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First Report of Neoscytalidium dimidiatum Causing Root Rot in Sweet Potato in Brazil

Mello, J. F., Brito, A. C. Q., Motta, C. M. S., Vieira, J. C. B., Michereff, S. J., Machado, A. R.
Plant disease 2019 v.103 no.2 pp. 373
Bayesian theory, Neoscytalidium dimidiatum, anamorphs, black root rot, cassava, conidia, cultivars, dieback, ethanol, fungi, hyphae, inoculum, internal transcribed spacers, mangoes, microscopy, mycelium, necrosis, paper, pathogenicity, peptide elongation factors, phylogeny, pycnidia, ribosomal DNA, roots, sodium hypochlorite, species identification, sporulation, sweet potatoes, Brazil
In January 2018, a large number of tuberous roots of sweet potato were lost in a production field in Jaíba, State of Minas Gerais, Brazil. Immediately after harvest, the roots showed dark spots on the surface, extending to the center, causing an internal, brown to black necrosis. Two symptomatic roots (cultivar Canadense) from different parts of the field were subjected to fungus isolation by removal of fragments between symptomatic and healthy tissue, followed by surface sterilization in 70% ethanol and then in sodium hypochlorite (1%) for 1 and 3 min, respectively, rinsing in sterile water, and incubation in PDA plates for 25°C in the dark. A gray to black, fast-growing fungus with abundant arthroconidia in aerial mycelia was isolated from all samples after 4 days of incubation. Hyphal tips were excised and transferred to new PDA plates, and two pure cultures (isolates) were obtained. Based on these characteristics, the fungus was identified as Neoscytalidium. For morphological analysis, culture sporulation was induced as described by Machado et al. (2014). Microscopic examination of the fungal structures identified arthroconidia with one to two septa, released by hyphal fragmentation, 4.2 to 9.5 × 3.2 to 5.3 μm, and a pycnidial synanamorph with hyaline, ellipsoid to nearly fusiform conidia, 7.4 to 12.7 × 4.2 to 6.3 μm. The morphological characteristics were similar to Neoscytalidium dimidiatum (Phillips et al. 2013). To confirm species identification, genomic DNA was extracted, and sequences of the internal transcribed spacer of the rDNA and translation elongation factor 1-α (TEF1-α) were obtained (Machado et al. 2014) and deposited in GenBank (accession nos. MH636327, MH636328, MH349427, and MH349428, respectively). According to a BLAST search, the TEF1-α sequences showed 99% identity with N. dimidiatum (accession no. KF553902). A combined phylogenetic tree obtained by Bayesian inference using MrBayes version 3.1.1 (Ronquist and Huelsenbeck 2003) grouped the isolates within the N. dimidiatum clade, confirming the species identification. One representative isolate (URM 7888) was deposited in the culture collection “Micoteca URM Profa. Maria Auxiliadora Cavalcanti” at the Universidade Federal de Pernambuco (Recife, Brazil). The pathogenicity test was performed by inoculating asymptomatic roots (cultivar Canadense), previously sterilized on the surface with 0.5% sodium hypochlorite and unwounded or wounded with a sterile scalpel at two equidistant points, with conidia suspension. Arthroconidia suspension (20 μl, 2.4 × 10⁶ conidia/ml) obtained from 7-day-old colonies was pipetted onto the inoculation sites. Sterile distilled water was used for the control instead of inoculum. Five roots were used for each treatment. The inoculated roots were maintained in plastic boxes containing moistened paper towels at approximately 25°C for 15 days. After 7 days, it was possible to observe initial symptoms of internal brown to black rot, which increased in size after 15 days. The fungus was reisolated, and pathogenicity was confirmed. The control and unwounded roots remained asymptomatic. In Brazil, N. dimidiatum has been reported causing collar and root rot in physic nut (Machado et al. 2012), black root rot in cassava (Machado et. al., 2014), and dieback in mango (Marques et al. 2013). To our knowledge, this is the first report of N. dimidiatum causing root rot of sweet potato in Brazil.