Effects of inoculum potential, shading and soil temperature on root infection of oil palm seedlings by the basal stem rot pathogen Ganoderma boninense

Ganoderma boninense causes severe losses to oil palm in South East Asia. The disease typically manifests itself as basal stem rot, but there remains controversy over the route of infection and source of inoculum. Using isolates differing in aggressiveness, infection via roots was confirmed; it was also shown that large inoculum provided as Ganoderma ‐infested palm‐ or rubber‐wood blocks (12 × 6 × 6 cm) is necessary for soil infection of seedlings after 6–8 months. Smaller blocks (3 × 3 × 3 cm) produced rapid (≤ 3 months) infection of roots and lower stem when physically attached to roots. Therefore fragmentation of infested palm wood from a felled, mature plantation before subsequent replanting may provide inoculum. Failure of G. boninense to grow through non‐sterile soil or organic debris from frond bases, suggests it is a poor competitor and that roots must contact inoculum directly. Severe disease occurred after 8 months on inoculated seedlings under shade, but not on seedlings exposed to sun. Soil temperatures in sunlight frequently rose above 40°C and reached 45°C, whereas in shade they never exceeded 32°C. Ganoderma boninense is probably inhibited in exposed soil since optimal growth in vitro was 25–30°C, and there was no recovery from 45°C. Soil temperature may explain why symptoms often first appear in mature plantations when canopy formation creates shade. Infection is not peculiar to senescing palms but can occur throughout the growth cycle.