First Report of Rhizoctonia spp. Causing a Root Rot of the Invasive Rangeland Weed Lepidium draba in North America.

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 its biomass (4). 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. An association of Rhizoctonia spp. with root tissue of plants galled by the weevil has been documented in Europe (3). The possible presence of soilborne pathogens similar to those found in the native range has been the subject of L. draba surveys in the United States. 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 diseases on L. draba in the native range of the weed (2). 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 non-sporulating fungus was isolated from three plants. Colonies of the isolates on PDA were 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 (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 ten 85-cm-diameter pots of pasteurized soil mix infested with Rhizoctonia-colonized barley grain that had been dried and milled. An inoculum level of ~8 CFU/g (1) 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 20-25°C. The test was repeated. Typically, R. solani caused mortality of six to eight plants, from which it was re-isolated, whereas binuclate isolates caused stunting and lower dry weight of L. draba. Control plants remained asymptomatic. This is the first report of R. solani and binucleate Rhizoctonia spp. on L. draba in North America.