Yam / Taro

Colocasia spp., Dioscorea spp., Alocasia spp. and Xanthosoma spp.

1. Identification and origin

The term “yam” or “taro” is used to designate various species of tropical plants producing edible rhizomes or tubers. The main ones belong to the genera Colocasia, Alocasia and Xanthosoma (Araceae family), as well as to the genus Dioscorea (Dioscoreaceae family).

• Colocasia esculenta (L.) Schott (true taro) is native to Southeast Asia and the Pacific.
• Xanthosoma spp. and Alocasia spp. originate from Tropical America and Southeast Asia.
• Dioscorea spp. (true yams) are native to Africa, Asia and Tropical America.

These crops are widely cultivated in tropical and subtropical regions for human and animal food.

2. Economic importance

The rhizomes and tubers are consumed boiled, roasted or processed into flours, purees and traditional products. In many tropical regions they constitute essential subsistence crops. Countries such as Nigeria, Ghana, Ivory Coast, China, the Philippines and the Pacific islands are major producers. Commercial interest has increased due to the nutritional value and tolerance to poor soils and high humidity.

3. Botanical characterisation

Despite the diversity of genera, they present common characteristics:

• Araceae (Colocasia, Alocasia, Xanthosoma): perennial herbaceous plants, with large, cordiform or sagittate leaves, rhizomes or corms rich in starch and a typical spadix inflorescence.
• Dioscoreaceae (Dioscorea): herbaceous climbers with twining stems, opposite or alternate leaves and underground tubers rich in starch.

The storage organs vary between rhizomes (taro), corms (some Alocasia) and tubers (Dioscorea yams).

4. Soil and climate requirements

They are humid tropical climate crops, with ideal temperatures between 20–30 °C.

• Colocasia and Xanthosoma prefer humid or even waterlogged soils (lowland taro).
• Dioscorea requires well-drained and aerated soils, avoiding waterlogging.
• Alocasia tolerates partial shade and soils rich in organic matter.

The ideal pH lies between 5.5 and 7.0. Production is favoured by high humidity and regular rainfall.

5. Main pests

• Soil beetles (Coleoptera): boreholes in rhizomes and tubers.
• Aphids (Aphis gossypii, Myzus persicae): sap sucking and virus transmission.
• Mites (Tetranychus spp.): chloroses and reduction of vigour.
• Nematodes (Meloidogyne spp., Pratylenchus spp.): root galls, necrosis and rots.
• Defoliating caterpillars (Lepidoptera): reduction of the leaf area.

6. Main diseases

• Taro leaf spot / taro leaf blight (Phytophthora colocasiae): the most serious disease in Colocasia, causing extensive necrosis on the leaves, rapid dehydration and tissue death.
• Root rots (Pythium spp., Phytophthora spp.): common in waterlogged soils, causing wilting and rotting of the underground organs.
• Bacterial soft rots (Erwinia spp.): rapid rotting in humid and hot conditions.
• Viruses (Taro bacilliform virus, Dasheen mosaic virus): mosaics, leaf deformations, reduction of vigour and production.
• Anthracnose (Colletotrichum spp.): leaf spots, necrosis and premature leaf drop.

7. General crop management

It includes the choice of species and varieties adapted to the local climate, planting in soils suitable for the genus (humid for Colocasia, drained for Dioscorea), the use of healthy propagation material, control of excessive humidity, crop rotation to reduce nematodes and fungi, balanced fertilisation and monitoring of pests and diseases. Harvesting should be carried out when the storage organs reach commercial size and the leaves begin to yellow.

Bibliographic references

• EPPO Global Database. Colocasia esculenta, Dioscorea spp. – pests and diseases.
• CABI Invasive Species Compendium. Colocasia, Dioscorea, Xanthosoma – datasheets.
• Lebot, V. (2009). Tropical root and tuber crops. In: Crop Production Science in Horticulture. CABI.
• Ramanatha Rao, V., et al. (2010). Taro genetic resources. In: Kole, C. (Ed.), Wild Crop Relatives. Springer.
• Coursey, D. G. (1967). Yams. In: The Tropical Agriculturalist. Longman.