Reproducible infection of intact roots of oil palm (Elaeis guineensis) with Ganoderma boninense, the cause of basal stem rot, showed penetration followed by rapid longitudinal progression of hyphae and colonization of the lower stem (bole). Light and transmission electron microscopy showed invasion of the root cortex, with no evidence of selective progression through the vascular system or lacunae. In newly colonized tissue the fungus behaved as a hemibiotroph, with numerous, wide, intracellular hyphae occupying entire host cells that possessed intact cell walls and contained discernible cytoplasm and organelles. In the bole this phase coincided with a complete depletion of previously abundant starch grains in advance of invasion. Subsequently, in the roots and colonized stem base, widespread necrotrophic, enzymatic attack of all layers of the host cell walls occurred. Hyphae were intra-and intercellular and intramural and associated host cell wall degradation was often at a distance from hyphae, resulting in cavities within cell walls. A third developmental stage was the formation of an extensive, melanized, tough mycelium or pseudo-sclerotium which surrounded roots and comprised many very thickwalled cells encasing more typical thin-walled hyphae. Macroscopic observation of and isolation from the bole of randomly felled, commercial palms provided confirmatory evidence that multiple infections originated in the roots before spreading into the base of long-established palms.
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.
Basidiospores are implicated in the distribution and genetic diversity of Ganoderma boninense, cause of basal stem rot (BSR) and upper stem rot (USR) of oil palm (Elaeis guineensis). Measurement of aerial basidiospores within plantations in Sumatra showed continuous and high production over 24 h (range c. 2-11 000 spores m ) with maximum release during early evening. Basidiospores applied to cut surfaces of fronds, peduncles and stems germinated in situ. Equivalent, extensive wounds are created during plantation harvesting and management and represent potential sites for formation of infective heterokaryons following mating of haploid basidiospore germlings. Use of spore-sized micro-beads showed that basidiospores could be pulled up to 10 cm into severed xylem vessels, where they are relatively protected from dehydration, UV irradiation and competing microflora. Diversity of isolates from five locations on two plantations was assessed by RAMS fingerprinting. Isolates from within individual palms with USR were identical and represent single infections, but different USR infections had unique band patterns and revealed separate infections. Some BSR-affected trees contained more than one isolate, and thus had multiple infections. There was one example of adjacent BSR palms with the same isolate, indicating vegetative spread, but there were no identical genets from BSR infections and adjacent fallen palms. Isolate diversity was as great within a plantation as between plantations. It is evident that basidiospores play a major role in spread and genetic variability of G. boninense. Evidence for direct basidiospore infection via cut fronds, indirectly through roots via colonized debris and less frequently, infection by vegetative, clonal spread is considered.
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