Acacia mangium Willd. is an important economic tree species with diverse uses and high ecological value, used in agriculture, restoration of degraded lands, for ornamental purposes, and commercial forestry (Leroy et al. 2009). This tree is widely cultivated in tropical and subtropical regions of China, such as Hainan, Guangdong, and Guangxi Provinces. In July 2021, a leaf blight of A. mangium saplings was observed with widespread distribution in the cultivation base of Hainan University (20° 3' 33″ N, 110°18' 56″ E) in Hainan Province, China. The initial symptoms were brown-to-black, irregular-shaped lesions on the leaf margin or tip. The entire leaf wilted and turned brown, leading to death and defoliation in the later periods of the disease (Fig. 1A-C). Of the 50 trees surveyed, the incidence rate was 68%. Small pieces of diseased tissue were collected from six different infected trees. The tissue surface was disinfested with 75% ethanol for 30 s, and 1% mercury chloride for 1 min, then rinsed three times with sterile distilled water. Infected tissues were placed onto potato dextrose agar (PDA) containing 0.01% streptomycin and incubated at 25°C for 7 days. Seventeen isolates were obtained by single-spore isolation on fresh PDA plates, and 15 isolates were identified morphologically as Fusarium spp. (Leslie et al. 2006). The colony surface was white, and the reverse side yellowish (Fig. 1D-E). Microconidia were oval or kidney-shaped, 6.2 to 9.6 × 2.5 to 6.3 μm (n=60). Macroconidia were sickle-shaped, with three to five septa, 19.5 to 41.2 × 3.7 to 6.8 μm (n=60) (Fig. 1F). For molecular identification, genomic DNA of three isolates (HNKFS01, HNKFS02, and HNKFS03) was extracted using E.Z.N.A.® HP Plant DNA kit (Omega Bio-Tek) and the internal transcribed spacer of rDNA (ITS), translation elongation factor 1-α (EF1-α), RNA polymerase II beta subunit (RPB2), and β-tubulin (TUB) regions were amplified and sequenced using primers ITS1F/ITS4, EF-1/EF-2, RPB2-5F2/11aR, and T1/T2, respectively (O'Donnell et al. 2010; O'Donnell and Cigelnik 1997; White et al. 1990). Sequences were deposited in GenBank for ITS (OM289152 to OM289154), EF1-α (OM289155 to OM289157), RPB2 (ON193365 to ON193367), and TUB (ON193314 to ON193316). Using BLASTn, ITS sequences matched 98.88% (532/538 bp) to F. falciforme (MT114705), EF1-α sequences matched 99.58% (703/706 bp; MK752502), RPB2 sequences matched 98.31% (988/1005 bp; MW691193), and TUB gene sequences matched 99.10% (332/335 bp; OK087483). Polyphasic identification with ITS, EF1-α, RPB2, and TUB sequences revealed a 98.51% match with F. falciforme (NRRL 32729) sequences in Fusarium-ID databases. Pathogenicity of 15 isolates was determined by using healthy one-year-old A. mangium seedlings. Three leaves from each seedling were selected to test. Mycelial plugs of each isolate were inoculated on one leaf (three plugs per leaf). An agar plug without the fungus was placed on a leaf as a control. The pathogenicity tests were repeated twice. After 7 days of incubation in a greenhouse, all leaves inoculated with the pathogen showed black lesions with white flocculent hyphae (Fig. 1H), while control leaves were asymptomatic (Fig. 1G). Typical colonies of F. falciforme were isolated from all inoculated leaves, and identified by morphology and ITS, EF1-α, RPB2, and TUB sequence analysis, fulfilling Koch´s postulates. To our knowledge, this is the first report of F. falciforme causing leaf blight in A. mangium in China and the world. This disease can lead to death of A. mangium, which seriously restricts commercialization, requiring that management strategies be adopted.
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