3.7.3S
GANODERMA DISEASES OF PERENNIAL CROPS

M HOLDERNESS1, PD BRIDGE1, J FLOOD1, H ROLPH1, RNG MILLER1, FR SANDERSON2, Y HASAN3, C PILOTTI2, F ABDULLAH4, R WIJESEKARA5, GF CHUNG6, PM KIRK1 and S MEON4

1CABI Bioscience, Egham, Surrey TW20 9TY, UK; 2PNG OPRA, Box 36, Alotau, PNG; 3PTPP London Sumatra, Bah Lias Res. Stn., PO Perdagangan 21184, Indonesia; 4Universiti Pertanian Malaysia, PO 43400, Serdang, Selangor, Malaysia; 5Coconut Research Institute, Bandirippuwa Estate, Lunuwila, Sri Lanka; 6Sime Darby Plantations, Batu Tiga 40706 Selangor, Malaysia

Background and objectives
Diseases caused by Ganoderma spp. are of major concern to oil palm growers in SE Asia, causing stand decline as pathogens of roots and stem bases, with progressively increased significance over successive replantings. Disease management is constrained by limited understanding of the Ganoderma strains involved and of mechanisms of disease establishment and spread. CABI Bioscience has been involved for several years in research on the disease, in partnership with various research and industry organizations across the Asia/Pacific region. Control currently relies on sanitation through removal of infected plants at replanting, but this is costly and of dubious efficacy. The advent of molecular analyses has enabled new insights into fungal variability and the programme has used these to elucidate mechanisms of pathogen dissemination and evaluate disease management procedures. Ganoderma has been reported as a pathogen on coconut in South Asia, but is only saprobic on coconut in SE Asia, so isolates from the two regions have been compared.

Materials and methods
The programme has used a range of techniques to investigate variability among isolates on various scales, including different hosts and geographic regions as well as local variation in single fields. These have included RFLPs of mitochondrial DNA, AFLPs of genomic DNA, isoenzyme analyses, somatic compatibility and mating group studies and pathogenicity determination.

Results and conclusions
Isolates from palms show common functional characters, notably a uniform pectinase isoenzyme profile [1]. However, within this group, widespread variation was found in other molecular and physiological characters. Somatic compatibility studies on systematic samples from Malaysian oil palm blocks showed that field populations consisted of numerous distinct individuals, nearly all mutually incompatible with those from neighbouring palms. Analysis of mt DNA RFLPs similarly revealed heterogeneity between isolates from neighbouring palms and in some cases within individual palms. Research in Papua New Guinea has shown the mating system of Ganoderma on oil palm to be tetrapolar, favouring outcrossing. Replanting studies in Indonesia have shown that infection of replants arises predominately from contact with diseased debris and stumps. The variation found between heterokaryons in a field may thus be indicative of basidiospore dispersal and infection or a heterogenous long-term residual inoculum in debris from previous plantings, both of which may occur within the disease cycle. The value and economic implications of disease management practices are being investigated under different environmental conditions to determine appropriate strategies. Research on disease management has included the possible use of biodegradative organisms to decompose oil palm debris in advance of its colonization by Ganoderma strains. These agents have been assessed using a range of approaches and various candidate organisms identified. Preliminary evidence from mtDNA RFLP and AFLP analyses of populations on coconut and betel palms in Sri Lanka suggests that Ganoderma populations in these areas may be less variable than those from oil palm. This may reflect genetic isolation or differences between populations adapted to coconut and a 'new-encounter' disease on oil palm.

References
1. Miller, R.N.G., Holderness, M., Meon, S., Bridge, P.D., Paterson, R.R.M. & Hussin, M.Z. 1995 EPPO Bulletin 25, 81-87.