First report of root rot in Trichosanthes kirilowii (Chinese cucumber) caused by Fusarium sulawesiense in China.

Abstract

Trichosanthes kirilowii (Chinese cucumber) is one of the important perennial herbaceous vines in China, with putative pharmacological activities including anti-tumor and lowering blood lipids. In July 2022, T. kirilowii plants with brownish roots and chlorotic leaves were observed in several orchards in Qianshan, Anhui province, China (30°34'N, 116° 30'E). The disease incidence reached approximately 10% within an area spanning 20 ha, and was higher in poorly drained orchards. To investigate this root rot disease, five symptomatic plants were collected from the diseased orchards in Qianshan. Subsequently, small sections of the diseased roots were surface sterilized using 1% sodium hypochlorite and 75% ethanol for 45 seconds each. Then, sterilized roots were placed onto PDA (20% diced potato, 2% glucose, and 1.5% agar, and distilled water) and incubated at 28℃ in the dark for 6 days. A total of eight isolates with similar morphology were obtained and purified by single spore culturing. Two representative isolates (QSJ4 and QSJ5) were chosen for further analysis. When grown on PDA, the surface of each colony was white with dense aerial mycelium and pale orange color in the center with a white edge on the reverse side. Macroconidia produced on carnation leaf agar plates were falcate, slightly curved, and 3 to 5 septate, with papillate apical cells and indistinct basal cells. Macroconidia were 17.4-42.3 × 2.4-5.8 μm (n = 100). Microconidia were ellipsoidal in shape, slightly curved or not curved, and most were 1-septate, 9.6-16.7 × 1.5-3.8 μm (n = 40). The identity was determined by sequencing four loci (i. e., ITS, CAL, EF1-α and RPB2) from two representative isolates (Liu et al. 1999; O'Donnell et al. 1998, 2000; Reeb et al. 2004; White et al. 1990). Sequences were deposited in GenBank [ITS (OR267397, OR267398), CAL (OR296634, OR296635), EF1-α (OR296637, OR296638) and RPB2 (OR296640, OR296641)]. A phylogenetic analysis was performed with three loci (CAL, EF1-α, RPB2) comprising a concatenated dataset of 68 strains in the Fusarium incarnatum-equiseti species complex (Han et al., 2023). The results showed that isolates QSJ4 and QSJ5 clustered closely together with reference strains of F. sulawesiense. Pathogenicity tests were conducted by inoculating three-week-old healthy T. kirilowii seedings (cv. Wanlou No. 9) cultivated in substrate soil in pots with a diameter of 17 cm and a height of 10.5 cm. A 20 mL aliquot of spore suspension (106 conidia/mL) of F. sulawesiense was inoculated to the roots of potted seedlings by irrigation. Each strain was inoculated onto three seedlings. The potted seedlings were inoculated with sterile water as the negative control. Inoculated seedlings were incubated in a growth chamber at 25℃ and 75% relative humidity. After one week, typical symptoms of root necrosis and leaf chlorosis were observed on the inoculated seedlings. Disease symptoms were not observed on the control seedlings. All seedlings showing root necrosis and leaf chlorosis caused by the inoculations were subjected to fungal isolation, and the results showed that the reisolated colonies matched the inoculated ones for morphologies and ITS sequences. Fusarium sulawesiense has been previously reported to cause disease on Cucumis melo L. in Brazil (Medeiros Araujo et al. 2021), Musa acuminata Colla in south Sulawesi (Maryani et al. 2019), Luffa aegyptiaca Miller and Musa nana Lour. in China (Wang et al. 2019). To our knowledge, this is the first report of F. sulawesiense causing Fusarium root rot of T. kirilowii in China.