This project has amalgamated two earlier ACIAR projects - 8402 and 8403 - which investigated coconut cadang-cadang viroid (CCCVd) and coconut foliar decay virus (CFDV) respectively. Common methodologies and control procedures may be developed for both diseases and it is thus most efficient to survey and investigate them together.

Cadang-cadang disease causes major losses of coconut palms in the central Philippines - about 0.5 million palms a year. Its viroid agent (CCCVd) occurs in several molecular forms, which appear in a pattern that can be used to indicate the approximate time elapsed since infection. A similar disease in Guam is caused by the closely related coconut tinangaja viroid (CTiVd). CCCVd and CFDV cannot be controlled. Their mode of transmission is unknown, and testing of many coconut cultivars has failed to reveal any with tolerance or resistance to CCCVd.

The development of assay systems for CCCVd based on polyacrylamide gel electrophoresis (PAGE) and molecular hybridisation has facilitated surveys to determine the viroid's distribution. These will supersede surveys previously done on the basis of disease symptoms only. The results will be used to map distribution and rate of spread of CCCVd and to determine the incidence of latent infections. They will also serve as a basis for testing eradication as a control measure. A mobile laboratory has been built to test palm samples on field trips using PAGE.

Some evidence suggests that other monocotyledonous plant species contain nucleic acids similar to CCCVd and more species are still being tested. Continuing development of assay methods is important, both for evaluation of the disease in the Philippines and for transfer of survey methodologies to other countries, and the research programs on viroid pathology, epidemiology and host resistance at the Albay Research Centre should therefore continue.

Scientists in Adelaide will isolate, purify and characterise CCCVd-like molecules in alternative hosts, particularly to determine their degree of relatedness and possible role in pathogenesis. This may lead to the inoculation of specific variants of CCCVd to look for interference and/or cross protection.

Foliar decay disease has been recognised since 1964 in Vanuatu, where an agent transmitted by the cixiid plant hopper, Myndus taffini, kills introduced palms, whereas the local cultivar shows no evidence of disease. Project 8403 identified an unusual virus, now described as coconut foliar decay virus (CFDV), as the cause of this disease, and probes consisting of radioactively labelled cloned fragments of the virus DNA have been developed for diagnosis and experimental studies. In recent work, the viral DNA was detected in the local symptomless coconuts, showing that they are tolerant rather than resistant to the disease. Research is continuing on the possible use of tolerance in coconut breeding and on the host range, epidemiology, and distribution of CFDV. Molecular diagnostic tests for CFDV, CCCVd and related pathogens will be used to screen embryolings produced through ACIAR project 8442, and so facilitate quarantine for intercountry movement of coconut germplasm.

The recent detection of CCCVd-like sequences in oil and coconut palms in a number of areas outside the Philippines could indicate that similar viroids occur widely in the Pacific region. An early evaluation is needed to determine whether CCCDv, CFDV and other related viroids or viruses are damaging coconut and other palms in the Pacific Islands. A survey of Solomon Islands is planned and further surveys will be undertaken in the region, involving local authorities and the South Pacific Commission.

Cost-benefit ratios will be high if successful control measures can be developed. Provision of viroid and virus-tested material for new planting areas would have very high value. Moreover, the research will contribute to the general understanding of viroid and virus diseases of crop plants world-wide.