LAUSR

Apricot (Prunus armeniaca) is one of the most important economic fruit species in China, which is highly prized for its excellent quality and high nutritional value. In June 2020, fruit rot was observed on 10 to 15% of apricot fruit at the full fruit stage in an orchard of Yiyang County (34.54° N, 112.28° E), Luoyang City, Henan Province, China. Initially, the symptoms appeared as brown lesions. With the aggravation of the disease, the lesions enlarged and appeared white hyphae on the fruit surface, which eventually turned soft and brown rots. To identify the pathogen, the surface of symptomatic fruit was sterilized with 2% sodium hypochlorite for 2 min followed by 70% ethanol for 30 s and rinsed 3 times with sterilized water. Tissues (5 × 5 mm) from the margin of the necrotic lesion were cut and cultured on potato dextrose agar (PDA) and incubated for 7 days at 28±1°C for morphological identification. Five pure isolates were obtained from single spores. The isolates had abundant white aerial mycelia initially and turned light violet on the third day. Macroconidia were falciform, 2-5 septate, straight or slightly curved, 15.27 to 37.18 × 1.97 to 3.99 µm (n = 50). Microconidia predominated and were elliptical or oval, hyaline, 0-1 septate, 5.08 to 14.42 × 1.65 to 4.10 µm (n = 50). Chlamydospores were spherical, intercalary or terminal. According to these morphological characteristics, the fungus was initially identified as Fusarium species (Leslie and Summerell 2006). To confirm the identification, The ITS, cal, RPB2, EF and ACT genes were amplified and sequenced with the primers ITS1/ITS4 (White et al. 1990), CL1C/CL2C (O'Donnell et al. 2000), fRPB2-5F/fRPB2-7cR (Liu et al. 1999), EF-1Ha/EF-2Tb and ACT-512F/ACT-783R (Carbone and Kohn 1999), respectively. Compared with the sequence in GenBank, it showed 99-100% homology to F. oxysporum (GenBank accessions No. MT560381, LS423442, AB986568, MN417202, MK001023), and the results of sequences were deposited into GenBank with accession No. MW812394, MW849863, OK067318, MW792498, MW849864. A maximum likelihood (ML) phylogenetic analysis based on ITS, EF and RPB2 sequences using MEGA7.0. (Mao et al. 2021), revealed that the isolates were placed in the F. oxysporum species complex. To confirm pathogenicity, a spore suspension was prepared from the cultures grown on PDA for 7 days at 28±1°C. Twenty healthy P. armeniaca fruit were disinfected with 75% ethanol. Half of the disinfected fruit was wounded using a sterile needle and the other half remained non-wounded. These wounded and non-wounded fruit were inoculated with spore suspension (1.0 × 106 conidia/ml). Fruit inoculated with sterilized water served as the controls. After 5 days of incubation at 28±1°C and 90% relative humidity (12 h light/dark), all the inoculated fruit developed typical symptoms similar to the original diseased fruit, whereas the control fruit remained healthy. The pathogenicity test was repeated two times and F. oxysporum was re-isolated from the inoculated fruit, fulfilling Koch's postulates. Although the pathogenicity of F. oxysporum has been previously studied in apricot fruit in vivo (Seyed and Raeisi 2018), this is the first report of F. oxysporum causing fruit rot on P. armeniaca in nature. This finding will help develop effective disease management strategies.