First Report of Lily Root Rot Caused by Thanatephorus cucumeris AG 2-1 in the United States

A survey was undertaken on April 2016 to profile soilborne fungal pathogens causing root rot and lesions on lily (Lilium longiflorum) cv. Nellie White in Brookings, Curry County, Oregon. Diseased roots were either blackened or rotted. Sections of diseased roots including the epidermis were surface sterilized and placed on sterile water agar plates containing 100 μg/ml chloramphenicol. From the emerging mycelia, pure cultures were obtained by collecting hyphal tips and placing them on potato dextrose agar (PDA) medium. Recently, we demonstrated the presence of pathogenic Fusarium oxysporum, F. tricinctum, and Ceratobasidium sp. AG-I isolates in eight randomly selected fungal isolates from diseased roots of lilies (Lakshman et al. 2017). In this investigation, three additional fungal isolates showed four to six nuclei per cell under the microscope with lactophenol staining at 400× magnification. The hyphae were branched at right angles, and septa were present near the point of branching. The respective fungal isolates produced dark brown microsclerotia of 2 mm or less in diameter, mostly at the edge of PDA plates. Total DNA from each of the three cultures was PCR amplified with ITS1 and ITS4 ribosomal DNA (rDNA) primers (White et al. 1990) and cloned into a TOPO TA cloning vector (Thermo Fisher Scientific, Waltham, MA). Nucleotide sequences of a randomly chosen clone from each of the three isolates were 706 bp long and identical in sequence. Hence, all were assumed to have originated from a single haplotype and numerated as ELRF 11. The rDNA amplicon deposited in GeneBank (accession no. MF972486) showed 100% homology with the corresponding region of two isolates of Rhizoctonia solani (Teleomorph: Thanatephorus cucumeris) anastomosis group (AG) 2-1 (gb|KP662692 and gb|JQ676881). For pathogenicity tests, 5-mm-diameter plugs from the margin of a 5-day-old culture were placed near the roots of axenically grown lily seedlings, two replicates per treatment and repeated two times, for 10 days at 22°C. Necrotic brown lesions appeared on the roots of such seedlings whereas control inoculation with only PDA plugs did not produce any symptoms on roots. Lactophenol staining and microscopy showed both inner and outer mycelia colonization on infected roots. Additionally, the ELRF 11 caused necrosis and rot upon inoculation of roots of diploid strawberry runners (Hawaii4 F7-3) and necrotic spots on detached leaves of Nicotiana glutinosa, N. benthamiana, and N. paniculata. The onset of necrotic symptoms on those plants occurred 4 to 7 days after inoculation. Fungus with identical morphology was reisolated from root lesions, thus proving Koch’s postulate. No fungus could be isolated from respective roots and leaves of control seedlings. AG 2-1 isolates are reported to be pathogenic on roots, bulbs, and leaves of various hosts, including strawberry and Nicotiana spp. (Yang and Li 2012). Furthermore, both pathogenic and nonpathogenic isolates of AG 2-1 were reported on bulbous crops in the Netherlands (Schneider et al. 1997). To our knowledge, this is the first report of T. cucumeris AG 2-1 causing root rot of lily in the field plots of Oregon or anywhere else in the United States. As various AGs may differ in sensitivity to fungicides and biocontrol agents (Amaradasa et al. 2014), a systematic survey of Rhizoctonia and other pathogens infecting lily under field conditions is required in order to develop effective disease management strategies.