Kava dieback in the South Pacific

• Ministry of Agriculture, Fisheries and Forests, Fiji
• Ministry of Agriculture and Forestry, Tonga
• Department of Agriculture and Horticulture, Vanuatu

Kava (Piper methysticum) is a traditional crop and a very important cash commodity in the island nations of the South Pacific. It is grown by many small landholders and yields better returns per hectare than alternative crops. A member of the pepper family, the multi-stemmed shrub does not set seed and is propagated vegetatively by placing stem cuttings into the soil. Kava is usually harvested 2-5 years after planting and the roots and rhizome are used to prepare the traditional beverage of the South Pacific. This has been used in the South Pacific for centuries without obvious ill effects. It is drunk at social, political and religious ceremonies and on welcoming visitors. It is also used as a medicine.

A serious dieback disease of kava occurs in Fiji, Tonga, Vanuatu and Samoa. Plant pathologists in Fiji estimate that the disease causes annual production losses of around 60%. Several attempts have been made to establish the cause without success. However, results of an earlier ACIAR project (8548) indicated that dieback is caused by a pathogen, spread either by producing airborne inoculum or through transmission by an airborne vector.

The objectives of the project are first to determine the cause of kava dieback and then to develop disease management strategies to reduce the impact of the disease on kava production. To establish the cause, scientists will use:

. the conventional approach, where relationships between the disease and the presence of pathogens are studied;
. the chemical approach, where plants grown in the field are treated with various chemicals and antibiotics (which have activity against specific groups of plant pathogens) and their responses monitored;
. the method-of-spread approach, where pattens of spread of the disease within kava gardens are monitored.

The collaborators will also screen kava cultivars to determine whether they exhibit varying resistance to the disease.

In the conventional approach, attempts will be made to isolate potential pathogens from diseased tissue, to identify and culture candidate organisms, and to test them for their pathogenicity on kava. This approach will be successful only if the causative organism can be grown on artificial media. Attempts will also be made to transmit the disease using methods such as grafting and insect vectors.

Tissues from diseased plants will be studied in Australian laboratories to determine whether virus particles, mycoplasma-like organisms or other pathogens are present. The collaborators will seek assistance for this aspect of the study from the diagnostic services sections of the Institute of Plant Sciences, Burnley, Victoria and the Plant Pathology Branch of the Queensland Department of Primary Industries. The Plant Pathology Laboratory at the University of New England has obtained a quarantine permit to study organisms isolated from kava and is well equipped for such studies.

Plants for chemical trials were planted in Tonga, Fiji and Vanuatu in February and March 1990. The incidence and severity of dieback in the chemical trials will be monitored until the plants are harvested at 3.5 years. The various chemicals and antibiotics will be applied either as soil drenches or as foliar applications. Trials to screen cultivars for their resistance were also set up early in 1990.

Mr I.R. Davis, a Research Fellow associated with the project, has been stationed at the Plant Pathology Laboratory at Tonga's Vaini Experiment Farm, and will visit experimental sites at Fiji and Vanuatu each year. The Project Leader, Dr J.F. Brown of the University of New England, will visit all three countries a number of times during the course of the project.

If the cause of dieback is found and the disease can be controlled, there will be a marked increase in productivity and efficiency. The same yield will be produced by fewer plants, and the total yield per hectare will increase. This would benefit small landholders, because most kava is grown by traditional farmers. Family incomes would increase, and countries such as Fiji, which now have to import kava, may become self-sufficient, or even come to export the commodity.

The project will contribute towards research training of the collaborators in the developing countries and will provide research experience for the Australian scientists.