Ganoderma steyaertanum as a root-rot pathogen of forest trees

Two Acacia seed orchards, one each of A. mangium and A. auriculiformis, in Central Java have suffered high losses from root rot associated with a species of Ganoderma that is distinct from G. philippii, which causes extensive losses in A. mangium plantations in Sumatra and Kalimantan. Ganoderma steyaertanum was consistently isolated from roots of affected trees in the Javanese orchards and pathogenicity was confirmed by satisfying Koch's postulates. The mode of spread of the pathogen was examined by analysing the distribution of tree deaths in the A. mangium seed orchard and conducting somatic incompatibility tests among G. steyaertanum isolates from diseased trees. Spatial analyses indicate that, after an initially random disease distribution, the pattern of tree deaths was clustered. This is consistent with spore colonisation of thinned stumps followed by vegetative spread through root-to-root contact, as has been observed for other root pathogens such as Heterobasidion annosum. Somatic incompatibility tests demonstrated high genetic variability of the pathogen with root-to-root spread also confirmed.