First Report of Rhizoctonia solani AG 2-1 Causing Root and Bulb Rot on Hymenocallis glauca in Mexico.

Abstract

In Mexico, there are 29 native species of the genus Hymenocallis, including H. glauca, which is characterized by a bulb that stores carbohydrates, giving energy for the emergence of foliage and floral scapes; it is the most cultivated species and holds economic value as a potted plant and cut flower (Leszczyñska-Borys and Borys, 2001). In September 2024, a survey was conducted at the Center for Research in Horticulture and Native Plants (18°55'55.6"N 98°24'01.4"W) at UPAEP University, where there was an average temperature of 25 °C and 75% relative humidity (RH) for 10 consecutive days. Approximately 30-day old H. glauca seedlings exhibited symptoms of root and bulb rot in a 0.4 ha area, with a 45% disease incidence. Symptoms included root and bulb rot with constriction at the base of the bulb and the presence of brown mycelia. Symptomatic tissues from 50 seedlings were collected, cut into 5 mm pieces, sterilized with 3% NaClO for a minute, rinsed with sterile distilled water, and placed in Petri dishes with potato dextrose agar (PDA) medium. Samples were incubated in the dark for six days at 28 °C. An isolate was obtained from each diseased seedling using the hyphal tip method. After six days, the colonies consisted of white mycelium that turned brown with age. Right-angle branching hyphae were observed, with slight constriction at the base of the branches. The hyphae were multinucleate, containing four to nine nuclei per cell. After 15 days, some isolates produced dark brown sclerotia. Based on these morphological characteristics, isolates were tentatively identified as Rhizoctonia solani Kühn (Parmeter, 1970). To confirm the anastomosis group (AG), two isolates (RsHg4 and RsHg8) were selected for molecular identification. Genomic DNA was extracted using the CTAB protocol. The ITS region was amplified and sequenced (White et al. 1990) in both isolates, and the sequences were identical. Thus, only the sequence of isolate RsHg8 was deposited in GenBank (PQ524600). BLAST analysis of the partial ITS sequence (639 bp) showed 99.84% similarity with R. solani AG 2-1 isolate (GenBank: JF792354) (Mercado et al. 2012). Phylogenetic analysis of AGs sequences allowed assignment of the isolate RsHg8 to the AG 2-1 clade. Pathogenicity was confirmed by inoculating 50 30-day old H. glauca seedlings, grown in pots with sterile substrate. A 5 mm diameter PDA plug colonized with mycelium from the RsHg8 isolate was placed on each bulb, 10 mm below the soil surface. For control treatment, a PDA plug without fungal growth was placed on the bulb of 25 seedlings. The inoculated seedlings were incubated in a greenhouse at 28 °C and 90% RH. After six days, inoculated seedlings showed root and bulb rot with constriction at the base of the bulb. No symptoms were observed in controls. Fungus was re-isolated from the inoculated seedlings and characterized both morphologically and molecularly, yielding identical results as described above and identified as R. solani AG 2-1, thereby fulfilling Koch's postulates. Pathogenicity tests were repeated thrice. R. solani AG 2-1 has been reported infecting Allium tuberosum in Hokkaido, Japan (Misawa and Kuninaga, 2013), and Allium cepa in Morrow, Oregon (Patzek et al. 2013). To the best of our knowledge, this is the first report of R. solani AG 2-1 causing root and bulb rot in H. glauca in Mexico. Data on diseases affecting this plant is scarce, highlighting the importance of this research in developing integrated management strategies and preventing pathogen spread.