1PNG OPRA, PO Box 36, Alotau, Milne Bay Province, Papua New Guinea; 2CABI Bioscience, Egham, Surrey, UK

Background and objectives
Field practices currently employed to control basal stem rot of oil palm caused by Ganoderma sp. involves complete removal of all infected palms from the plantation including a significant portion of the root mass below ground level. These methods are costly and time consuming and were based on the assumption that initial infection arose from infected debris within the soil and that secondary spread away from the initial infection site was by root to root contact. Work at both IMI (1) and PORIM on the population dynamics of Ganoderma sp. in single fields of oil palm using somatic compatibility, RFLPs of mitochondrial DNA and isoenzyme analyses indicated that these field populations consisted of numerous distinct individuals, nearly all mutually incompatible with those from neighbouring palms. Although this work does not preclude primary infection arising from the soil, it does not support root to root contact as the method of secondary spread. The mating system of Ganoderma sp. on oil palm in PNG has been shown by us to be not only tetrapolar but based on multiple alleles and therefore strongly encourages outcrossing. This highly developed mating system along with the heterogeneity of the Ganoderma population suggests that the basidiospore has an important role in the life cycle of the disease. The disease patterns in the field (in PNG) show a random spread of infection which is not usual for a crop under high spore load pressures. There are many publications (2) which describe varying degrees of resistance to Ganoderma. This is not surprising since plants used to establish an oil palm plantation are usually the progeny of different species or varieties of oil palm (eg. Elaeis guineensis Deli dura x tenera or E. guineensis x E. oleifera) and as such will be segregating populations. It is not unreasonable to expect that these populations will be segregating for characters which confer susceptibility to Ganoderma.

Results and conclusions
Over the three year period 1995-97, four surveys for basal stem rot were carried out in Milne Bay covering an area of 6215 ha. During this time 2111 palms were recorded with symptoms of basal stem rot and control strategies were implemented and modified to achieve a zero incidence of Ganoderma sporophores within the plantation.

The symptoms of infected palms fell within three broad categories covering a range from highly susceptible to those with a high level of tolerance to the disease. (i) 6 and 8 year old palms became stunted, wilted and died within about 12 months; (ii) 8-10 year old palms developed 'typical' symptoms (i.e. yellowing of crown, collapsed fronds, male flowers only, some internal rot); (iii) those palms with no top symptoms, still producing fruit bunches and extensive basal rot or Ganoderma brackets. If the spores are playing an important role in the disease epidemiology and the oil palm population is segregating for susceptibility, then the infection process is screening the host population for susceptibility and the pathogen population for aggressiveness. This will inevitably lead to earlier infection times and higher disease incidence. Brackets of Ganoderma only developed on between 18-68% of palms and it is these palms that the control strategy targets for removal. The infected base is removed from the plantation and the root ring protruding from the soil is broken into small pieces. The root ball however, is left in the ground and covered with soil. Tolerant palms without brackets are left in production.

1. Miller, R.N.G., Holderness, M. Bridge, P.D., Paterson, R.R.M., Hussein, M.Z., (1995), EPPO Bulletin 25, 81-87.
2. Purba, R.Y., Purba, A.R., Sipayung, A. (1994), Bulletin Pusat Penelitian Kelapa Sawit, 2, 81-88.