First Report of <i>Rhizoctonia solani</i> AG-3 Causing Elephant Hide of Potatoes in Colorado

Potato (Solanum tuberosum) is one of the most important food crops globally and a key component of U.S. agricultural production. The San Luis Valley (SLV), Colorado, accounts for nearly 95% of the state’s potato production and ranks as the nation’s second largest producer of fresh-market potatoes. During the 2022, 2024, and 2025 growing seasons, most notably in September, potato cultivars Soraya and Russet Norkotah were observed in multiple commercial fields across the San Luis Valley exhibiting characteristic elephant hide symptoms. Affected tubers showed rough, netted, and corky cracking on the periderm. To identify the causal agent, affected skin samples from fifteen Soraya tubers, approximately 10 mm² in size, were surface sterilized in 70% ethanol for 30 s, rinsed twice with sterilized water, air-dried, and placed onto potato dextrose agar (PDA) medium. Five plates were incubated in the dark at 25°C for one week with a 12 h photoperiod, after which Rhizoctonia-like colonies were observed and subsequently subcultured. Fast-growing, pale brown colonies producing sclerotia developed from eleven symptomatic tubers. Hyphal tip isolates displayed uniform morphology characterized by multinucleate cells, right-angle branching with slight constriction and septation near branch points, formation of monilioid cells, and irregular light brown sclerotia averaging 1.8 mm in diameter (n=30), features typical of R. solani AG-3 (Tsror, 2010). For molecular identification, DNA was extracted from infected tubers and fungal cultures using the DNeasy PowerSoil Pro Kit (Qiagen, MD, USA). PCR amplification with universal primers ITS1/ITS4 (White et al. 1990) and species-specific primers ITS1/ITS2 (Lees et al. 2002) produced amplicons of approximately 700 bp and 450 bp, respectively, while no amplification was obtained from healthy tubers. Further confirmation by qPCR with the primer-probe set RsTqF1/RsTqR1/RQP1 (Woodhall et al. 2007) verified the presence of R. solani in both symptomatic tubers and hyphal isolates. The ITS1/ITS4 and ITS1/ITS2 sequences were deposited in GenBank under accession numbers PX232606 and PX232607. BLASTn analysis revealed 100% identity with R. solani AG-3 sequences in GenBank, including accession no. KX631314 (Yang et al. 2017). Pathogenicity tests were conducted in the greenhouse (24 ± 2°C) using the potato cultivar Soraya. A single representative isolate of R. solani AG-3 was used for inoculation. Ten plants were inoculated, and four served as controls. Barley grains incubated with 2-week-old cultures of R. solani (AG-3) were applied at planting by mixing 5 g of inoculum with soil around each tuber, followed by an additional layer placed 1 cm above the tuber and covered with soil to ensure uniform distribution. Control plants received sterile barley grains incubated with PDA plugs. After three months, plants and progeny tubers were scored for elephant hide, black scurf, and stem canker. Out of total 24 progeny tubers from inoculated, 8 developed elephant hide (33%), and 16 had black scurf (67%), while stem canker was observed on 90% of stems. Tubers and stems of control plants remained symptomless, and no pathogen was isolated from the control plants. The pathogen was successfully re-isolated, and its identity was reconfirmed by morphology, PCR, and qPCR from tubers with elephant hide symptoms. Elephant hide can be a serious disease of potatoes capable of causing significant quality losses to potato production, in particular those produced for the fresh potato market. To our knowledge, this is the first report of R. solani causing elephant hide on potatoes in Colorado. Further studies are needed to understand the epidemiology of elephant hide in order to develop management practices.