Location: Foreign Disease-Weed Science
Title: First report of Pilidium concavum causing leaf necrosis on Fallopia japonica in the United States Authors
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 21, 2012
Publication Date: January 1, 2013
Citation: Bruckart, W.L., Eskandari, F., Coombs, E.M., Rossman, A.Y., Palm, M.E. 2013. First report of Pilidium concavum causing leaf necrosis on Fallopia japonica in the United States. Plant Disease. 97(1):146. Interpretive Summary: Japanese knotweed, an invasive weed, occurs in large areas that cannot be sprayed with herbicide or mowed down. For this reason, we are looking for a plant disease that will control it. Recently, such a disease was found in Oregon on Japanese knotweed. This report describes the identification of the pathogen that causes the disease. It is caused by a fungus called Pilidium concavum, because of its distinct morphology and DNA that matches other fungi in this species. Inoculation of plants with this fungus caused the same symptoms that were seen in the field in Oregon. Specimens of the fungus have been sent to herbaria in Beltsville, Maryland, and in the Netherlands. This is the first report of this disease on this plant in North America. Surveys for additional diseased plants in Oregon are planned for the summer of 2012 to learn if there are more diseased plants in nature and to learn if they are damaged or killed by the disease.
Technical Abstract: Fallopia japonica (Houtt.) Ronse Decr. (= Polygonum cuspidatum Siebold & Zucc.; Japanese knotweed) is an invasive perennial forb in the Polygonaceae. It has been identified as a target for biological control in many parts of the world, including the USA. Plants with an unknown disease were collected on August 20, 2007, from field plots at the Oregon Department of Agriculture, Salem (44.9299 N, -122.9936 W). Leaves of symptomatic plants had large, brown, necrotic spots, 1–3 cm diameter, in some cases occupying up to 30 percent of the leaf area. Hemispherical and discoid conidiomata with gloeoid spore masses developed in necrotic areas of leaves placed in moist chambers. Conidia developing on leaves were transferred to acidified potato dextrose agar. After 4 weeks, both types of the fruiting structures formed on the surface of the agar. This isolate was designated FDWSRU 07-116 as deposited at the Centraalbureau voor Schimmelcultures (CBS 132725). The discoid conidiomata had dark, pedicellate bases subtending a fimbriate disc on which pale brown to brown gloeoid conidial masses were produced. The hemispherical conidiomata were black, circular, sessile, and somewhat flattened, within which similar conidial masses were produced. Conidia were unicellular, cylindrical to fusiform, hyaline, and 4.5–7.2 × 0.9–1.8 um (5.7 + 0.14 × 1.33 + 0.93; mean + confidence interval, P= 0.05). The distinctly different conidiomata and the size range of conidia are characteristic of Pilidium concavum (Desm.) Hohn. A sequence of the ITS1-5.8S-ITS2 region of this isolate was identical to that of a P. concavum isolate (BPI 1107275; GenBank Accession No. AY487094) from Rosa sp., based upon an NCBI BLAST comparison. Fallopia japonica plants from Luckiamute River, Oregon, were inoculated with a conidial suspension of 1,000,000 spores per ml and incubated at 25 C for two 16 hr dew periods separated by an 8-hr “day” treatment. Large necrotic areas, similar to those observed in the field, developed on the inoculated leaves after a minimum of 2 months. Symptoms continued to develop on new leaves without additional inoculation or dew treatment. Pilidium concavum was recovered both from inoculated leaves and from symptomatic new growth. A dried culture specimen has been also deposited in the US National Fungus Collection (BPI 883546), and sequence data have been deposited in GenBank (JQ790789). To our knowledge, this is the first report of P. concavum causing disease on a member of the Polygonaceae in North America.