First report of Fusarium brachygibbosum causing root rot on soybean in Northeastern China.

In August 2017, soybean root rot plants exhibiting root rot were observed in Baiquan County (47°60'N, 126°10'E), Heilongjiang province, China. The disease occurred on approximately 65% of soybean (cv. Heihe43) plantsroots in five fields (>10 ha). The disease resulted in yellowing or wilting and smaller sized leaves, absence of lateral roots and black lesions on tap roots. Infected root tissues from 10 individual plants (2 plants/each field) were surface disinfested with 0.5% NaOCl for 2 min, rinsed three times in sterile distilled water, placed on potato dextrose agar PDA, and incubated at 26℃ for 3 days. Eight fungal isolates were obtained by transferring hyphal tips.isolated and subcultured by transferring hyphal tips. Colonies on PDA were initially white to rose, then yellow in color with abundant aerial mycelium. The fungal colonies grew to a size of 7.4 cm in diameter four days after inoculation. Macroconidia were scarce and scattered, measuring 19.7 μm× 3.5 μm (n = 50) on carnation leaf agar. Typical macroconidium had 3-5 septa, slightly sharp apices with a distinct basal foot cell. Microconidia had 0-2 septa, and were slightly curved, measuring 10.7 μm × 3.2 μm (n = 50). Spherical chlamydospores had a mean diameter of 13.7μm (n = 50), were terminal and intercalary on PDA. According to these morphological characteristics, the fungus was identified as F. brachygibbosum (Padwick1945). Genomic DNA of a representative isolate P13-1was extracted. The Ef-1α, RPB1 and RPB2 regions were amplified using primers ef1/ef2, Fa/G2R and 5f2/7cr (O'Donnell et al. 2010).The consensus sequences (accession nos. MH748277, MH748278 and MH748279) showed 98.65%, 98.91% and 99.54% identity to the sequences of F. brachygibbosum strain NRRL 34033(accession no.GQ505418.1, HM347172.1 and GQ505482.1). Isolate P13-1was preserved in Agricultural Culture Collection of China, Stock ID number is ACCC 39715.To confirm pathogenicity of P13-1, soybean (cv. Heihe43) seeds were grown in 15-cm pots containing a commercial potting mix (5seeds per pot,3 pots/ treatment). Sorghum seeds (10 g) fully colonized by F. brachygibbosum (Li et al., 2018)were uniformly distributed in each pot and then covered with a 0.5-cm layer of sterile potting soil. , Sterilized sterilized sorghum seeds(10 g) were added to control pots. , incubated in a growth chamber at 25°C (12h day) / 20°C (12h night). 10 Ten days after inoculation , all inoculated plants showed symptoms consistent with those observed in the fields. The experiment was repeated two times. F. brachygibbosum was reisolated from diseased plants and identified as F. brachygibbosum based on morphological and gene sequences analysismolecular characteristics. No fungal pathogens were isolated from nontreated controls. To our knowledge, this is the first report of F. brachygibbosum on soybean in China. The soybean is the prime oil seed crop and the source of protein cultivated in Northeast China and this disease seriously affects the seedling growth. So, our findings are very important for the establishment of control strategies and breeding for resistance to soybean root rot.