Location: Pest Management Research Unit
Title: First Report of Rhizoctonia spp. causing a root rot of the invasive rangeland weed Lepidium draba in North America. Authors
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 15, 2014
Publication Date: May 21, 2014
Citation: Caesar, A.J., Lartey, R.T., Caesar, T., Gaskin, J.F. 2014. First report of Rhizoctonia spp. causing a root rot of the invasive rangeland weed Lepidium draba in North America. Plant Disease Notes. 98(9):1278. DOI: 10.1094/PDIS-03-14-0300-PDN. Interpretive Summary: The association of soilborne pathogens with root damage caused by insect natural enemies has been documented extensively in previous research by the present author and cooperators. Further, the contribution of a soilborne plant pathogen in combination with insects that damage roots of invasive species leading to mortality has also been shown experimentally. A prospective means of biological control of the invasive rangeland perennial species white top are negative effects on the weed following successful establishment and feeding action of a weevil highly specific to white top. Soilborne fungi infect the roots after larvae of the weevil that causes galls to create an exit hole when migrating into the soil in the spring. While this insect/pathogen association has been shown to exist in the European native range of white top, the pathogen key to the association had not been found infecting the roots or existing in the root zone of white top searched over several years in North America. The findings presented in this paper resulted from finally detecting symptoms of disease caused by this key fungus, Rhizoctonia. The presence of this fungus in the present site and other infestations may be essential to successful biological control if and when the weevil is released. Populations (infestations) of white top harboring associated Rhizoctonia would be the best sites for initial releases of the weevil, since a synergistic interaction would likely result.
Technical Abstract: The exotic, invasive perennial rangeland weed Lepidium draba spreads rapidly and reduces native species diversity. The extensive root system of L. draba constitutes 76% of plant biomass. Thus searches have been done for biocontrol agents that target root tissue or that may interact with a weevil, Ceutorhynchus assimilis, that causes galls in the crown area of L. draba, should it be released for biocontrol of L. draba. An association in Europe of Rhizoctonia spp. with root tissue of plants galled by the weevil has been documented. The possible presence of soilborne pathogens similar to those found in the native range has been the subject of surveys of infestations of L. draba in the US. One such survey in 2008 detected a few plants with reddened and chlorotic foliage in a stand near Shepherd, MT. Such symptoms typically indicate the occurrence of soilborne disease on L. draba in the native range of the weed. The site had shown a gradual increase in the range of detectable pathogens beginning with foliar pathogens in 1997. In 2010 at the Shepherd site, L. draba plants with similar (but more severe) symptoms to those seen in 2008 were noted in a different area of the stand. Excavation of the roots in both years revealed brown, sunken crown and root cankers. Pieces of root tissue were excised from the lesions and plated on acidified PDA and Ko and Hora medium. A nonsporulating fungus was isolated from three such plants. Colonies of the isolates on PDA were broadly typical of known Rhizoctonia spp. The 2010 isolates were determined to be multinucleate using DAPI and were paired with 14 tester (including subgroups) isolates of AG-1 to AG-4 on water agar. Anastomosis was observed between the multinucleate isolates and the AG-2-1 tester isolate. Sequence analysis of ITS of the rDNA of a multinucleate isolate (GenBank KJ545577) indicated 99% similarity with an accession of R. solani AG 2-1 (GenBank AB547381). The 2008 isolates were binucleate. A binucleate isolate, KJ545578, had 100% similarity with an isolate of Rhizoctonia spp. AG-A (AY927356). Pathogenicity tests consisted of planting 6-week-old seedlings of L. draba, one per pot, in 10 85-cm-diameter pots of pasteurized soil mix infested with Rhizoctonia-colonized barley grain that had been dried and milled. An inoculum level of ca 8 CFU/g of air-dried soil was established by most probable number calculations from fourfold dilutions of infested soil. Controls were the same number of plants in pasteurized potting mix. Results were recorded after 3 months in a greenhouse at 20-25º C. The test was repeated. Typically, Rhizoctonia solani caused mortality of 6-8 plants from which it was reisolated, whereas binuclate isolates caused stunting and lower dry weight of L. draba. There was no mortality among controls. This is the first report of R. solani and binucleate Rhizoctonia spp. on L. draba in North America.