Mirabilis jalapa L., known as ʻmarvel of Peru’ or ʻfour o’clock flower’, belongs to the family Nyctaginaceae and is native to the arid tropical regions of the North, Central, and South American continents, including Mexico, Guatemala, Chile, and Peru. This plant has been introduced for ornamental purposes and has become naturalized in tropical, subtropical, and temperate regions worldwide [1]. Commercial cultivars are available with various flower colors. Despite its global distribution and worldwide planting as an ornamental plant, records of phytopathogenic fungi on this plant are scarce. A powdery mildew fungus, Pseudoidium nyctaginacearum (Hosag.) U. Braun & R.T.A. Cook from eight countries [2] and the leaf spot fungus Cercospora mirabilis Tharp from six countries are the two major pathogens of M. jalapa [3–5]. Phytopathogens that are rare and confined to a specific region include Pseudocercospora oxybaphi (Ellist & Halst.) U. Braun & Crous from the USA and Phytophthora mirabilis Galindo & H.R. Hohl from Mexico, with some minor records requiring confirmation [6].

Cercospora mirabilis was first described in 1917 on M. jalapa in Texas, USA [7]. Additional records of this fungus have been obtained from China, India, France, Sierra Leone, and Korea [6]. However, C. mirabilis could not be included in a recent extensive study on Cercospora species that used advanced molecular phylogenetic approaches, presumably because of the lack of living cultures [8]. Although most literature sources have listed C. mirabilis without a morphological description, Kim and Shin [9] have provided observations and illustrations of this fungus. However, the morphology and ecology of this fungus have not been sufficiently studied. To the best of our knowledge, no living cultures of C. mirabilis are available in culture collections [https://wfcc.info/]. Therefore, this study aimed to provide information on the morphological characteristics and molecular sequence of C. mirabilis on M. jalapa and to ensure the deposition of the first fungal isolate in an authentic culture collection center.

During the course of this study, six specimens were collected from various parts of Korea and preserved at the Korea University Herbarium (Seoul, Korea) for morphological examination. These were as follows: KUS-F14452 (17 Oct 1997, Seodun-dong, Suwon), F15599 (28 Oct 1998, Wabu village, Namyangju), F317159 (22 Oct 1999, Deokso farm, Namyangju), and F32007 (4 Oct 2020, Naksan Park, Seoul). To obtain monoconidial isolates of C. mirabilis, two additional samples were collected from Seoul and Jeonju in October 2024 and later deposited as voucher specimens in the fungorium of Jeonbuk National University (accession numbers JBNU-F0570 and JBNU-F0575).

Detailed morphological characteristics of the fungus were observed using fresh samples under an Olympus BX50 microscope (Olympus, Tokyo, Japan). Photomicrographs were captured using a Zeiss AX10 microscope equipped with an AxioCam MRc5 camera (Carl Zeiss, Oberkochen, Germany). For fungal isolation, conidia were collected from young lesions, mounted in a drop of sterile water, and streaked onto 2% water agar (WA; Junsei, Tokyo, Japan) plates supplemented with 100 mg/L streptomycin sulfate. The inoculated plates were then incubated at 25℃. After two days, the germinating conidia were transferred onto 2% potato dextrose agar (PDA; Difco, Detroit, MI, USA) plates to obtain a pure culture.

Leaf spots were amphigenous, scattered, distinct, small, circular to subcircular, mostly not exceeding 4 mm in diameter, rarely reaching up to 10 mm when confluent, initially pale brown to tan, and later the center became dull gray due to plentiful sporulation, with or without blackish-brown margins (Fig. 1A). Caespituli were amphigenous. Stromata were small to medium, composed of swollen hyphal cells, brown to dark brown, subglobular to angular, and 20–42 μm in diameter. (Fig. 1B). Conidiophores were 4 to 18 in a divergent fascicles, arising from substomatal stromata or emerging through the cuticle, olivaceous brown to grayish brown throughout, usually uniform in width, straight to mildly curved, rarely 2 to 3 times geniculate, 1–3(–4)-septate, and 50–140 × 4–5 μm; conidial scars were large, 2–3 μm wide, conspicuous, apical or on shoulders of conidiogenous cells (Fig. 1C). Conidia were solitary, filiform to cylindrical, straight to slightly curved, 2–10-septate, hyaline, non-constricted at the septa, obtuse at the apex, subtruncate to obtuse at the base, 24–110 × 3.5–5.5 μm; hila were conspicuously thickened, darkened, and non-protuberant (Fig. 1D). These characteristics are consistent with previous descriptions of C. mirabilis [9–11].

The culture characteristics of C. mirabilis have not been described previously. Two isolates obtained in this study showed the same growth pattern on PDA. Two-week-old colonies grown on PDA at 25℃ were 25–30 mm in diameter, with an entire margin. The obverse surface was irregularly folded with prominent network of ridges, pale olivaceous to pinkish color with clear undulate, dark pink margin, the reverse of colony was dark olive, with blood red margin (Fig. 1E). The two isolates were deposited in the Korean Agricultural Culture Collection (KACC) under the accession numbers KACC 410991 and KACC 410992.