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Significance

Taro, Colocasia esculenta, is grown under flooded or dry land conditions on over 1.8 million ha (4.5 million acres) and is the fourteenth most consumed vegetable worldwide. It is an important staple crop throughout the tropics and part of the traditional culture in places like Hawaii and the Samoan Archipelago. The leaves are eaten cooked, and the corm is baked, boiled, fried, pounded into a paste (poi), or made into flour.

Marian Raciborski first reported Phytophthora colocasiae, the cause of taro leaf blight disease, in Java in 1900. From its origin, possibly in eastern India or Indo-Malaysia, it spread throughout the tropical climates of Southeast Asia and the Pacific, Africa, the Caribbean, and the Americas. Reductions in corm yield of 25-50% have been reported in the Pacific and 25-35% in the Philippines. Losses may be greater among highly susceptible cultivars. Leaf yield losses of 95% were reported for susceptible varieties in Hawaii.

The most recent epidemic of taro leaf blight occurred in the Samoan Archipelago in 1993-1994. In 1993, the taro export market for Independent Samoa was US$3.5 million, accounting for 58% of Samoa’s exports. By 1994 leaf blight had swept the islands and exported taro was valued at less than US$60,000. American Samoa, 90 km (75 miles) to the east, produced 357,000 kg (786,000 lb) of taro in 1993. This dropped to about 22,000 kg (50,000 lb) in 1994 and 5,000 kg (11,000 lb) in 1995. The severity of this epidemic was mainly due to extensive cropping with a single, susceptible cultivar throughout the archipelago.

The warm, humid days and cool, wet nights of the tropics are ideal for reproduction and spread of P. colocasiae. During rainy weather, leaves on taro cultivars that normally live for 30-40 days may be destroyed in less than 20 days. Therefore, a healthy plant that carries 5-7 functional leaves may have only 2-3 leaves when infected. This reduces net photosynthesis, resulting in a reduced corm yield. Highly susceptible cultivars appear to be melting in the field, producing smaller and smaller leaves on shorter and shorter petioles.