Vascular wilt caused by the ascomycete fungal pathogen Fusarium oxysporum f. sp. cubense (Foc) is a major constraint of banana production around the world. The virulent race, namely Tropical Race 4, can infect all Cavendish-type banana plants and is now widespread across the globe, causing devastating losses to global banana production. In this study, we characterized Foc Subtropical Race 4 (STR4) resistance in a wild banana relative which, through estimated genome size and ancestry analysis, was confirmed to be Musa acuminata ssp. malaccensis. Using a self-derived F₂ population segregating for STR4 resistance, quantitative trait loci sequencing (QTL-seq) was performed on bulks consisting of resistant and susceptible individuals. Changes in SNP index between the bulks revealed a major QTL located on the distal end of the long arm of chromosome 3. Multiple resistance genes are present in this region. Identification of chromosome regions conferring resistance to Foc can facilitate marker assisted selection in breeding programs and paves the way towards identifying genes underpinning resistance.

Fusarium wilt of banana is a devastating disease that has decimated banana production worldwide. Host resistance to Fusarium oxysporum f. sp. Cubense (Foc), the causal agent of this disease, is genetically dissected in this study using two Musa acuminata ssp. Malaccensis segregating populations, segregating for Foc Tropical (TR4) and Subtropical (STR4) race 4 resistance. Marker loci and trait association using 11 SNP-based PCR markers allowed the candidate region to be delimited to a 12.9 cM genetic interval corresponding to a 959 kb region on chromosome 3 of ‘DH-Pahang’ reference assembly v4. Within this region, there was a cluster of pattern recognition receptors, namely leucine-rich repeat ectodomain containing receptor-like protein kinases, cysteine-rich cell-wall-associated protein kinases, and leaf rust 10 disease-resistance locus receptor-like proteins, positioned in an interspersed arrangement. Their transcript levels were rapidly upregulated in the resistant progenies but not in the susceptible F₂ progenies at the onset of infection. This suggests that one or several of these genes may control resistance at this locus. To confirm the segregation of single-gene resistance, we generated an inter-cross between the resistant parent ‘Ma850’ and a susceptible line ‘Ma848’, to show that the STR4 resistance co-segregated with marker ‘28820’ at this locus. Finally, an informative SNP marker 29730 allowed the locus-specific resistance to be assessed in a collection of diploid and polyploid banana plants. Of the 60 lines screened, 22 lines were predicted to carry resistance at this locus, including lines known to be TR4-resistant, such as ‘Pahang’, ‘SH-3362’, ‘SH-3217’, ‘Ma-ITC0250’, and ‘DH-Pahang/CIRAD 930’. Additional screening in the International Institute for Tropical Agriculture’s collection suggests that the dominant allele is common among the elite ‘Matooke’ NARITA hybrids, as well as in other triploid or tetraploid hybrids derived from East African highland bananas. Fine mapping and candidate gene identification will allow characterization of molecular mechanisms underlying the TR4 resistance. The markers developed in this study can now aid the marker-assisted selection of TR4 resistance in breeding programs around the world.

Passion fruit (Passiflora edulis Sims), which is native to South America, is an important fruit crop in tropical and subtropical countries. Passion fruit growing areas have increased rapidly in sou...

Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense (Foc), poses a significant threat to global banana production. This study used a GFP-tagged subtropical race 4 strain of Foc (GFP-Foc-STR4) to trace the pathogen’s movement in different banana cultivars. These include a race 4 resistant cultivar FHIA25 and the Cavendish somaclone ‘GCTCV119’, as well as susceptible cultivars including ‘Lady Finger’, ‘FHIA02’, and ‘Williams’ Cavendish. GFP localization revealed that GFP-Foc-STR4 was able to infect all tested cultivars, moving from the roots to the rhizome and aerial parts of the plant. Tyloses formation in root and rhizome vasculature, visualised with GFP autofluorescence and confirmed by scanning electron microscopy, was found to restrict Foc within the xylem vessels, slowing its spread but not fully preventing infection. This containment mechanism contributes to the host tolerance of ‘FHIA25’ and ‘GCTCV119’, though it does not confer complete immunity. The use of the fluorescently tagged Foc strain provides valuable insight into the infection process, and supports efforts in the integrated management of Fusarium wilt of banana.

