First Report of Ceratobasidium sp. (AG-A and AG-G) Causing Root Rot and Stem Canker of Common Bean in Mexico.

Common bean ( Phaseolus vulgaris L.), is a grain legume widely cultivated worldwide for its edible dry seeds and pods. In February 2021, root rot symptoms were observed in two common bean (cv. Azufrado Higuera) fields located in Ahome (25º96´19¨N, 109º33´42¨W) and Guasave (25º71´85¨N, 108º78´50¨W) municipalities in Sinaloa, Mexico. Diseased plants showed reduced growth, dark brown canker at the base of the stem, root rot, as well as the absence of secondary roots. The disease incidence was estimated up to 35%. For fungal isolation, symptomatic roots were surface sterilized with 1% sodium hypochlorite for 2 min, rinsed with sterilized distilled water two times, and blotted dry on sterile filter paper. Small fragments of diseased roots were placed on PDA medium and incubated at 25°C in darkness for 3 days. Rhizoctonia -like colonies were consistently obtained and 10 isolates were purified by the hyphal-tip method. Colonies on PDA were white initially and then turned brown after 15 days of incubation. The septate hyphae were 3.9 to 6.3 μm in width and branched at right angles with a septum near the point of branching. Microscopic examination by Safranine-O staining showed two nuclei per cell. The morphological features of the isolates resembled those of Ceratobasidium (Sneh et al. 1991). The two Ceratobasidium isolates were selected for molecular analysis and pathogenicity tests. The isolates were deposited in the Culture Collection of Phytopathogenic Fungi of the Faculty of Agriculture of Fuerte Valley at the Sinaloa Autonomous University (Accession nos. FAVF395 and FAVF396). For molecular identification, genomic DNA from each of the two isolates was extracted, and the internal transcribed spacer (ITS) region and partial fragments of the second largest subunit of RNA polymerase II ( rpb2 ) gene were amplified and sequenced with the primer pairs ITS5/ITS4 (White et al. 1990) and RBP2-980F/RPB2-7cR (Liu et al. 1999), respectively. The ITS sequences (accession nos. ON630914 and ON630915) showed 99.66 and 99.01% identity with Ceratobasidium sp. (AG-A) from the USA (OM045887) and Ceratobasidium sp. (AG-G) from China (HM623627), respectively. Whereas, the rpb2 sequences (OM258171 and OM258172) showed 94.10 and 95.74% identity with Ceratobasidium sp. (AG-A) from Serbia (MT1267888) and Ceratobasidium sp. (AG-G) from Japan (DQ301701), respectively. A phylogenetic tree based on Maximum Likelihood and including combined ITS and rpb2 sequences data for Ceratobasidium spp. was generated. The phylogenetic tree grouped the isolates FAVF395 and FAVF396 within the Ceratobasidium sp. AG-A and AG-G clades, respectively. Pathogenicity tests for each isolate were performed by inoculating 10 healthy common bean seedlings (15-day-old) grown in pots. A total of 50 ml of a mycelial suspension adjusted to a concentration of 1 × 105 mycelial fragments/ml were directly placed on the stem base of each plant. Five uninoculated common bean seedlings were used as control. All plants were kept in a greenhouse for 15 days at temperatures ranging from 22 to 32°C. Root rot and stem canker symptoms appeared on inoculated seedlings after 10 days, whereas control plants remained symptomless. Fungi were reisolated from the infected roots and found to be morphologically identical to the isolates used for inoculation, thus fulfilling Koch's postulates. Ceratobasidium sp. has been previously reported as the causal agent of root rot of watermelon in Sonora, Mexico (Meza-Moller et al. 2014; Farr and Rossman 2022). To our knowledge, this is the first report of Ceratobasidium sp. causing root rot of common bean in Mexico. Further monitoring should be performed to quantify yield impacts and develop effective management strategies for this disease.