First Report of Stalk Rot Caused by Phytophthora tentaculata on Aucklandia lappa in China

Phytophthora tentaculata causes root and stalk rot of Chrysanthemum spp., Delphinium ajacis, and Verbena spp. in nurseries in the Netherlands and Germany (2). In later years, P. tentaculata was isolated from Verbena hybrids (3) and lavender cotton (Santolina chamaecyparissus) in Spain (1). In August 2007, stalk rot symptoms were observed on Aucklandia lappa (Asteraceae), an economically important Chinese medicinal plant, in some fields in Yunnan Province of China. Small groups of infected plants were randomly distributed throughout the fields. Plants showing stalk rot and wilting died rapidly. Diseased tissues were cut into 10-mm pieces and plated onto Phytophthora selective medium, P₅ARP (2), to obtain the pure cultures. Seven isolates were obtained, and five isolates were grown on solidified LBA (60 g of lima bean powder and 15 g of agar per 10,000 ml of distilled water) and 10% V8 juice liquid medium for examination of morphological and physiological characteristics (4). The colony surface texture was uniform and formed sparse, loosely branched mycelium on LBA medium. Radial growth rate was 2 to 3 mm per day at 24°C on LBA. In water, relatively small swellings were formed at hyphal branches. Sporangia were spherical or ovoid to obpyriform and some were distorted and papillate with a narrow exit pore. Approximately 10% of the sporangia were caducous with a short pedicel. Sporangial dimensions were 28 to 47 (35) × 21 to 36 (29) μm, length/breadth ratio 1.2. Chlamydospores formed on LBA after 1 week and were terminal, spherical, thin walled, and 21 to 31 (27) μm in diameter. The isolates were homothallic. Oogonia abundantly formed on LBA and were 25 to 36 (31) μm in diameter. One or two paragynous antheridia (15 × 10 μm) were attached to the oogonia. Oospores were spherical, hyaline, aplerotic, and 20 to 32 (25) μm in diameter. The minimum temperature for mycelium growth was 8°C and maximum temperature was 34°C. The internal transcribed spacer (ITS) region was amplified and sequenced and agreed 100% with sequences of four P. tentaculata isolates deposited in GenBank (Accession Nos. AJ854302, AY881001, DQ335634, and AF266775). Pathogenicity was assessed by flooding three potted A. lappa plants with a 10⁴ ml^-1 zoospore suspension and incubating at 20 to 22°C. As controls, two potted A. lappa plants were flooded with deionized water. All three inoculated A. lappa plants exhibited stalk rot after 15 days, from which the pathogen was reisolated using selective medium, P₅ARP. Controls remained healthy 15 days after inoculation, To our knowledge, this is the first report of P. tetaculata H. Kroeber & R. Marwitz infection of A. lappa in China. We speculate that the pathogen might have been introduced from other countries on seeds. References: (1) L. A. Álvarez et al. Plant Dis. 90:523, 2006. (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (3) E. Moralejo et al. Plant Pathol. 53:806. 2004. (4) X. B. Zheng. Methods in Phytophthora. Chinese Agriculture Press, Beijing, China, 1995.