Eye-spot of Rudbeckia laciniata caused by Corynespora cassiicola in Brazil

Rudbeckia laciniata (common names: cutleaf coneflower in USA or margarida-amarela in Brazil) is a member of the Asteraceae and native to North America. It is traditionally used in the USA as a medicine, as well as being grown widely as an ornamental in the USA and abroad. During 2004 and 2005, R. laciniata plants cultivated at the Campus of the Universidade Federal de Viçosa (Minas Gerais, Brazil) were found attacked by a severe foliage disease. Primary leaf symptoms are circular-to-elliptical, dark brown-to-nearly-black leaf spots (eye-spots), 1.0-7.0 x 1.5-17.0 mm diameter and surrounded by a yellow halo. The spots coalesced, leading to extensive leaf necrosis with occasional shot-holes (Figs 1-2). Petiole and stem lesions are brown, elongated and irregularly dispersed.

A dematiaceous hyphomycete with the following morphological features was consistently found associated with the lesions: conidiophores formed singly (150.0-262.5 µm long x 7.5-12.5 µm thick), 3-8 septate and brown; conidia fusiform, obclavate to subcylindrical, straight or curved (62.5-154.0 x 18.0-22.5 µm), subhyaline to brown, 5-16 distosseptate (Figs 3-8). The organism conformed most closely to Corynespora cassiicola, a highly polyphagous fungus (Ellis, 1971). A sample was deposited in the local herbarium (VIC 28791).

Pure cultures were cultivated from sporulating leaf lesions on V8 juice agar. Three unwounded healthy plants were brush-inoculated with a conidial suspension containing 4.4 x 105 conidia per ml and left for 24 hours in a humid chamber. Similar symptoms to those observed in the field were seen after 7 days on inoculated plants (Fig. 9). C. cassiicola was re-isolated from these lesions. Control plants showed no leaf spots. This is the first record of C. cassiicola attacking R. laciniata worldwide (Farr et al, 1989).