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Symptoms and Signs

Phytophthora colocasiae is primarily a foliar pathogen, but it also affects petioles and corms. The first symptoms on taro (Colocasia esculenta) are small, dark brown flecks or light brown spots on the upper leaf surface (Figure 2). These early spots often occur at the tips and edges of leaves where water accumulates. They enlarge rapidly, becoming circular, zonate, and purplish-brown to brown in color (Figure 3). On the lower leaf surface, spots have a water-soaked, or dry gray appearance (Figure 4). As spots increase in size they coalesce and quickly destroy the leaf (Figure 5). In dry weather, or on some resistant cultivars, the centers of lesions become papery and fall out, producing a “shot-hole” appearance. Dead leaves often hang on their long petioles like flags (Figure 6).

Figure 2	Figure 3
Figure 4	Figure 5
Figure 6

Bright orange or reddish-brown plant exudate oozing from infection sites is another symptom of leaf blight disease in taro (Figure 7). The presence of yellow tissue around lesions (Figure 8) is not well understood, but could be a cultivar specific reaction or a response to dry weather. Infected corm tissue is brown, firm, and develops rapidly after harvest. A prominent sign of P. colocasiae is the white ring of sporangia around the edge of lesions (Figure 9).

Figure 7	Figure 8
Figure 9

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