First Report of Fusarium avenaceum Causing Canker Disease on Kiwi Tree in China.

Abstract

In May 2021, canker symptoms were detected on 'Xuxiang' kiwi trees in southwestern Shaanxi (Hanzhong municipality; 107.27° E, 33.23° N) in China. Seven-year-old trees exhibited black necrotic lesions and cracked areas in the trunk (Figure 1). The symptoms were observed in approximately 10% of the trees in 6 orchards (31 ha in total). Application of commercial fungicides did not control the advancement of the pathogen, and infected trees were removed to control the spread. Three samples, approximately 1 cm2 in size, of symptomatic tissue were collected and surface sterilized in 2% NaOCl for 1 min, and washed with sterile ddH2O. Four isolates showing white mycelium with yellow pigmentation were obtained after 4 days of incubation on PDA, containing chloramphenicol (50 µg/mL), at 28 ºC. The pathogen was isolated from all collected samples. ITS, EF1-α, TUB2, RPB1 and RPB2 genes were amplified using ITS1/ITS4, EF1-728F/EF1-986R, T1/T22, RPB1-5F/RPB1-8R and RPB2-5F/RPB2-7cR (strain NJC06), or RPB2-c7F/RPB2-11aR (strains NJC07 and NJC08), primers, respectively. Two isolates shared the same sequences (strain NJC08). Obtained sequences were submitted to GenBank under accession numbers MZ669205 and OL347898-OL347899 (ITS), OL439731-OL439733 (EF1-α), OL439734-OL439736 (TUB2), OL439737-OL439739 (RPB1), and OL439740-OL439742 (RPB2). The sequences shared >99% (ITS; F. avenaceum CBS 128538, MH864972), >99% (EF1-α; F. avenaceum 55-2, MN473124), 100% (TUB2; F. avenaceum SICAUCC 18-0001, MK253102), >98% (RPB1; F. avenaceum NRRL 26911, MG282372), and >98% (RPB2; F. avenaceum SICAUCC 18-0001, MK396098; or F. avenaceum FRC R-09495, CQ915486) homology to multiple F. avenaceum strains. Molecular phylogenetic tree (Figure 2) was constructed using MEGA7 with Fusarium strains found causing rot in various hosts (Wang et al. 2015), and other fungal species, such as Cadophora nalorum, Diaporthe ambigua, D. australafricana, and Neofusicoccum parvum, which were reported to cause cordon dieback on kiwi tree in Chile (Diaz et al. 2021). Microscope observations after cultivation of all isolates on barley-honey-tryptone medium (Song et al. 2020) showed the presence of septate mycelium, fusiform microconidia (8-15 µm in length, containing between 0 and 3 septa; n = 77) and chlamydospores (n = 21), and agree with the morphology of F. avenaceum (Zhao et al. 2020). To confirm pathogenicity, a sterilized spatula was used to make wounds (3 mm diameter, 1 mm depth) on the trunk of 3-months-old 'Xuxiang' kiwi trees. Solutions containing 1 × 106 spores/mL (20 µL) of the isolates were injected in the wounds. Sterile ddH2O was used for the control experiment. Inoculated plants were maintained in a growth chamber at 28 °C and 80% relative humidity for 4 days. The pathogen was recovered from the canker lesions, which were similar to those observed in the orchards, and its identity was confirmed by sequence analysis. The pathogen only infected wounded trees, and probably invaded the orchards during the pruning in February 2021. F. avenaceum was reported to cause canker on almond tree (Stack et al. 2020), stem rot on Anthoxanthum aristatum and Polygonatum cyrtonema (Pieczul et al. 2018; Xu et al. 2019), and root rot on carrot, Coptis chinensis and wheat (Le Moullec-Rieu et al. 2020; Mei et al. 2020; Ozer et al. 2020). Recently, F. avenaceum was found causing fruit blotch in kiwi fruit in Anhui (China) (Zhao et al. 2020). Here, F. avenaceum was found causing canker disease in kiwi tree, demonstrating the host and tissue promiscuity of this pathogen. Kiwi is an important crop in China with nearly 1.5 million tons produced in 2019. This report will help to better understand the pathogens reducing kiwi production in China.