First Report of Cercospora apii s. lat. causing a leaf spot on Cardinal Flower (Lobelia cardinalis) in the United States
Trigiano, R. N., S. L. Boggess, and K. D. Gwinn.  2018.  Plant Disease, Volume 102 (1): 252.

Cardinal flower (Lobelia cardinalis L.: Campanulaceae) is a wildflower common throughout much of the eastern United States and grows in a variety of habitats. It is used as an ornamental plant because its bright red flowers appear in the fall and attract hummingbirds. The foliage is generally free of disease when the plant is not flowering, although a few leaf and stem lesions may be present during the growing season. In August/September 2016 and 2017, plants established in eastern Tennessee developed numerous spots on stems and leaves. The round- to lenticular-shaped lesions had dry, white, papery necrotic centers surrounded by reddish-purple host tissue. There were no signs of a pathogen. Leaves were washed in running water for 5 min, surface disinfested with 1.05% NaClO for 15 min, and rinsed three times in sterile water. Lesions and some green tissue from leaves were transferred to half-strength potato dextrose agar (PDA) augmented with 10 mg rifampicin/liter, and incubated in the dark. Mycelium was initially white and cottony, then brown-reddish, and became gray/black and appressed to the medium with time. Conidia were not formed on any of several media. Mycelium and agar from four isolates were air-dried at room temperature and the red pigment extracted with 100% ethyl acetate. The UV-visible spectrum of the extracted pigments was compared with a cercosporin standard and also verified using color changes associated with addition of acid or base. Genomic DNA was isolated from mycelium and amplified with ITS1 and ITS4 primers for partial 18S rRNA subunit (White et al. 1990) and sequenced. The sequence was 100% identical to Cercospora cf. malloti (KT193692.1) and deposited in GenBank as KY940304. Sequences amplified by TEF1 (Carbone and Kohn 1999) (MF687454) and H3-1 primers (Glass and Donaldson 1995) (MF687455) aligned well with a number of Cercospora species, but did not provide resolution to a single species. This fungus has been variously described under names including C. lobelia (Kellerman and Swingle 1889), a synonym of C. cf. malloti, which Crous and Braun (2003) reduced to synonymy with C. apii s. lat., a complex of Cercospora species. Symptom- and sign-free leaves of L. cardinalis were surface disinfested and placed on 1.5% water agar in Petri dishes. The upper surfaces of seven leaves were inoculated with ∼9 mm2 plugs of mycelia grown on PDA for 7 days. Mycelial plugs also were placed adjacent to the leaves. Controls included placing a blank PDA agar plug on seven leaves, and noninoculated leaves. All cultures were incubated in darkness at ∼22°C for 7 days. Leaves paired with the fungus had necrotic lesions with slight reddening of surrounding tissue within 5 days, whereas leaves in the two noninoculated controls remained green and healthy. The same fungus was reisolated from lesions and we concluded from the completion of Koch’s postulates that the disease was caused by a species in the C. appi s. lat. complex. This fungus does not kill plants, but can cause extensive defoliation and cosmetic damage on ornamental plantings, especially during flowering