First Report of Rust Caused by Phakopsora nishidana on Creeping Fig, Ficus pumila, in South Africa.

Abstract

Phakopsora nishidana has recently been reported as the causal organism of rust on edible fig, Ficus carica in South Africa (SA) (Boshoff et al. 2022). This contradicted reports by Doidge (1927, 1950) and Verwoerd (1929) who listed Cerotelium fici as the causal organism of the disease in SA. Similarities in urediniospore morphology and differing taxonomic interpretations most likely contributed to the use of both pathogen names as the causal agents for fig rust (Boshoff et al. 2022; Padamsee and McKenzie 2024). In January 2023 rust was commonly observed on nursery specimens of creeping fig, Ficus pumila, a popular evergreen and fast-growing garden plant in SA. Ficus pumila is native to southern China, Indochina, and eastern Asia but has been introduced to many countries worldwide (https://powo.science.kew.org/ accessed 20 March 2024). The F. pumila rust isolate collected in Somerset West (34°07'00.70"S, 18°52'41.75"E; Western Cape (WC) province) was identified using the sequenced 5.8S rRNA-ITS2-28S rRNA locus. When used in a BLAST analysis, this F. pumila isolate (GenBank accession number OR835538) shared the best homology with the Vermont P. nishidana isolate (34°24'33.37"S, 19°08'59.24"E) (MZ047090; e-value 0.0, 99.7% identity, 1400/1404 bp) reported on edible fig by Boshoff et al. (2022). Other P. nishidana isolates with excellent homology were KY764080 (e-value 0.0, 99.8% identity, 1012/1014 bp), KY764081 (e-value 0.0, 99% identity, 882/885 bp) and MF580676 (e-value 0.0, 99.3% identity, 981/988 bp). Accessions were used in a phylogenetic study that included four other rust samples collected from F. carica trees in Somerset West, WC (OR835534; 34°03'37.26"S, 18°51'02.18"E), Onrus River, WC (OR835535; 34°24'43.10"S, 19°09'57.99"E), Elgin, WC (OR835536; 34°14'31.37"S, 19°03'05.38"E), and Bloemfontein, Free State province (OR835537; 29°05'05.1"S 26°09'09.5"E). Reference sequences were as described in Boshoff et al. (2022). Despite limited variation, all six South African isolates, including the F. pumila isolate, grouped with the three P. nishidana reference isolates, but separate from the three P. myrtacearum isolates collected from eucalyptus trees (Maier et al. 2016). Uredinia on F. pumila leaves were mostly hypophyllous and surrounded by a halo of brown, necrotic tissue. On the upper leaf surface, the lesions appeared as small, dark leaf spots with infrequent sporulation. Urediniospores were echinulate, mostly obovoid or elliptical and their cell walls pale-yellow to yellowish-brown. The mean width and length of urediniospores were 16.7 x 21.3 µm. No telia were observed. Applying standard rust inoculation and incubation procedures (Boshoff et al. 2022), F. pumila plants inoculated with P. nishidana isolate PREM63073 produced sporulating uredinia on the abaxial surface of leaves. Likewise, inoculation of F. carica cv. Kadota leaves with the F. pumila isolate OR835538 yielded sporulating lesions containing urediniospores typical of P. nishidana, on the lower surface. Based on both the DNA sequence data and controlled infection studies, our study confirmed the host status of F. pumila for P. nishidana. The occurrence of rust on creeping fig plants in nurseries, the evergreen status of the creeper, and thus the potential to harbor the pathogen during winter, are likely to contribute to the spread of fig rust in SA.