The comparative inportance of different oil palm tissues as infection sources for basal stem rot in replantings

The efficacy of sanitation at the time of replanting oil palm (Elaeis guineensis) as a means of minimising the subsequent incidence of basal stem rot (BSR), caused by Ganoderma spp., has sometimes been doubted due to variable results following attempted clean celaring. Using bait seedlings under controlled conditions, this was investigated in North Sumatra by assessing the relative importance of plam stumps, trunks and roots as potential sources of BSR. All trials showed that stumps were the most highly infective tissues. Symptoms appeared in bait seedlings after about six months, with a mean symptom appearance time of about 14 months. Larger stumps (50 cm high) become infection foci more slowly than shorter (20 cm) stumps. Stumps derived from healthy palms become BSR sources long after BSR stumps are no longer infective and continue to be infection foci for years. Disease could be contracted by bait seedlings up to 60 cm beneath a BSR point, with inoculum amount clearly diminishing with increasing depth. Although trunk sections became infection foci to a lesser extent than stumps, they are still of considerable practical significance, e.g. where they are windrowed as a part of replanting technique. Their rate of decomposition is much greater than that of stumps. The great diversity of fungi non-pathogenic to oil palm developing on trunks, particularly when poisoned before felling, points to the possibility of biological control of Ganoderma. Trunk tissues pieces of size similar to those when palms are shredded as a replanting method could also give rise to BSR, especially when buried. Aggregates of infected roots comprise a small, but still significant, source of BSR for a replant. Pre-felling poisoning by paraquat accelerated both oil palm decomposition and infection of bait seedlings. Legume cover over tissues of old palms may also hasten infection. No BSR tissues were found to be infective after two years, suggesting a fallow period, if feasible, would reduce later BSR incidence. The results indicated that the times of greatest practical significance for BSR infection in a replant are (1) soon afater planting when suitable inocula remain in the ground, giving the potential for fungicidal control at this stage, and (2) later, when root contact is made with infective tissues in windrows. In the absence of other measures, pre-felling poisoning, removal or destruction of the old palm tissues, deep tining, a fallow period and planting away from windows and former planting points are collectively indicated as means of minimising the incidence of BSR in a replanting.