Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/16/2009
Publication Date: 2/1/2010
Citation: Caesar, A.J., Caesar, T., Maathuis, M. 2010. Pathogenicity, characterization and comparative virulence of Rhizoctonia spp. from insect-galled roots of Lepidium draba in Europe. Biological Control. 52(2): 140-144.
Interpretive Summary: The soilborne fungus Rhizoctonia consistently infects the root tissue of plants of white top (aka hoary cress) in association root galls caused by populations of the cabbage weevil which are highly specialized to this invasive perennial. This insect-disease association occurred throughout the European of white top range surveyed. The comparative virulences of 13 isolates of the fungus were assessed. Using a statistical method with origins in medical and public health research, the isolate were shown to vary substantially in virulence. The method, which is freely and readily accessible may have utility equivalent to more specialized and less accessible methods such as calculation of area under the disease progress curve. The importance of insect/pathogen interactions in maintaining an aggressive perennial in its native range as widely scattered, relatively sparse stands is demonstrated. This indicates that the potential of prospective biocontrol agents should be ranked by criteria that include the propensity for such interactions.
Technical Abstract: The association of Rhizoctonia spp. with insect-damaged and diseased tissue of the invasive perennial Lepidium draba was documented throughout the range of L. draba that was surveyed in Europe, including Hungary, Austria, Switzerland and France. Samples that could be both maintained under cooled conditions after collection and promptly processed consistently yielded R. solani, 11 isolates of which anastomosed with AG-4, one with AG-2-1 and one isolate was determined to be binucleate. Comparative virulence among the isolates was analyzed using nonparametric survival analysis of interval-censored data. This analysis indicated that there were significant differences among the isolates. These findings provide further evidence that both root-attacking insects and soilborne pathogens are associated with the lower density of a perennial invasive species in its native range. Prioritization of prospective biocontrol agents should include the ability to engage in insect/plant pathogen synergisms. The application of nonparametric maximum likelihood estimators to interval censored data for the assessment comparative virulence among several strains of a plant pathogen is more accessible than calculation of area under the disease progress curve.