Current Thinking on Infection and Control Strategies of Basal Stem Rot Disease of Oil Palm in Malaysia

Ganoderma causes basal stem rot and upper stem rot infection of oil palm, and the former is the most frequently found and devastating disease in oil palm cultivation in Indonesia and Malaysia. The primary route of infection through root contact has been generally regarded as the sole mode of infection by the pathogen. Other root-infecting basidiomycete diseases of tree and perennial crops are spread by vegetative growth. After basidiospores of Heterobasidion spp. colonize stump surfaces, the subsequent spread of the fungus from infected stumps to neighbouring trees occurs via root contacts. This vegetative infection of trees creates a clonal spread over a wide area. Armillaria species cause serious root diseases of a wide range of trees and other woody species. For example, in Kenya and Indonesia they affect tea plantations. This fungus can spread by root contact and by cord-like rhizomorphs, which can directly penetrate healthy roots to cause disease. In contrast, various studies using vegetative compatibility and molecular analysis of Ganoderma isolates indicate high genetic variability within oil palm plantations and raise an argument on the issue of disease epidemiology. Currently there are two opinions on the mode of infection: (i) Healthy palm roots encounter a source of inoculum, such as infected roots from adjacent palms; this become infected and mycelial growth progresses along or through the root towards the bole leading to stem rot of the base of the palm. (ii) Basidiospores are dispersed throughout the plantations onto wounded palm tissues and cause direct infection, for example via wounds created on harvesting or pruning of palm petioles (fronds). (iii) Basidiospores land on and colonize palm debris to create an inoculum to be contacted by roots. There is evidence to support the first two routes, which complicates disease control. Therefore, understanding G. boninense infection and disease spread are critical to develop a successful disease management against BSR.