First Report of Ceratocystis fimbriata Causing Postharvest Fruit Rot of Cucumber in China

Cucumber (Cucumis sativus L.) is a widely cultivated vegetable in China, which is the largest planting area in the world. During June and July of 2016, a fruit rot disease was observed on cucumbers in two farmers markets of Kunming City, Yunnan Province, China. Symptoms on cucumber initially appeared as yellow, brown, and irregular necrotic spots in the center or lower part of the fruit epidermis. The lesions then enlarged gradually and developed into grayish-brown, necrotic tissue on the pulp. Eventually, the diseased fruits were hollow, rotten, and deformed. A fungus was consistently isolated from the pulp when symptomatic tissue was incubated between two slices of fresh carrot root (Moller et al. 1968). Spore masses were removed from the apices of perithecia, transferred to potato dextrose agar medium (PDA), and incubated at 25°C. After 5 days, a fungus was consistently observed growing from the tissue. Six single-spore isolates were obtained. In culture, mycelium was initially white, and it turned to a shallow celadon and gradually to greenish or brown. Measurements were made 7 days after formation of perithecia. Observed perithecia were black, globose, 185.36 to 265.24 × 184.53 to 249.52 µm, and showed a long black neck, 720 to 986 µm. Ascospores were hat shaped and 2.7 to 4.73 × 2.9 to 4.05 µm. Cylindrical conidia 9.46 to 22.97 µm × 2.7 to 5.41 µm were found. No barrel-shaped conidia were observed. Morphological characteristics of the fungus were consistent with the description of Ceratocystis fimbriata. Three of six isolates were used for molecular identification, and genomic DNA was extracted from isolates (Hgk4 to Hgk6) using the cetyltrimethylammonium bromide method (Lee and Taylor 1990). The universal primers ITS1F and ITS4 were used to amplify and sequence the ITS1, 5.8S, and ITS2 rDNA region (Thorpe et al. 2005). The sequence was deposited in the NCBI database (accession nos. MH535908 to MH535910). Using NCBI’s BLASTn tools, MH535908 and MH535909 show 100% similarity to MH540144 (C. fimbriata isolate SP2PYP), and MH535910 shows 100% similarity to KT963153 (C. fimbriata isolate XJm3). Thus, the fungus was identified as C. fimbriata based on morphological and molecular characteristics. Pathogenicity tests were conducted using three isolates. Three green cucumber fruits were inoculated with each isolate. The cucumber fruits were wounded using a 1-cm hole punch and inoculated with mycelial agar plugs, with PDA as the control. Rotten symptoms were observed 3 to 4 cm in length 1 week after inoculation. C. fimbriata was consistently reisolated from the inoculation point. No symptoms developed on the control. The cucumber fruits were maintained in a controlled greenhouse at 25°C. The test was repeated three times. Three days after inoculation, cucumber fruits showed dark brown to black rot in the injected parts and produced necrotic tissue on the pulp. As the disease progressed, all cucumber fruits were rotted in most tissue after 7 days of inoculation, whereas control fruits remained symptomless. Based on symptoms, morphological characteristics, rDNA-ITS sequence analysis, and pathogenicity, this fungus was identified as C. fimbriata. Although postharvest fruit rot of passion fruit caused by C. fimbriata was reported in Brazil before (Firmino et al. 2016), to our knowledge, this is the first report of C. fimbriata on cucumber in China. Because C. fimbriata is one of the most aggressive plant pathogens on a wide variety of perennial as well as agronomic crop plants worldwide, it is critical to deploy appropriate management strategies to limit the fungus spread.