First Report of root rot Caused by Fusarium solani and F. breve on kiwifruit in China.

Abstract

China is considered as the main producer of kiwifruit (Actinidia spp.) in the world. During 2020-2021, root rot (~8000 plants, ~5% disease incidence) of 3-year-old kiwifruit (cv. Xuxiang) was observed in Lujiang County (117°24'E, 31°15'N), Anhui, China. This disease usually occurred in fields with poor drainage in hot and humid summers. Symptoms started on leaves showing dehydration and curling, the last root of diseased plant turned black and died. Dig out the skin on rotten root was cracking and flaking and white mycelium covered on surface. Twenty rotten tissues from ten plants were cut and surface disinfected with 1% NaOCl for 5 min, rinsed in sterile water, and cultured on potato dextrose agar (PDA) at 25 ± 2°C in the dark. Fifteen fungal isolates were obtained. The first type (KWRR1, 3-10) was cotton-like, reverse with white outer margin, and light brown inner region on PDA. The second type (KWRR2, 11-15) was cotton-like on PDA but appeared pale yellow in reverse. On oatmeal agar, the KWRR1 colony was flat with little aerial hyphae and was red, while KWRR2 was hyaline. On carnation leaf agar (CLA), microconidia of the KWRR1 and KWRR2 isolates were reniform, fusiform or oblong, 0-1 septate, and measuring 1.9-4.3×8.4-15.7 µm and 3.0-3.8×8.2-16.7 µm, respectively (n=50). The macroconidia of KWRR1 were straight or moderately curved, 3-5 septa (2.7-4.6×21.5-52.6 µm in size, n=50). For KWRR2, the macroconidia were straight or slightly curved and with 3-4 septate, 4.1-4.8×26.1-30.8 µm (n=50). Chlamydospores of the KWRR1 and KWRR2 isolates were 1-2 celled, irregular globose, measuring 4.5-8.5 µm and 7.6-9.0 µm diam, respectively (n=50). To identify the isolates, four DNA fragments (RPB1, RPB2, ITS and TEF-1α) were amplified and sequenced from all isolates (O'Donnell, et al. 2012; White et al. 1990; O'Donnell et al. 2022). BLAST analysis of the RPB1, RPB2, ITS and TEF-1α sequences of the KWRR1 isolates (OL474057, OL474055, OL468550, OP382187) showed highest identity with F. solani (NRRL66304; MW218134, KT313623, KT313633, KT313611) at 98.2%-99.8%, while KWRR2 (OL505579, OL474056, OL468551, OP382188) showed that their homology with F. breve (NRRL28009; HM347149, EF470136, DQ094351, DQ246869) at 98.2%-99.4%. F. solani and F. breve belong to clade 3 of the F. solani species complex (FSSC) (Geiser et al. 2021). Phylogenetic analysis based on RPB2, ITS and TEF-1α sequences with MEGA7 software (Šišić et al. 2018), placed the KWRR1 sequences with F. solani (FSSC5), while there of KWRR2 nested with F. breve (FSSC15). One-year-old seedlings (n=6) of 'Xu Xiang', growing in a greenhouse (at 28℃, relative humidity 80%), were inoculated by drenching the soil with a conidial suspension with one of the two isolates (30 ml, 106 conidia/ml). Control plants (n=6) were inoculated with sterilized water and the pathogenicity assay was repeated three times. One month post-inoculation, the leaves of inoculated plants became chlorotic, wilted and died, whereas the controls were disease-free. F. solani and F. breve were successfully reisolated from diseased samples (n=6) and verified based on morphology and sequencing as described above, fulfilling Koch's postulates. Members of the FSSC cause root rot on many hosts (Coleman. 2016; Schroers et al. 2016), but this is the first report of F. solani and F. breve causing root rot disease on kiwifruit in China. The result will serve as the foundation for management of root rot of kiwifruit.