Occurrence of Crown Rot Disease Caused by Fusarium incarnatum on Cucumber (Cucumis sativus) in China

Cucumber (Cucumis sativus L.), an important vegetable with high economic value, has become one of the most economically important domestic vegetables in China. In April 2019, a previously undescribed crown rot with about 80% incidence was observed in many vegetable greenhouses in Huaian city, Jiangsu Province. Typical symptoms of the infected fruits appeared to be that young fruit withered and were covered with abundant white mycelia. The symptoms were similar to fruit rot caused by Botrytis cinerea. It is possible that this similarity has caused this disease to be overlooked. The pathogen was isolated as follows: diseased tissues were washed with sterile water, and potato dextrose agar (PDA) plates were inoculated with mycelium from diseased fruit. After incubation for 7 days at 25°C in the dark, the spores were isolated from each colony, and a single spore was isolated for pure culture. After single-spore isolation, 56 isolates with similar morphological characteristics were obtained. After incubation for 7 days at 25°C, the colony with abundant whitish to yellowish aerial mycelia produced pale brown to dark brown pigment and numerous falciform macroconidia (20.4 to 59 × 2.9 to 5.1 μm) in PDA plates. Moreover, macroconidia (three to five septa) exhibited pronounced dorsiventral curvature, with a tapered and elongated apical cell and prominent foot-shaped basal cell. These morphological characteristics of the isolates are consistent with Fusarium incarnatum described in previous studies (Gai et al. 2016; Ramdial et al. 2016). For molecular identification, the internal transcribed spacer (ITS) region of ribosomal DNA, a partial elongation factor-1 alpha (EF-1α) gene, Histone 3 gene (H3) and β-tubulin gene of 16 isolates were amplified and sequenced using the primer pairs ITS4/ITS5 (Lu et al. 2014), EF-1/EF-2 (O’Donnell et al. 1998), H3-1a/1b, and Bt2a/2b (Glass and Donaldson 1995), respectively. The sequences of each gene for 16 isolates were completely consistent and deposited in NCBI GenBank nucleotide database (MN227262 for the ITS region, MN233576 for β-tubulin gene, MN233577 for EF-1α gene, and MN233578 for H3 gene) and showed 99.65, 100, 100, and 100% identity to F. incarnatum species (MH979697 for ITS region, KJ020861 for β-tubulin gene, JX269001 for EF-1α gene, and MH979696 for H3 gene), respectively. To confirm the pathogenicity of the isolates, 10 μl of conidial suspension (10⁶ conidia/ml) of each isolate was inoculated on nine healthy fruits wounded with a sterilized needle, and another nine fruits were inoculated with sterilized water as controls. Six isolates and two cucumber cultivars were tested. All the inoculated fruits were placed under 25°C and 90% relative humidity. After about 5 to 7 days, they were similar to the symptoms observed on cucumber fruit in the greenhouses, but no symptoms were observed on the control fruits. The experiment was performed three times. Furthermore, F. incarnatum was reisolated from the infected cucumber fruits, fulfilling Koch’s postulates. To our knowledge, this is the first report of F. incarnatum causing crown rot on cucumber in China. Although cucumber crown rot is reported as an emerging vegetable disease, it is serious in cucumber greenhouses, causing severe economic losses to the cucumber industry in China.