Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2009
Publication Date: 12/1/2009
Citation: Caesar, A.J., Lartey, R.T., Caesar, T. 2009. First Report of a Root and Crown Disease caused by Rhizoctonia solani on Centaurea maculosa in Russia. Plant Disease. 93(12): 1350-1350.
Interpretive Summary: Spotted knapweed (SKW) is a highly invasive weed of Eurasian origin that infests large areas of rangeland in the northwestern U. S. and western Canada. Annual economic losses due to knapweed are estimated at $42 million. A potential means of increasing the impact of the 5 insects that cause crown and root damage to SKW among the 12 insects released for biological control is to apply soilborne plant pathogens. In pursuit of possible augmentation of insect releases with fungi which are associated with insect damage to roots and crown of SKW in it’s native range, and the future use of possible indigenous strains of the same pathogens, searches were done in Europe. In Russia, SKW occurs as small groups of 4-5 plants, nearly all of which are colonized by one or more of Pterolonche, Cyphocleonus, Pelochrista, and Agapeta. We isolated, identified to subspecies and proved pathogenicity of the soilborne fungus Rhizoctonia solani was associated with root and crown damage to SKW by larvae of the insect Cyphocleonus.
Technical Abstract: Spotted knapweed (SKW) (Centaurea maculosa Lamarck) is a non-indigenous species that is invasive over large areas in the U.S., especially in the western U. S. and Canada. It has been estimated that infestations of SKW cause $42 million in direct and indirect economic losses annually 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 soilborne pathogens indigenous to the U.S., which could be applied in combination with insects, we undertook studies in the native range of SKW to survey for plant pathogens associated with damage caused by any of the 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”) 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 PDA and Ko and Hora media were confirmed under microscopic examination to be Rhizoctonia solani by the occurrence of robust, thick-walled golden hyphae, with branching at right angles and constrictions at the branch points. The anastomosis grouping of the SKW 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, we determined pathogenicity by planting 12 week-old seedlings of SKW, one per pot, into twenty 150 cm diameter pots of a steamed greenhouse soil mix composed of sphagnum peat, sand, and Bozeman silt loam (1:1:1, v/v), pH 6.6 infested with R. solani-colonized barley grain which had been dried and milled. An inoculum level of 8 cfu/g of air-dried soil was established as determined by most probable number calculations from four-fold dilutions of infested soil. Controls were planted into non-infested soil. In two greenhouse tests, of 8 months duration, the isolate caused either mortality or a 93 % mean fresh weight reduction on surviving plants, compared to controls. R. solani was reisolated from root and crown tissue of dead and stunted plants but not from controls. This is the first report of R. solani occurring on SKW in Europe.