First Report of Fruit Rot on Postharvest Pumpkin Caused by Fusarium acuminatum in China

Pumpkin (Cucurbita maxima L.) is grown on approximately one million hectares, producing about 40 million tons annually in China, accounting for 40% of the world’s production. In November 2017, fruit rot was observed on pumpkins that were harvested from local pumpkin fields in Harbin, China, and stored at 8°C in a commercial food processing facility in Harbin, China. Disease incidence was approximately 10% on the fruit. Symptoms appeared on the rind of the fruit as soft, water-soaked lesions, slightly sunken, irregularly shaped with abundant and clearly visible white to pink mycelia. Each fruit had three to eight lesions that could cover 30 to 60% of the fruit surface at the later stages of storage. Fruit flesh under the lesions became soft and rotted. Eight pieces of tissue, one from each diseased fruit, were surface disinfested by soaking in 0.1% HgCl₂ for 2 min, rinsed with sterile distilled water, and then transferred to potato dextrose agar (PDA). Hyphal tips of fungal cultures growing around the tissues were transferred to PDA plates after 2 days of incubation at 26°C in the dark. Eight isolates with similar morphological characteristics were obtained from tissue samples of eight fruit, and single-spore cultures were prepared from each sample as described previously (Leslie and Summerell 2006). The isolates produced white aerial mycelium on PDA, which gradually developed a rose pigmentation with a brownish color in the center and grayish rose at the periphery of the colony. Microconidia were not observed. Macroconidia were produced from monophialides on branched conidiophores, which were slender, equilaterally curved, and measured 29.4 to 54.6 μm in length and 3.1 to 5.2 μm in width, with three to five septa. Chlamydospores were usually formed in chains. All isolates were identified as Fusarium acuminatum based on the morphological characteristics (Gerlach and Nirenberg 1982; Leslie and Summerell 2006). Genomic DNA was extracted from two isolates, and the internal transcribed spacer regions (ITS) and translation elongation factor 1-alpha genes (TEF-1a) were amplified in a polymerase chain reaction using the primers ITS1and ITS4 (Yin et al. 2012) and EF1-728F/EF1-986R (Carbone and Kohn 1999), respectively. Sequences of the two isolates were identical, and DNA sequence of a representative isolate NG1 was deposited in GenBank (accession nos. MK287977 and MH025841, respectively). MegaBLAST analysis of the ITS and TEF-1a sequences showed 99% similarity with F. acuminatum strain DJ11-2-1 (KJ737377) and strain NRRL 52789 (JF740857), respectively. Pathogenicity of isolate NG1 was assessed by fulfilling Koch’s postulates. Five pumpkin fruit (cv. Xiangli23) were stab inoculated by piercing into 1-mm depth using a fine needle and applying 100 μl of conidial suspension of the isolate (1 × 10⁵ spores/ml) to the surface of the wounded area. Five fruit treated with an equal volume of sterile water were used as the control. The fruit were placed in a humidity chamber (>95% relative humidity, 8°C) for 48 h after inoculation and then in an incubator at 8°C. All inoculated fruit showed symptoms similar to those observed in the food processing facility 20 days after inoculation, whereas no symptoms were observed on the control fruit. The pathogen was reisolated from the diseased fruit and identified as F. acuminatum by morphological characteristics and DNA sequencing. F. acuminatum was reported to cause fruit rot on preharvest pumpkins in the United States (Elmer 1996; Wyenandt et al. 2010). To the best of our knowledge, this is the first report of F. acuminatum causing postharvest fruit rot on pumpkin in China. Pumpkins are widely grown for food and other uses in China. The occurrence of this new disease needs to be carefully monitored, and effective disease management approaches need to be implemented during pumpkin storage and transportation.