First report of Alternaria alternata causing leaf spot on Polygonatum kingianum in China.

ABSTRACT

Polygonatum kingianum is a Chinese herbal medicine that belongs to the genus Polygonatum of the family Liliaceae. In June 2023, Polygonatum kingianum Coll. et Hemsl. in nurseries in Qujing, Yunnan Province, China, showed irregular brown spots on the leaves, whole leaf necrosis, and plant death in serious cases, with an incidence of 10-20% (Fig. S1). To identify the pathogens of P. kingianum, six diseased samples were collected from nurseries with 0.6 acre. These diseased sample leaves were soaked in 0.1% HgCl2 for 1 min and 75% ethanol for 2 min and then rinsed thrice with sterile water. Treated leaves were cut into small pieces (5×5 mm) and cultured on potato dextrose agar (PDA) for five days at 28°C. Total thirteen fungal strains were isolated from PDA medium. The nuclear ribosomal internal transcribed spacer of ribosomal DNA (ITS rDNA) region of these 13 strains was amplified by polymerase chain reaction (PCR) using universal primers ITSI/ITS4 (White et al. 1990). Sequencing and BLAST of the ITS region on NCBI showed that 11 out of 13 fungal strains belonged to the genus Alternaria, with an identity ≥99%. We selected one of the Alternaria strains, HJ-A1, for further study. The HJ-A1 colony appeared grayish brown white-to-gray with a flocculent texture on the front side and a dark gray underside on the PDA medium (Fig. S1). The conidiophores appeared brown, either single or branched, and produced numerous short conidial chains. The conidia were obclavate to obpyriform or ellipsoid in shape and contained 1-4 transverse septa and 0-2 oblique septa. The conidial diameter was 27.30µm in length and 12.27µm in width. (Fig. S1). To further determine the species of HJA1, the genomic DNA of HJ-A1 was extracted using the Lysis Buffer for PCR (AG, Hunan, China). Four Alternaria genomic DNA regions including the ITS, translation elongation factor 1-α gene (TEF1-α), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and Alternaria major allergen gene (Alt a1) were amplified by PCR using the primers as previously reported (Woudenberg et al. 2013, Hong et al. 2005). Sequence analysis revealed that the ITS (484bp) of HJ-A1 (NCBI No. PP082633), TEF1-α (267bp) of HJ-A1 (NCBI No. PP419893), GAPDH (582bp) of HJ-A1 (NCBI No. PP419892), and Alt a1 (522bp) of HJ-A1 (NCBI No. PP228046) shared the highest identity with A. alternata respectively (99≥%). A maximum likelihood phylogenetic tree was constructed with the combined sequence data sets of ITS, GAPDH, TEF, and Alt a1 using MEGA 7. The results showed that HJ-A1 strain clustered with A. alternate (Fig. S2). The pathogenicity of HJ-A1 was tested according to Koch's postulates by inoculating HJ-A1 conidia suspension (2×105 conidia/mL) into leaves of 1-year-old P. kingianum, with sterile water as a control. Each treatment group included 3 plants with 3 replicates. The tested plants were planted in a phytotron at 28℃ and 90% humidity. Three days after inoculation, symptoms similar to those under natural conditions were observed in the HJ-A1-inoculated plants, whereas no symptoms were observed in the control plants (Fig. S1). The same fungal strains were re-isolated from inoculated leaves and identified by morphologically and sequence of ITS. Previous studies showed that Alternaria alternata funji cause many plant diseases, such as fig fruit rot (Latinovic N et al. 2014)，daylily leaf spot (Huang D et al. 2022), fruit blight on sesame (Cheng H et al. 2021)，leaf spot of Cynanchum atratum Bunge (Sun H et al. 2021) and so on. To our knowledge, this is the first report of A. alternata causing P. kingianum leaf spot in China. The discovery of this pathogen will help to guide the protection and control of P. kingianum disease.