Occurrence of Southern Blight Caused by Athelia rolfsii on Pinellia ternata in China and Worldwide

Abstract

Pinellia ternata (Thunb.) Breit. is an herbaceous plant (family Araceae) native to China. The tuber of P. ternata is used in traditional Chinese herbal medicine as an antiemetic and antitussive (Marki et al. 1987). In June 2018, a soilborne disease on P. ternata plants was initially observed in Zhongxiang City (31°17′N, 112°58′E), central Hubei Province, China. The incidence of the disease was 25 to 30% in two commercial fields, and the total area affected by the disease was approximately 200 ha. Symptomatic plants initially had small, water-soaked, sunken lesions on the stems near the soil line that developed symptoms typical of southern blight. As the disease progressed, white hyphae were observed on infected stem lesions in the presence of abundant moisture. Numerous tan-to-brown, spherical sclerotia developed on the infected basal stem and on soil surfaces around the infected plants. After 1 month, plant foliage became wilted, followed by a complete collapse of the plant. Sclerotia from symptomatic basal stems of plants were surface disinfested in 2% sodium hypochlorite for 1 min, washed thrice with sterilized distilled water, air dried, and placed on potato dextrose agar (PDA). The cultures were incubated at 28 ± 1°C in the dark. Numerous globoid sclerotia formed on PDA after 12 days of growth. The sclerotia, 2 to 4 mm in diameter, were white at first and became melanized, turning brown with age. The white mycelium formed typical clamp connections after 5 days of growth. Aerial fungal growth was composed of many narrow hyphae 3 to 8 μm wide. The isolate was identified as Athelia rolfsii on the basis of mycelial and sclerotial characteristics (Punja and Damiani 1996). To confirm the identification, the strain NB1 was chosen for DNA sequencing. The internal transcribed spacer (ITS) region of rDNA was amplified using the ITS1 and ITS4 primers (White et al. 1990) and sequenced, and the resulting sequences of 650 bp were deposited in GenBank (accession no. MN071107). BLAST analysis of the sequences showed 99.8% identity to A. rolfsii (imperfect stage of Sclerotium rolfsii) (MN328049.1). Moreover, the isolate was further identified to the species level as A. rolfsii by amplifying RPB2 gene sequences (accession no. MN509436) and EF1-α gene sequences (accession no. MN509438), which also exhibited 100% similarities with A. rolfsii (JF267830 and JF267817) (Remesal et al. 2013). Based on morphological and molecular characteristics, the fungus was identified as A. rolfsii. NB1 was chosen to fulfill Koch’s postulates; 10 healthy 30-day-old P. ternata seedlings were planted in plastic pots with sterilized mixture of peat moss and vermiculite (3:1). Each seeding was inoculated with one sclerotium, which was placed on the soil surface against the stem of the plant. Three noninoculated plants served as controls. The inoculated and noninoculated plants then were placed in a greenhouse at 28 ± 1°C, with ambient lighting and relative humidity of 70%. By 10 days after inoculation, all inoculated plants showed symptoms identical to those observed in the field, and no symptoms were observed on control plants. The fungus was reisolated from the inoculated plants and identified by the above method. The experiment was repeated thrice with similar results. To our knowledge, this is the first report of southern blight caused by A. rolfsii on P. ternata in China and worldwide. There is a need to develop effective management measures to reduce the occurrence of this disease.