First Report of Green Mold Disease Caused by Penicillium ochrochloron on Medicinal Plant Pseudostellaria heterophylla in China.

Abstract

Pseudostellaria heterophylla is one of the Chinese herbal medicines with high medicinal and economic values. From 2019 to 2021, postharvest green mold disease was observed with an incidence of 2~5% on the tuberous roots of the harvested P. heterophylla at eight locations in Zherong county, Fujian Province, China. The symptoms were as follows: white mycelial growth on the tuberous roots surface initially, then green mold layers forming, and the tuberous roots decaying finally. To identify the causal agent, a total of 20 symptomatic tuberous roots were collected. Small pieces (5 mm×5 mm) were treated by surface disinfestion with 75% ethanol and 1% NaOCl, then rinsed 3 times with sterile distilled water. These treated pieces were transferred onto potato dextrose agar (PDA) and incubated at 25°C in the dark for 7 days. Twenty pure cultures were obtained by single-spore isolation method. Colonies on PDA medium initially appeared as white mycelium that developed grayish-green conidia with white margins. Mycelium was septate and colorless. Conidiophores were predominantly monoverticillate, occasionally biverticillate. Stipes was long and slender. Phialides were ampulliform to almost cylindrical with collula, 11.25 (7.80-23.50) µm long (n=50). Conidia were smooth walled and pale green, with globose to ellipsoidal shape, 2.75 (2.37-3.27)× 2.47 (2.18-3.13) µm (n=50). Based on these morphological characteristics, the isolates matched the description of the genus Penicillium. Genomic DNAs from two representative isolates (FJAT-32578 and FJAT-32579) were extracted with a fungal genomic DNA extraction kit. The rDNA ITS region and partial β-tubulin gene (BenA) were amplified using the primers ITS1/ITS4 (White et al. 1990) and Bt2a/Bt2b (Glass and Donaldson 1995), respectively. The sequences of isolate FJAT-32578 and FJAT-32579 were deposited in GenBank (ITS, OM920986 and OM920987; BenA, OM953825 and OM953826). All sequences showed above 99% similarity to P. ochrochloron type strain CBS357.48 (ITS, NR111509; BenA, GU981672). In multilocus phylogenetic analysis (ITS + BenA), the two isolates from this study clustered together with other strains of P. ochrochloron with 100% bootstrap support. The two isolates were thus identified as P. ochrochloron based on both morphological and molecular characteristics. Pathogenicity tests were conducted in triplicate by inoculating the aseptic wounds with 10 µl of conidial suspension (1×106 conidia/ml) of the two isolates in the each healthy tuberous root (cv. Zheshen No.1). The experiment was conducted twice. All the inoculated tuberous roots were placed in sterilized Petri dishes with moistened filter paper, and incubated at 25 ± 2 °C. Fifteen days after inoculation, all inoculated tuberous roots demonstrated the same symptoms as those observed in the field conditions. The re-isolated fungi from the artificially infected tuberous roots were confirmed as P. ochrochloron using the method described above, while the control tuberous roots treated with sterile water did not develop symptoms, fulfilling Koch's postulates. To our knowledge, this is the first report of P. ochrochloron causing green mold disease on P. heterophylla in China, which would be a potentially new threat to the medicinal plant.