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First Report of Fusarium cerealis Causing Root Rot on Soybean

Abdelmagid, A., Hafez, M., Lawley, Y., Adam, L. R., Daayf, F.
Plant disease 2018 v.102 no.12 pp. 2638
Fusarium cerealis, agar, air drying, biological safety cabinet, chlorosis, color, conidia, cultivars, culture media, developmental stages, fungi, genes, greenhouses, growth retardation, hyphae, inoculum, leaves, loam soils, microscopy, mycelium, mycotoxins, night temperature, nitrogen fixation, nucleotide sequences, pasteurization, pathogenicity, pathogens, peptide elongation factors, polymerase chain reaction, potatoes, relative humidity, root rot, root systems, roots, seeds, sieves, sodium hypochlorite, soybeans, stems, streptomycin, sulfates, Manitoba
In 2017, soybean plants (cultivar 24-10RY, R4 growth stage) with poor standing, stunting, and leaf chlorosis were collected at Carman, Manitoba. They had few nitrogen-fixing nodules and lateral roots left on the root system and reddish brown-to-black lesions on tap and lateral roots. Roots and lower stems had cortical decay symptoms when split in half. Root pieces (1 to 2 cm) from 48 plants were surface sterilized in 0.5% NaOCl, rinsed twice in sterilized water, air dried on sterilized filter papers, and placed on potato dextrose agar (PDA) amended with 100 mg of streptomycin sulfate. Plates were incubated for 3 days at 26°C and 12 h light/12 h dark. The growing hyphae were transferred using the hyphal tip method to new PDA plates. Out of 240 single spore isolates, 12 were identified as Fusarium cerealis based on morphological characteristics and microscopic examination. On PDA, cultures grew and produced profuse, fuzzy, aerial mycelium with dark red and yellowish color around the center of the cultures within 5 days. On Spezieller Nährstoffarmer agar medium at 25°C for 10 days, macroconidia were stout, thick walled, apical and basal cells curved, usually with four to six septa, and 23.9 to 41.2 × 4.8 to 7.3 μm (Leslie and Summerell 2006). The identity of all isolates was confirmed by sequencing the translational elongation factor 1-alpha (TEF1) gene. TEF1 was amplified by polymerase chain reaction using the universal primers EF1 and EF2 (O’Donnell et al. 1998). The online resource Basic Local Alignment Search Tool (BLAST; confirmed the fungus identity as 100% F. cerealis. The TEF1 sequence for the original F. cerealis strain Carm17 isolated from field-infected soybean plants was deposited to GenBank with accession number MH151080. To confirm pathogenicity, three 5-mm plugs from 1-week-old culture of isolate Carm17 were placed in 1-liter conical flask containing a sterilized potato and soil mix (500 g of loamy soil, 50 g of finely chopped potatoes, and 100 ml of sterilized H₂O to keep the mix fairly wet) (Ko and Hora 1971). The inoculum was kept at 25°C with 12 h light/12 h dark for 7 days and shaken once after 4 days to allow the fungus to grow throughout the medium. The inoculum was air dried in laminar flow cabinet overnight, ground with mortar and pestle, and sieved with 2-mm sterilized sieves. Five grams of this inoculum was placed 3 cm below the surface of pasteurized soil in 6-in pots. In control pots, 5 g of noninoculated sterilized potato and soil mix was used. Five seeds of cultivar 24-10RY were seeded per pot, and five pots were used for inoculated and control pots. Pots were kept in the greenhouse with 24/16°C day/night temperature, 13 h light/11 h dark cycle, and 80% relative humidity. Disease symptoms similar to those observed in the field were visible on the root system of all inoculated soybean plants after 40 days. Noninoculated plants had root rot symptoms. The pathogen was reisolated from infected roots and identified as F. cerealis (strain Carm20) as described above. TEF1 gene has been sequenced. The TEF1 gene sequence of Carm20 (accession no. MH151081) was identical to that of Carm17. The experiment was repeated two times. To our knowledge, this is the first report of F. cerealis causing root rot on soybean anywhere on the globe. This information will have an impact on future scientific research on soybean root rots and on F. cerealis, especially with regard to this pathogen’s prevalence in soybean fields, its effects on crop productivity, mycotoxin production, and further interaction with other species to form root rot pathogen complexes.