First report of Pestalotiopsis paeoniicola causing twig blight on Paeonia suffruticosa in North America.

Native to China, tree peony (Paeonia suffruticosa Andrews) is a perennial valued for its showy, often fragrant flowers. In May 2007, samples received from two field-grown tree peonies from Torrington, CT exhibited twig blight characterized by small, black spots on the bark of living or dead branches, and associated with subsequent death and loss of the branches. Colonies on potato dextrose agar (PDA) attained 5.5 to 6.5 cm in diameter in 9 days at 25°C and produced abundant, white, aerial mycelium, later turning pink and forming scattered, wet, black, acervular conidiomata containing abundant conidia that were five-celled, fusiform, smooth, straight, or slightly curved, and 24.0 ± 1.8 × 7.1 ± 0.5 μm (n = 20); three intermediate cells were dark brown and end cells were hyaline; two to four hyaline whip-like appendages on the apical cell, 26.6 ± 4.1 μm long, and one appendage on the basal cell, 6.7 ± 1.1 μm long. We identified the fungus as Pestalotiopsis paeoniicola (Tsukam. & T. Hino) J.G. Wei & T. Xu (= Pestalotia paeoniicola Tsukam. & T. Hino) based on morphology and host and have deposited a culture with CBS (CBS 124745). Originally described from Paeonia suffruticosa in Japan (1,3), the fungus is also found in China on Paeonia lactiflora (Pall.) (2). To confirm pathogenicity, two 5-year-old potted plants of Paeonia suffruticosa cv. Shichifukujin were inoculated with the fungus as follows: a 2-week-old PDA culture was used to produce a conidial spore suspension in sterile water; incisions (5 mm) were made with a sterile scalpel; 4 μl of either the conidial suspension or water were applied; and the wounds were wrapped with Parafilm. Each plant received three replicates each of the treatment and the control. Plants were loosely covered with plastic bags, kept on laboratory benches with ambient light for 1 month, and transferred to a greenhouse for an additional 2 months. Subsequent inspection revealed irregular or elongate, grayish brown lesions at the treatment inoculations, while the controls remained symptom free. Lesions gradually girdled the branches and the infected cortex turned dark brown. At advanced stages, small, black acervuli developed. Treatment and control tissues were cut into four pieces, surface sterilized, and placed on malt extract agar in petri dishes, four per dish. These were incubated for 9 days at 25°C under ambient light. Reisolation of P. paeoniicola only from tissues that had been treated with the pathogen, not from control inoculations, confirmed that the causal agent was P. paeoniicola. DNA sequences were obtained from the β-tubulin gene (1,512 bp) and the internal transcribed spacer (ITS1 and ITS2) and 5.8S regions of the rDNA (550 bp) and were deposited in GenBank (Nos. FJ975603 and FJ997645, respectively). Neither ITS nor β-tubulin sequences distinguished this P. paeoniicola isolate from Pestalotiopsis spp. whose sequences are deposited in GenBank. Morphological characteristics identify the fungus as P. paeoniicola, constituting, to our knowledge, the first report of this pathogen in North America. Discovery of P. paeoniicola in field-grown plants warrants further monitoring by growers because of the uncertain level of threat this pathogen may pose to the tree peony industry. References: (1) E. F. Guba. Monograph of Monochaetia and Pestalotia: 224. Harvard University Press, Cambridge, MA, 1961. (2) F. Tai. Sylloge Fungorum Sinicorum: 1021. Sci. Acad. Sin, Peking, 1979. (3) E. Tsukamoto et al. Ann. Phytopathol. Soc. Jpn. 4:183, 1956.