Target Spot on Menispermum dauricum Caused by Streptobotrys (≡Streptotinia) caulophylli, a New Disease in China

Abstract

Menispermum dauricum (moonseed) (family Menispermaceae), a perennial twining vine, is an ornamental plant traditionally used in Chinese medicine. M. dauricum is distributed mostly in northeastern, northern, and eastern China, Japan, Korea, and southern Siberia (4). Rhizoma menispermi is the dry root of M. dauricum, which has detoxifying and dehumidifying effects, and is mainly used for the treatment of sore throat, enteritis, diarrhea, and rheumatism. From June to September in 2012 and 2013, target spots were observed on moonseed plants, with an incidence above 30% in the medicinal herb garden of the Institute of Special Animal and Plant Sciences at the Chinese Academy of Agricultural Science (44°02′ N, 126°05′ E) in Jilin Province. Lesions on the leaves were roughly circular, forming concentric rings of alternating light and dark brown bands with yellow halos at the margins, and up to 50 mm in diameter. Lesions coalesced to a large area capable of destroying the leaf. Under humid conditions, the lesions enlarged rapidly. Occasionally, grayish-white mycelia appeared on the lesions. Subsequently, grayish-brown conidiophores arose, singly or in dense groups, up to 700 μm high, with large loose conidial heads. Side branches and branchlets were tightly twisted, brown and 7 to 11 μm in width. Conidiogenous cells were inflated at the apex of the branches and delimited by a septum. Conidia were globose to subglobose, pale brown, unicellular, minutely echinulate, and rounding to 7.8 to 16.9 μm in diameter. Four isolates were obtained from necrotic tissue from leaf spots and cultured on potato dextrose agar (PDA) at 25°C. Mycelia grown on PDA were sparse, whitish-gray, and produced small black sclerotia within 3 to 5 days. Sclerotia were round or oval to oblong and 0.3 to 1.7 × 0.8 to 1.7 mm. No conidiophores or conidia were produced on PDA. The similar species Streptobotrys streptothrix had smooth conidia and small sclerotia. So, all isolates were identified as S. caulophylli based on their morphological and cultural characteristics (2,3). The internal transcribed spacer (ITS) region was amplified by using the primers ITS4 and ITS5 (1). The ITS sequences (529 bp) were identical in these four isolates (Accession No. HG918042). Pathogenicity tests were performed on healthy 2-year-old moonseed plants. Ten leaves were inoculated with a 0.6-cm diameter mycelial plug from 3-day-old PDA cultures for each isolate, and the inoculation sites covered with moistened sterile absorbent cotton. Another 10 leaves were inoculated with sterile PDA plugs as controls. All plants in the experiments were covered with plastic bags and maintained in a greenhouse at 20 to 25°C for 24 h. After 3 days, dark brown spots were observed on all leaves inoculated with these isolates. After 7 days, the classical symptoms were evident, while control plants remained healthy. The re-isolated pathogen was identified as S. caulophylli based on morphological analysis. The pathogenicity test was repeated with similar results. Currently, the economic importance of this disease is limited, but it may become a more significant problem with the cultivation area of M. dauricum increasing. To our knowledge, this is the first report of S. caulophylli causing target spot on M. dauricum in China. References: (1) D. E. L. Cooke et al. Mycol. Res. 101:667, 1997. (2) M. E. Elliott, Can. J. Bot. 40:1197, 1962. (3) S. K. Hong et al. Plant Pathol. J. 20:192, 2004. (4) Y. H. Liu. Page 39 in: Flora Republicae Popularis Sinicae, vol. 30. Sciences Press, Beijing, 1996.