Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/8/2006
Publication Date: 6/1/2006
Citation: Eskandari, F., Bruckart, W.L., Littlefield, J., Mcmahon, M.B. 2006. First report of leaf spot caused by a cercosporella sp. on acroptilon repens in the united states. Plant Disease. 90:833(on-line as DOI: 10.1094/Pd-90-0833B). Interpretive Summary: Fungi that may be useful to control weeds in the United States are collected where the weed came from overseas, because plants in the U.S. aren’t sick. These are studied in a containment greenhouse until they are proven safe to use around valuable U.S. plants, like crops and native plants. One of these fungi, called Cercosporella acroptili, has been studied because it may be useful to control Russian knapweed, a very important weed in the Western U.S. The fungus under study came from Turkey. Recently, samples of sick Russian knapweed came from Montana, and symptoms were found on leaves that looked like the same pathogen from Turkey. When the Montana fungus was purified, it caused the same symptoms and was the same as the isolate from Turkey in appearance, shape, and chemistry. It was identified as C. acroptili. This is the first report of this fungus on Russian knapweed in the U.S.
Technical Abstract: Acroptilon repens (L.) DC. (Russian knapweed, RK) is a major weed pest in the western United States and has been target of biological control research in recent years. Recently, leaf spots were observed on an RK sample collected at the Charles M. Russell National Wildlife Refuge in Montana. Circular to sub circular brown spots, 1 to18 mm in diam., with indefinite margins, sometimes surrounded by a thin purple to rose-colored border, were noted on most leaves in the sample. Conidiophores and conidia developed in moist chambers within 48 hours and enabled isolation of the fungus. The fungus was re-isolated from similar symptoms that developed on healthy RK after inoculation. The fungus has subcuticular, pale yellowish stroma, conidiophores that are hyaline, 0 – 2 septate, thin-walled, smooth, unbranched, 31- 91 x 2.8 – 5.6 um, and conidia that are solitary or in secondary short chains, ovoid, obovoid, subclavate, 13 – 52 x 3.2 – 6.8 um, 0 – 3 septate, hyaline, and thin-walled. Conidial scars and hyla are umbronate, somewhat thickened and refractive, but not darkened. The fungus is identified as Cercosporella acroptili (Bremer) U. Braun, based on morphology (1) and comparisons with the type specimen and an isolate from Turkey (FDWSRU 98-001). ITS 1 and 2 sequences (GenBank #779164) also were identical to a known isolate of C. acroptili. A specimen has been submitted to the USDA, ARS, SBML (BPI #871029).