging 26.2 ± 3.3 × 12.7 ± 1.8 μm (n = 40), and 2 to 4 transverse and 0 to 3 longitudinal septa. Six Stemphylium-like isolates formed light gray aerial mycelium with whitish to pinkish margins. Conidiophores were erect and brown, each bearing a single conidium. Conidia were light to dark brown, oblong to ovoid, averaging 27.7 ± 3.9 × 16.7 ± 2 μm (n = 40), with 2 to 6 transverse and 1 to 3 longitudinal septa. No ascocarp development with ascospores was observed. Four representative Alternaria isolates were sequenced for Alt a1, ATPase, and cmdA, and three Stemphylium isolates for ITS, cmdA, and gapdh (Lawrence et al. 2013; Woudenberg et al. 2017). Nucleotide BLAST identified the isolates as Alternaria alternata and Stemphylium vesicarium, with 90-100% identity to type strains EGS 34.015 and CBS 715.68, respectively. Maximum likelihood and maximum parsimony analyses based on concatenated three genes sequences confirmed that the isolates clustered in well-supported clades with reference strains of A. alternata and S. vesicarium. Sequences were deposited in GenBank for A. alternata isolates (Alt a1: PX505968 to PX505971; ATPase: PX649406 to PX649409; cmdA: PX649410 to PX649413), and for S. vesicarium isolates (ITS: PX650066 to PX650068; cmdA: PX655600 to PX655602; gapdh: PX655603 to PX655605). Pathogenicity tests were conducted on ripe ‘Packham's Triumph’ and ‘Cossia’ fruits (n=160). Twenty surface-disinfected pears were wounded (3 mm) and inoculated with mycelial plugs from each of the isolates Alt-Ac6, Alt-Ac10, and Stemph-Ac3 and incubated at 20°C for 10 days and at 0°C for 30 days under 80% relative humidity. Control fruits received sterile PDA plugs (n= 20 fruits). Inoculated fruits developed rot identical to natural infections at 0°C and 20°C. Controls remained healthy. Fungal isolates reisolated from the symptomatic fruits were identified by molecular analyses (cmdA loci), thus fulfilling Koch´s postulates. To our knowledge, this is the first report of A. alternata and S. vesicarium causing pear rot during postharvest in Chile. These pathogens have previously been reported to cause rots in pears in Oregon (DeShield and KC 2021) and Argentina (Tempirini et al. 2022). This funding provides a scientific basis for managing for storage of pear fruit during postharvest in Chile.