Phenotypic and pathogenic characterization of Pseudocercospora cladosporioides, causal agent of cercospora leaf spot of olives

Abstract

Ninety-two samples of olive leaves showing cercospora leaf spot (CLS) symptoms were collected from 1995 to 2003 in Spain and Morocco. Additionally, leaves were also collected from four carob groves showing CLS symptoms. Five representative isolates of Pseudocercospora cladosporioides, the causal agent of CLS, were recovered from the affected olive leaves and used to evaluate the effect of different culture media on mycelial growth and conidial production. Conidia obtained from the leaf samples were used for morphological and pathogenic characterization. Slow mycelial growth was observed on most of the culture media tested, with Czapek Dox agar, malt agar and diluted PDA being the most successful. Colonies of P. cladosporioides showed two well-differentiated mycelial types. Production of conidia was not observed in any culture media. The conidia obtained from the leaves were hyaline, septate, straight or slightly curved, rounded at the apex and with truncated basal cells with a rounded end. Large differences in length and width were observed between the conidia of P. cladosporioides and those of P. ceratoniae, obtained from olive and carob, respectively. The optimum growth and germination temperatures ranged from 15 to 25 °C. Pathogenicity tests on detached olive leaves and fruit demonstrated that the minimal incubation period was 30 and 7 days, respectively. The length of the latency period was 11 months in the pathogenicity tests on the olive plants. Disease symptoms only developed on youngest leaves. This study provides relevant information regarding in vitro management of P. cladosporioides and its pathogenicity to olive.