Spotted knapweed (SKW), Centaurea stoebe L., is a nonindigenous species that is invasive over large areas in the United States, especially in the west. It has been estimated that infestations of SKW cause $42 million in direct and indirect economic losses annually (2), and the weed could potentially invade 13.6 million ha of rangeland in Montana alone. Extensive efforts toward the control of SKW have included the release of 12 insects for biological control, four of which attack the crowns and roots of this short-lived perennial. To focus efforts to select potential soilborne pathogens, which could be applied in combination with insects, we conducted a survey for plant pathogens in the native range of SKW associated with damage caused by any root-attacking insects. Stunted and chlorotic SKW plants, which were colonized by larvae of Cyphocleonus spp., were found in June 1994 near the Novomar'evskaya Botanical Sanctuary (45°08′49.87″N, 41°51′02.05″E) in the Caucasus Region of Russia. A nonsporulating multinucleate fungus was isolated from the lower stem, crown, and upper root tissue of one such plant. Colonies growing on potato dextrose agar and Ko and Hora media were examined microscopically and identified as Rhizoctonia solani by the occurrence of robust, thick-walled, golden hyphae with right-angled branching and constrictions at the branch points. The anastomosis grouping of the one isolate was determined to be AG 2-2 IIIB after pairing it on water agar with 11 AG tester isolates representing all subgroups of AG 1 to AG 5. The hyphal diameter at the obvious point of anastomosis was reduced and cell death of adjacent cells was observed. In 2007, pathogenicity was determined by planting 12-week-old seedlings of SKW, one per pot, into 20 15-cm-diameter pots of a steamed greenhouse soil mix composed of sphagnum peat, sand, and Bozeman silt loam (1:1:1, vol/vol), pH 6.6, infested with R. solani-colonized barley grain that had been dried and milled. An inoculum level of 8 CFU/g of air-dried soil was determined by most probable number calculations from fourfold dilutions of infested soil. Controls were planted into noninfested soil. In both greenhouse tests, the isolate caused either mortality or a 93% mean fresh weight reduction of surviving plants, relative to the controls, after 8 months. R. solani was reisolated from necrotic root and crown tissue of dead and stunted plants but not from the controls. To our knowledge, this is the first report of R. solani occurring on SKW in Europe. The characterization and pathogenicity of Fusarium spp. isolated from insect-colonized roots of SKW in Europe was reported previously (1).
References: (1) A. J. Caesar et al. BioControl 47:217. (2) S. A. Hirsch and J. A. Leitch, North Dakota Agricultural Economics Report No. 355. NDSU, Fargo. 1996.