First report of Ganoderma colossum on Ficus altissima and Delonix regia in Oman

Abstract

The lofty fig (Ficus altissima) and the flame tree (Delonix regia) are important amenity trees in the Sultanate of Oman. Commonly planted on roadsides, in parks and public gardens, they grow well under the high temperature conditions of the Gulf region. During the summer of 2002 a large number of trees, initially Ficus and later Delonix, on the campus of Sultan Qaboos University, Muscat, began to show symptoms of leaf yellowing and branch dieback. Sheets of white mycelium were seen under the bark of both species. From the trunks of affected trees, bracket-like, spongy, sessile basidiocarps appeared. These were initially rounded, pale brown above and cream-yellow below, eventually reaching a diameter of 20 cm and a thickness of 8 cm. Trees died within 6 months of symptoms first appearing. Fruiting bodies were sent to the Royal Botanic Garden, Kew, UK (RBGK) and identified as Ganoderma collosum (Peter Roberts, personal communication). The basidiospores were brown, ovate with a rounded base and a truncate to narrowly rounded apex, bitunicate, measuring 14–16 × 9–11 µm. The pathogen appears to be a pantropical species on a wide variety of woody hosts. It causes selective and extensive delignification and decay of wood. It has been reported as a root and stem rot pathogen of pine, Eucalyptus and Callitris in South Africa (Luckhoff, 1955). The species was originally described from Costa Rica and it has been reported on Phoenix canariensis, Ficus carica and Celtics laevigata in the USA (Adaskaveg & Gilbertson, 1988), and on Delonix regia trunk (with no symptoms) in Vietnam (Kleinwachter et al., 2001). The morphology of the basidiospores, pilocystidia and other basidiocarp characters of the fungus were compared with related species (Adaskaveg & Gilbertson, 1988). The fungus can be distinguished from other similar species by the formation of thick-walled chlamydospores and its ability to grow slowly at 45°C and survive exposure to temperatures up to 50°C (Adaskaveg & Gilbertson, 1989). Specimens of G. colossum are held at RBGK originating from Saudi Arabia (date palm), Malaysia and Sierra Leone (oil palm), Pakistan (bamboo), and on Gmelina arborea from Nigeria. Research is now underway at Sultan Qaboos University to examine implications for other amenity species and the economically important date palms (Phoenix dactylifera). We are grateful to Peter Roberts for identification of G. colossum and information about specimens held at RBGK.