Identification of Corynespora torulosa (Sydow) Cros Isolate SJ1, the Causal Agent of Leaf Spot Disease on Banana cv. Kluai Khai and Infection of the Pathogen

Abstract

Banana leaf spot is an important disease on the production of banana cv. Kluai Khai for export in
eastern region of Thailand. It causes a significant reduction in either standard-fruit size for export or weight
loss. Furthermore, it also affects health of the growers due to the continue use a lot of fungicide for controlling
the disease for long periods of time. Unfortunately, to recent times, there has been far too little study on
this disease in Thailand. This research aims to study on infection of the causal fungus and its conidial
germination. It also focuses on identification of the pathogen by molecular technique. The causal agent
was isolated from diseased leaves harvested from 3 banana growing areas: Chanthaburi, Trat and Rayong

provinces. The result revealed that the plant pathogenic fungus, Corynespora sp. isolate SJ1 was obtained.
This pathogen infected the leaf tissues and expressed the tiny spot symptoms within 3 days after inoculation.
The lesions later expanded and exhibited yellow haloes surrounding the spots, subsequently the necrotic
areas were exposed after 7 days after inoculation. Additionally, conidial germination of the pathogen was
assayed. The results showed that conidia could germinate within a 1-hour period on PDA. The best infection
on banana leaf by the pathogen was observed at the minimum conidial density of 2,400 conidia/cm2
. The
infection of pathogen on plant tissue was prevalent at the abaxial surface of the leaf. Moreover, identification
of the pathogen by nucleotide sequencing analysis of rDNA-ITS region was determined. It was shown that
the PCR product of Corynespora sp. isolate SJ1 was 590 base pairs, and it was similar to Corynespora
torulosa strain CPC15989 (Accession no. KF777154.1) and Corynespora cassiicola (Accession no.
GQ407100.1) with 99% identity. Phylogenetic tree analysis and morphological study of this pathogen were
also investigated. The result showed that Corynespora sp. isolate SJ1 was identified as Corynespora
torulosa. The knowledge obtained from this study will be useful for investigation of leaf spot disease of
banana for future research.