First Report of Alternaria linariae Causing Postharvest Fruit Rot of Pepino (Solanum muricatum) in China.

Abstract

Pepino (Solanum muricatum L.) is a popular solanaceous crop that is native to South America and is commercially grown in many countries including China for its attractive, sweet and flavorful fruits. In September 2023, a postharvest fruit rot was observed at an incidence of 7% to 10% on pepino at supermarket in Nanchang, Jiangxi, China (28.69°N, 115.81°E). Symptoms on fruits initially appeared as small black spots that later enlarged and became necrotic. To isolate the pathogen, symptomatic tissues were surface-sterilized using 75% ethanol for 15 s, then 1% sodium hypochlorite for 30 s, rinsed three times in sterile water, air dried, finally placed on potato dextrose agar (PDA) plates and incubated at 25℃ for 4 days. Ten strains (about 83% isolationfrequencyfromsymptomaticpepinofruits) with similar morphological characteristics were isolated. The colonies on PDA were initially white, gradually turning gray and eventually becoming black, and had abundant aerial mycelia. Conidia were fusiform to linetype, dark brown, measuring 50 to 100 × 10 to 28 μm (n = 30) with 5 to 10 transverse septa and 0 to 3 longitudinal septa. Based on the morphological characteristics, the pathogen was identified as Alternaria sp. (Ma et al. 2021). To further confirm species, two representative isolates (JXAL-1 and JXAL-2) were selected for molecular identification. The internal transcribed spacer (ITS) region, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), elongation factor 1-alpha (TEF1), Alternaria major allergen gene (Alt a 1) and DNA-directed RNA polymerase II core subunit (RPB2) were amplified and sequenced by using primers ITS5/ITS4, gpd1/gpd2, Alt-for/Alt-rev, EF1-728F/EF1-986R and fRPB2-5F2/fRPB2-7cR (Woudenberg et al. 2013; Woudenberg et al. 2014), respectively. These sequences were deposited into GenBank with accession number PP231808-PP231809 (ITS), PP238480-PP238481 (GAPDH), PP238482-PP238483 (Alt a 1), PP238484-PP238485 (TEF1) and PP238486-PP238487 (RPB2). A BLASTn homology search for these nucleotides showed 100% identity to ITS (KJ718182, 525 nt/525 nt), GAPDH (KJ718026, 579 nt/579 nt), Alt a 1 (KJ718694, 472 nt/472 nt), TEF1 (KJ718530, 334 nt/334 nt) and RPB2 (KJ718355, 772 nt/772 nt) sequences of Alternaria linariae CBS 107.61. The maximum likelihood analyses were performed for the combined ITS, GAPDH, Alt a 1, TEF1 and RPB2 using the IQtree web server (Trifinopoulos et al. 2016). In the phylogenetic tree, the isolates and isolates of A. linariae clustered together with 100% bootstrap support. Therefore, the fungus was identified as A. linariae. To evaluate pathogenicity, five healthy fruits of pepino were surface-sterilized with 75% ethanol, then wounded and a 5 mm diameter agar with isolate JXAL-1 was put on the wound. Another five fruits was inoculated with sterile agar plugs as control. All treated fruits were incubated at 25 ℃ with 80% humidity , and repeated twice. Five days later, all the wounded fruits inoculated with A. linariae showed similar symptoms and A. linariae was reisolated, while the control fruits remained healthy and no pathogen was isolated, fulfilling Koch's postulates. A. linariae is known as an important pathogen causing early blight of tomato and potato(Adhikari et al. 2020). To our knowledge, this is the first report of A. linariae causing postharvest fruit rot on S. muricatum in China, which expands the natural host range of A. linariae and will be helpful to develop efficient management strategies on pepino.