First report of Phytophthora nicotianae causing black shank of Schizonepeta tenuifolia in China

Schizonepeta tenuifolia (Lamiaceae) is a medicinal herb widely planted as an economically important plant in China. During the summer of 2008, plants with symptoms of black shank disease were observed in many fields of the municipalities of Beijing and Hebei provinces. Initially black-brown lesions were observed on the stems and roots, and then affected plants eventually wilted and died without dropping leaves. A survey found that more than 80% of S. tenuifolia fields were affected, resulting in serious economic loss.

Diseased stem and root samples were surface disinfected in 1% NaOCl for one minute and transferred to oomycete selective medium (Masago et al., 1977) and incubated at 22°C for five days. Single spore cultures were obtained and initially identified as Phytophthora nicotianae based on morphological and physiological features. (Stamps et al., 1990; Erwin & Ribeiro, 1996). Genomic DNA was extracted from the purified cultures using Nucleospin Plant Kit (Macherey-Nagel Inc., Düren, Germany). The internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS4 and ITS6. The amplicons were sequencedand showed 100% nucleotide and 100% amino acid sequence identity with the P. nicotianae reference sequence (844bp, GenBank Accession No. FJ215687).

Koch’s postulates were satisfied by dipping plants roots into the spore suspension (10⁵spores/ml) obtained from 10-day-old culture plates. Sterilized water was used for control plants. The inoculated plants were placed outside under natural environmental conditions (mean temperature 20°C). After 10 days, black-brown lesions were observed on the root and stem samples. P. nicotianae was reisolated from the lesions fulfilling Koch’s postulates. No symptoms were observed on the control plants. To our knowledge, this is the first report of black shank disease onS. tenuifolia caused by P. nicotianae in China.