First Report of Fruit rot Caused by Phytophthora nicotianae on Passion Fruit in Guangxi Province, China.

Passion fruit (Passiflora edulis) is an economically important fruit crop in many tropical and subtropical regions worldwide. In recent years, passion fruit was widely cultivated in Guangxi Province. In 2020, a rot disease occurred on immature fruit of passion fruit in several commercial orchards of Nanning, Guangxi, caused about 50% incidence. The first appeared as small, irregular, water-soaked, brown lesions on immature fruit. As the disease progressed, the lesions rapidly enlarged, causing fruit rot. A layer of sparse white mycelia appeared on the lesions at high humidity. The disease first developed in June, its peak periods from August to September. Five diseased fruits were collected from five different orchards. The edges of symptomatic fleshy mesocarp tissue were cut into pieces (5 mm × 5 mm), surface-sterilized in 75% ethanol solution for 60 s, rinsed three times with sterilized distilled water, and plated on potato dextrose agar (PDA). Plates were incubated at 25°C in the dark. After 5 days, similar white colonies with abundant aerial mycelia developed from all plated tissue samples. Five isolates were obtained, and they were identified as Phytophthora nicotianae based on morphological characteristics and DNA analysis. Spherical hyphal swellings were commonly produced. Numerous sporangia were formed in sterile soil extract. Sporangia were ovoid or obpyriform, papillate, and measured 25 to 58 μm (average 41 μm) × 21 to 45 μm (average 29 μm). Chlamydospores were spherical and 19 to 43 μm in diameter (average 30 μm) (Erwin and Ribeiro 1996). The genomic DNA of a representative isolate Seg2-5 was extracted from mycelia through modified CTAB method (Murray and Thompson 1980). The rDNA internal transcribed spacer (ITS) region, ypt1, and coxII were amplified and sequenced with primers ITS1/ITS4 (White et al., 1990), Yph1F/Yph2R (Schena et al. 2008), and FM75F/FM78R (Villa et al. 2006), respectively. BLAST searches of the ITS, ypt1, and coxII sequences (Accession No. MW470847, MW770870, and MW770871) showed 99 to 100% identity with sequences of P. nicotianae (Accession No. JF792540, MK058408, and MH551183). Based on morphological characteristics and phylogenetic analysis, isolate Seg2-5 was identified as P. nicotianae. To confirm pathogenicity, asymptomatic and immature fruits 'Mantianxing' of passion fruit were previously disinfested in 0.5% sodium hypochlorite. Mycelial plugs of isolate Seg2-5 were placed onto the surface of fruits by nonwounded and pin-prick inoculation. Blank plugs were used as negative controls. Each treatment had five replicates and the test was repeated twice. Fruits were maintained in plastic boxes at 28°C and the initial disease spots appeared at 3 dpi or 5 dpi with wounded or non-wounded inoculation. After 7 to 10 days, all inoculated fruits showed similar symptoms as observed initially in the field, whereas control fruits remained healthy. P. nicotianae was successfully reisolated and identified from the inoculated fruits based on morphological characters and ITS sequence, thus confirming Koch's postulates. P. nicotianae had been previously isolated from passion fruit in South Africa (Van and Huller 1970), Vietnam (Nguyen et al. 2015), and Fujian Province of China (Luo et al. 1993). To our knowledge, this is the first report of P. nicotianae infecting passion fruit in Guangxi Province, China.