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Title: Assessment of Boeremia exigua var. rhapontica, as a biological control agent of Russian knapweed (Rhaponticum repens)

Author
item Berner, Dana
item Cavin, Craig
item WOUDENBERG, JOYCE - Fungal Biodiversity
item TUNALI, BERNA - Ondokuz Mayis University
item BUYUK, ORHAN - Central Research Institute - Turkey
item KANSU, BAYRAM - Ondokuz Mayis University

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2014
Publication Date: 11/22/2014
Publication URL: http://handle.nal.usda.gov/10113/62494
Citation: Berner, D.K., Cavin, C.A., Woudenberg, J., Tunali, B., Buyuk, O., Kansu, B. 2014. Assessment of Boeremia exigua var. rhapontica, as a biological control agent of Russian knapweed (Rhaponticum repens). Biological Control. http://dx.doi.org/10.1016/j.biocontrol.2014.11.009. Biological Control 81 (2015) 65-75.

Interpretive Summary: Russian knapweed (RK) is a problematic invasive weed in the United States (U.S.) and is a target of biological control efforts. The fungus Boeremia exigua var. rhapontica (BEXR) kills RK plants in greenhouse and field tests and is specific to Rhaponticum spp., which are not native in the U.S. The objectives of this study were to 1) determine the genetic relatedness of BEXR to other Boeremia spp. and varieties; 2) determine environmental conditions favoring disease progress of BEXR on R. repens; 3) determine the host range of BEXR for its safety to release as a classical biological control agent of R. repens in the U.S.; 4) determine the damage potential of BEXR on R. repens. Results of controlled temperature tests in the laboratory and field tests in Turkey showed that the fungus grew best, sporulated the most, and caused the most disease at temperatures near 25 degrees C. Only three Rhaponticum spp. and a related non-native sp. were susceptible. The most notable, and significant damage to R. repens by B. ex. rhapontica was in reduction in root weights, a key to controlling R. repens. The host specificity and specific target damage should make this pathogen an ideal candidate to release for biological control of RK in the U.S.

Technical Abstract: Russian knapweed (Rhaponticum repens (L.) Hidalgo) is an herbaceous perennial weed that was introduced and has become invasive in the United States, particularly in the semi-arid west. It is characterized by its extensive root system, low seed production, and persistence. The weed has caused serious reductions in yields and crop value and may significantly devalue the land itself. Conventional control strategies have been inadequate because of the size of infestations and economic and environmental costs of control. Biological control has been a sought-after potential solution to this weed problem. In the summer of 2002, diseased R. repens plants were collected near Cankiri, Turkey, and the facultative saprophytic fungus Boeremia exigua isolate FDWSRU 02-059 was isolated from diseased plants. Bayesian analysis of the actin, beta-tubulin, calmodulin, elongation factor, and ITS genes, combined with binary host range data, of nine species of Boeremia and 61 isolates of 12 varieties of B. exigua, including FDWSRU 02-059, showed that the isolate is a unique genetic entity and was renamed B. exigua var. rhapontica Berner, Woudenb. & Tunali, var. nov. MycoBank MB809363. Results of controlled temperature tests in the laboratory and field tests in Ayas, Turkey showed that the fungus grew best, sporulated the most, and caused the most disease at temperatures near 25 C. Disease incidence and severity data from host-range determination tests, conducted at 25 C with adequate dew periods, were combined with a genetic distance matrix based on ITS sequences of 66 plant species related to R. repens. The combined disease and genetic data were analyzed by mixed model equations to produce best linear unbiased predictors (BLUPs), standard errors, and probability values, in t-tests against zero, for disease incidence and severity for each species. BLUPs of disease incidence were significantly different from zero only for three Rhaponticum spp. BLUPs of disease severity rankings were significantly different from zero, i.e., the BLUP indicated susceptibility, only for R. repens, R. carthamoides, R. uniflorum, and Leuzea berardioides. Best linear unbiased predictors for differences in above-ground dry weights, between control and inoculated plants of a subset of the species evaluated were not significant. However, above-ground damage by B. ex. rhapontica to R. repens was nearly twice that for any other species, except Rhaponticum species. The most notable, and significant damage to R. repens by B. ex. rhapontica was in reduction in root weights, a key to controlling R. repens.