Cercospora leaf spot in Toona ciliata: epidemiology and infection process of Cercospora cf. alchemillicola

Abstract

The biology of Cercospora leaf spot (Cercospora cf. alchemillicola) of Toona ciliata was examined in terms of the effects of temperature and duration of foliage wetting on the progress of disease and the mechanism of pathogen infection. To evaluate the effect of temperature on mycelial growth, cercosporin production and conidial germination, experiments were conducted in a 4 × 5 factorial design, with four isolates (HR01, HR02, HR03 and CML3059) at five temperatures (15, 19, 23, 26 and 30°C). The area under the disease progress curve (AUDPC) was calculated in a 3 × 5 factorial design at three temperatures (20, 23 and 29°C) and five leaf wetness durations (0, 12, 24, 48 and 72 hr). Penetration of C. cf. alchemillicola into leaves was examined by scanning electron microscopy. Maximum mycelial growth of all isolates occurred at 24.8°C. HR02 and HR01 produced higher quantities of cercosporin compared with CML3059 and HR03. Temperatures between 21.6 and 23.0°C favoured production of the toxin among isolates. Higher conidial germination rates occurred in isolates HR02 and HR01 compared with other isolates, with optimum germination occurring at 24.2–26.3°C. A leaf wetting period of 72 hr at 23.3°C gave the highest AUDPC value. No hyphal penetration was observed through natural openings. However, germ tubes grew inside wounds on the leaf epidermis. In addition, Phyllachora balansae (cause of tar spot) opened fissures on the leaf epidermis, thus creating wounds enabling C. cf. alchemillicola to infect.