First report of Geotrichum candidum causing sour rot of peach in China.

Peach (Prunus persica L. Batsch) is one of the most important fruit crops in China (Wang et al. 2011). Yangshan Town of Jiangsu Province is one of the four major peach producing areas in China, with a growing area of 2,000 ha (Tian et al. 2018). During June 2020, a postharvest disease presenting with brown necrosis and rot occurred on peaches in Yangshan Town. The estimated damage was more than 10% of the total harvest. The symptoms included soft rot, and the lesion appeared sunken, accompanied with sour odor and white mycelia. Twelve peaches with representative symptom were sampled for pathogen isolation. Pieces (about 5 mm × 5 mm) from the lesion edge of symptomatic fruits were dissected and surface disinfected (3% NaClO for 10 s and 75% ethanol for 30 s), then rinsed three times with distilled water, dried on sterile filter paper and transferred to Potato Dextrose Agar (PDA) media plates supplemented with 150 ng/mL streptomycin sulfate. The plates were incubated at 28 ℃ for 3 days. Forty-eight isolations were obtained from the plates and isolates were single-spored. All isolates presented white, flat, milky yeast-like colonies with radial mycelia. Hyphae under microscope were septate, branched, disarticulating into arthroconidia measuring 3.39 to 9.27 × 2.05 to 7.71 μm. The morphological characteristics are consistent with Geotrichum candidum (De Hoog et al. 1986). Internal transcribed spacer (ITS) and 18s nuclear ribosomal small subunit (SSU) of the 48 isolates were amplified and sequenced using the primers ITS5/ITS4, and NS1/NS4 for molecular identification (Schoch et al. 2012). The resulted sequences showed no difference among all the isolates. Alignment by blastn showed the sequence of ITS and SSU were 100% (accession number. GQ376093) and 99.7% identical (accession number. KY977411.1) to Geotrichum candidum, respectively. The sequences of ITS (accession number MW493646) and SSU (accession number MW493648) were submitted to the GenBank. Commercial ripe peaches with the size of about 15 cm × 15 cm × 10 cm was used for pathogenicity test. Peaches were surface disinfected with 75% ethanol, then a wound with 4 mm in diameter and 5 mm in depth was made on the surface of each fruit. Ten peaches were inoculated with 10 μL (1×105 spores /mL) of the isolate suspension. Another ten peaches were inoculated with 10 μL sterile water as the control. Peaches were incubated individually at 28 ℃and a relative humidity of about 85%. After three days, large scale of pits and necrosis appeared on every peach inoculated, and the symptoms were consistent with the diseased peaches in Yangshan Town, while no symptoms non-inoculated on the control peaches were observed. The pathogen was re-isolated from the diseased fruit and was identified again by sequencing of ITS and SSU. All the tests were conducted three times. Considering the evidence, we identified the pathogen as G. candidum. This pathogen has been reported to cause sour rot was reported in kiwifruit, strawberry, melon and other fruits (Alonzo et al. 2020; Cheng et al. 2020; Halfeld-Vieira et al. 2020). To our knowledge, this is the first report of G. candidum causing sour rot of peach in China, which may cause a great loss to peach industry of China.