An investigation of species associated with stem of cacao in Sulawesi under climate change

Phytopthora palmivora Butl. causes various diseases in cocoa, including Phytophthora stem canker (or cacao stem canker) and Phytophthora pod rot. However, in Sulawesi, another pathogen has been reported to infect cocoa causing stem canker and dieback. This study aimed to investigate the fungal/oomycetes species associated with stem canker lesions in Luwu Timur, Sulawesi, Indonesia and thereby identify the main putative pathogen causing stem canker of cocoa in the region. The research was carried out in the field and in the Laboratory of Plant Pests and Diseases of the Muslim University of Indonesia. The study consisted of several stages, namely sampling, preparation of various types of comparison media, isolation from tissue in stem canker lesions, culture and microscopic observation. The test plants consisted of 11 clones (9-10 year-old trees), including 5 clones showing symptoms of stem canker, namely M01, M06, Sul1, Sul2 and clone 45 (MCC02). Infected tissue excised from stem canker lesions was transferred to media plates (V8, PDA). Subcultures resulted in colonies producing hyphae and sporangia identified as Phytophthora spp., consistent with previous reports that P. palmivora causes stem canker in cacao. Furthermore, inoculation of healthy pods of four clones (both attached and detached) with infected canker tissue resulted in lesions demonstrating Phytophthora pod rot symptoms. Lesion diameter determined daily for 8 days, indicated that the largest lesions developed in M06 (101,8 cm) and the smallest in Sul1 (5.8 cm), demonstrating variation in clone resistance to P. palmivora. Re-isolation from the pod lesions caused by inoculations resulted in Phytophthora colonies and sporangia. These results are consistent with previous reports, namely that P. palmivora is the principle cause of cacao stem canker.

Keywords: cocoa, stem canker, phytopthora palmivora, climate change, plant disease