First Report of <i>Choanephora cucurbitarum</i> Causing Blossom Blight on Passion Fruit (<i>Passiflora edulis</i>) in China

Passion fruit (Passiflora edulis Sims), which widely grows in tropical and subtropical regions (Wang et al., 2023; Xu et al., 2023), is an economically important fruit with large-scale cultivation in Guangxi, China. In September 2022, symptoms of blossom blight were observed on passion fruit (cultivar Tainong No. 1) in the commercial planting areas in Guangxi (23°15'9''N, 108°3'30''E). The incidence of this disease was approximately 30% in a field with a 2.5 ha planting area. Initially, there were water-soaked spots on the infected flowers, which progressed to a wet rot under high humidity, and the diseased flowers were covered with white mycelia and dark masses of sporangiophores bearing brown to black sporangia and sporangiola. Three fungal isolates (C9-23-1, C9-23-2, and C9-23-3) were isolated from three symptomatic blossoms, respectively. Colonies on potato dextrose agar (PDA) medium were white and cottony, and later became pale yellow, covering the 90 mm plate in three days at 28°C. Sporangiophores were aseptate, hyaline, and unbranched. Sporangia were globular to ellipsoid, brown to dark brown, 96.1 to 102.1 µm in length, and 88.6 to 95.8 µm in width (n =15). Sporangiospores from sporangia were brown to dark brown, ellipsoid to ovoid, distinctly longitudinally striate, about 7.4 to 17.8 µm wide and 11.0 to 25.8µm long (n = 30). Sporangiola were brown to dark brown, oval or spindle-shaped, 7.4 to 17.8 µm wide, and 15.5 to 29.8 µm long (n=30). These characteristics were consistent with the descriptions of Choanephora cucurbitarum (Park et al., 2016). To confirm the identity, the internal transcribed spacer (ITS) region and the large subunit (LSU) of ribosomal DNA (rDNA) were amplified and sequenced with the universal primer ITS1/ITS4 and LROR /LR5 (White et al.1990; Rehner & Samuels 1994), respectively. The BLAST results showed that the ITS and LSU sequences shared 99.83% to 100% identity with the corresponding sequences of C. cucurbitarum strain CBS 178.76. Moreover, a phylogenetic tree based on the two genes revealed that all three isolates were closest to C. cucurbitarum. Based on these results, the isolates were identified as C. cucurbitarum. Then pathogenicity tests were conducted by spraying six flowers with a sporangiospore suspension (1 × 105 spore/mL), which were then kept in the greenhouse (28°C, 85% to 95% RH). Control flowers were sprayed with sterile water. After three days, sporulation of C. cucurbitarum was present on all symptomatic flowers similar to that was observed in the field, while control flowers were asymptomatic. The isolates from artificially infected plants were confirmed as C. cucurbitarum through morphological identification, thus fulfilling Koch's postulates. C. cucurbitarum has a wide range of hosts, causing fruit and blossom rot in cucurbits and other plants, including cabbage, ice plant, beans, potato, cauliflower, okra and lettuce, Pinellia ternata, Crotalaria spectabilis, Crotalaria breviflora, etc ( Alfenas et al., 2018; Min et al., 2017; Oliveira et al., 2021; Ryu et al., 2022; Wang et al., 2021). However, to our knowledge, this is the first report of C. cucurbitarum causing blossom blight on passion fruit in China and worldwide. Guangxi is the biggest producer of passion fruit in China. Many growers in the region are reporting similar symptoms in their passion fruit plantations. This study will provide valuable information for growers and plant pathologists to improve the management of blossom blight.