First Report of Penicillium cellarum Causing Rot Disease on Dioscorea polystachya in China.

Dioscorea polystachya (Chinese yam) is a kind of medicine and food homologous crop, the tubers as its main production organ, with high potassium, low fiber, high protein and rich nutrition characteristics. In 2022, at the Chinese herbal medicine planting experimental site in Anguo, Baoding City, Hebei Province, China, we found the symptoms of Chinese yam decay during the storage, with an incidence of 15%~25%. The diseased part of Chinese yam tuber rots expands from the outside to the inside and sags, with a brown or dark brown discoloration, and the surface covered with a thick grayish green mold. The diseased tissue was first rinsed with clean water to remove dirts from the surface. Thereafter, 3 to 4 mm Chinese yam pieces were picked from rotting edge with a sterilized forceps, sterilized with 75% alcohol for 30 s followed by 0.1% mercuric chloride solution for 1min, and then rinsed three times with sterile water. The sterilized pieces were cultured on potato dextrose agar (PDA). One isolated fungus was obtained, and conidia were observed after incubation for 5 days at 26°C. Pure cultures were isolated by single-spore isolation. Conidia were single spore, round or oval, colorless. Conidiophores produce several rounds of symmetric or asymmetric small stems after multiple branches, which were shaped like brooms. The length and width of 100 conidia were measured, and size ranged from 3 to 4×3 to 4 μm. On the basis of morphological characteristics, the isolate was identified as Penicillium spp. (Uy et al. 2022). To further assess the identity of isolated species, the genomic DNA of the fungal isolate (SYRF1) was extracted by CTAB protocol. The ribosomal DNA internal transcribed spacer (ITS) region and the ribosomal large subunit (LSU) were amplified and sequenced with primers ITS1/4, LR5/LROR respectively (White et al. 1990, Xu et al. 2010). The obtained ITS-rDNA region and LSU sequences (GenBank accession OQ707937 and OQ704185) of the isolate were more than 99% identity to the corresponding sequences of Penicillium cellarum in GenBank (KM249068 and MG714818). Phylogenetic results based on a maximum-likelihood analysis revealed that SYRF1 was grouped with P. cellarum. To determine the pathogenicity of the isolated fungi, tests were carried out by aseptic inoculation of fresh and healthy tubers. Before the experiment, the healthy tubers were washed, surface disinfected and dried. The tubers were then wounded with sterile inoculation needles, and the conidium-bearing hyphal discs (5 mm) were inoculated on the surface of the wounded tubers and covered with wet sterile cotton. Three tubers were inoculated repeatedly each time as the experimental group. Inoculate sterile PDA with three tubers as the control group. Each tuber was inoculated with four mycelium disks, and the pathogenicity test was repeated four times. The inoculated tubers were incubated at 26°C for 14 days with sterile PDA as control. After ten days, the inoculated points showed symptoms similar to those of the initial infection, whereas controls remained symptomless. The reisolated fungus matched SYRF1 based on morphological and sequence analyses, thereby fulfilling Koch's postulates. To the best of our knowledge, this is the first report of Penicillium cellarum as causative agent of postharvest rot of Chinese yam tubers in China. This finding will help inform the prevention and management of postharvest diseases of Chinese yam tubers.