DG HUNTER¹, P SlVAN¹, F AMOSA¹ and K POUONQ²

¹School of Agriculture, Alafua Campus, University of the South Pacific, Samoa; ²Ministry of Agriculture, Forests, Fisheries and Meteorology, Nu'u, Samoa

Background and objectives
The current outbreak of taro leaf blight, caused by Phytophthora colocasiae, in Samoa is a good example of a pathogen that has re-encountered its host. It is believed that both the disease and the taro (Colocasia esculenta) plant are of South-East Asian origin. However, when taro was taken to Samoa on the early migrations by Polynesian settlers, it arrived without Phytophthora colocasiae. Since that time farmers in Samoa have been able to cultivate the crop free from taro leaf blight. This situation changed drastically in 1993 when the first epidemic of taro leaf blight occurred in Samoa. By then the cultivar Niue, which is highly susceptible to the disease, dominated taro production.

The impact of this disease in Samoa has been dramatic. Prior to the arrival of leaf blight, taro was the major staple of Samoans. It was also the major export crop, having recently replaced cocoa and coconut. In financial terms the losses to farmers were devastating, and within a few months of the disease outbreak export earnings had fallen dramatically and taro production had dropped by over 95% [1]. The initial response of the Samoan government was to carry out large-scale spraying with fungicides and place restrictions on the movement of planting material. Despite these actions the disease spread to the nearby island of Savai'i. The rapid spread of the disease prompted the government to supply subsidized fungicides and spraying equipment to farmers. This was folowed by a major information and public awareness programme related to the disease and its control, which included the use of fungicides and crop sanitation. Unfortunately, the majority of traditional farmers in Samoa lack the resources to carry out these methods of control, and as a result taro production in Samoa is undertaken by a few commercial growers while subsistence growers have diversified into other crops. In the Samoan context, reliance on methods of control that require considerable inputs is not sustainable. The objective of current research on taro leaf blight focuses on varietal selection and the breeding of taro varieties with resistance to the disease. The use of cultural approaches, such as intercropprng, is also being investigated.

Results and conclusions
A number of varieties have been screened for resistance to taro leaf blight. Initial studies have shown that four varieties have some resistance to the disease. One of these varieties, PSBG2, originates from the Philippines while the others (Croantoal, Pastora arid, Pwetepwet) are from the Federated States of Micronesia. In addition, the taste and quality of these cultivars is quite comparable to the favoured variety, Niue. These varieties are being multiplied so that they may be released to farmers for testing. Other sources of resistance have been identified, but at present there are concerns about the importation of new pathogen problems with this material. Breeding blocks have been established at USP and Nu'u using a number of cultivars. Seedlings from a number of crosses are currently undergoing screening in the field [2]. It is hoped that the number of varieties in this breeding programme will be expanded in the future as constraints to germplasm movement are overcome.

Trials demonstrated that taro interoropped with maize had significantly lower taro leaf blight levels compared to taro as a sole crop. Future trials will examine the effect of Xanthamonas and Alocasia spp. as intercrops with taro.

References
1. Chan E, 1995. Western Samoa Farming Systems Project, Samoa.
2 Sivan P, Misipati, P, 1997. IRETAs South Pacific Agricultural News 14, 1-6.