The genus Passiflora, commonly known as passion fruit, originated in South America, is an economically important horticulture crop and widely distributed in the tropics and subtropics. Yellow passion fruit (Passiflora edulis f. flavicarpa) and purple passion fruit (Passiflora edulis f. edulis) are the two most planted species (Santos-Jiménez et al., 2022), which have been largely cultivated in southern China. The average annual production reaches 600,000 tons, of which yellow fruit accounts for more than 70% (Zhou et al., 2022). In 2022 to 2023, a disease caused flower rot severely in passion fruit plantations. The incidence rate was generally 10% in purple passion fruit, with an incidence up to 60% in yellow passion fruit 'Qinmi No. 9'. Flower rot occurs mainly in the rainy season, especially during periods of prolonged rain. Infected flowers had black patches that were water-soaked on the interior of the flower bud. The patches covered the entire flower bud, and fluffy mycelium and sporangia developed, which caused the flower bud rotten and abscised easily. Five symptomatic flowers from Wuhua, Guangdong (23°23'N, 115°18'E) and 8 symptomatic flowers from Shangsi, Guangxi (21°15'N, 107°98'E) of 'Qinmi No. 9' were collected during flowering period in 2022 and 2023. Diseased flower pieces were surface-sterilized with 70% ethanol for 2 to 3 min, rinsed with sterile distilled water 3 times, and placed on PDA medium at 25℃ in darkness. Four and 6 fungal isolates with similar morphology were isolated from the infected samples of Wuhua and Shangsi, respectively. Two isolates, PRFJ01 from Wuhua and PRGX02 from Shangsi, were randomly selected for further study. Purified fungal colonies at the age of 3 days accompany with diffuse cottony mycelia, turned white to gray later. The mycelia were hyaline and aseptate. Sporangiophores with 0.56 (0.22~1.10) mm in length and 6.1 (3.18~10.87) μm in width (n=100) were erect, light brown, and had rhizoids and stolons at their bases. Sporangia with 48.0 (23.45~92.85) μm in diameter (n=100) were dark-colored, near spherical and having dark ovoid sporangiospores with 3.56 (2.34~6.39) μm × 2.82 (1.73~4.70) μm (n=100). The morphology of the fungus were identical to Rhizopus stolonifer (Ehrenb.) Vuill (Haque et al. 2023). The two isolates were molecularly identified using genomic regions of 28S large ribosomal subunit (LSU) with NL1 and LR3 primers (Cruz-Lachica et al., 2018). The phylogenetic trees revealed the sequences of PRFJ01 (OR801560.1) and PRGX02 (OR801561.1) were 100% and 99% identical to R. stolonifer (MK705761.1 and KC412868.1), respectively. Pathogenicity tests were conducted on healthy flowers and leaves of 5-month-old grafted 'Qinmi No. 9' plants. Mycelial plugs with 5-mm diameter were placed on the flowers and leaves. Three plants were performed for each of the isolates, and the test was repeated twice. The inoculated plants were moisturized with plastic bags. Healthy flowers and leaves inoculated with sterile PDA plugs were used as control. Typical symptoms were observed on inoculated plants after 2 days. The dark grey mycelia and sporangia covered the entire flower after 4 days inoculation. The flower bud became putrid and the flower stalk split off. Lesions on leaves expanded accompany with numerous aerial mycelium. However, the controls were symptomless. R. stolonifer was reisolated from inoculated tissues. Previously, flower rot on passion fruit caused by R. stolonifer has only been recorded in Brazil (Ploetz, 2003). To our knowledge, this is the first report of R. stolonifer causing flower rot on passion fruit in 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.