What's Bad for Yam...

Research by the International Institute of Tropical Agriculture (IITA) has confirmed that a major phytoparasitic nematode previously known to only infect yam (Dioscorea spp.) has the potential to attack and destroy Irish potato. The pest, Scutellonema bradys, or simply "yam nematode," causes severe crop losses, suboptimal yields and the deterioration of yam tubers while in storage.

Nematode-infested (left) and healthy (right) yam tubers. Scutellonema bradys, or simply "yam nematode", is a major endoparasite which causes severe crop losses, suboptimal yields and deterioration of tubers while in storage.

Yam is a preferred staple tuber crop in West Africa. Annual production in sub-Saharan Africa is about 45 million tons, which is 95% of the world's total production. In West Africa, some 11 million tons of yams are lost annually because of damage in storage initiated by S. bradys. With West African yam exports rising, there is a heightened risk of S. bradys spreading to potato-producing countries in southern Africa, where potato production already suffers from other pests and diseases.

The discovery has important implications for trade between the two regions of Africa. The uncontrolled spread of this nematode is especially worrying for small-scale farmers in southern Africa, where potato is the most important commodity crop and about 50,000 hectares are planted to it.

Like other nematode species, S. bradys transmits viruses when it feeds on tubers, making them more prone to fungal and bacterial attacks that cause, for example, dry rot and wet rot. Yam nematodes reproduce and build up large populations in stored tubers, causing severe damage. The largest population recorded was 6,200 nematodes per gram of tuber.

The first recorded instance of the nematode attacking potato was in the key Nigerian potato-growing area of Jos. The sample potato specimens showed cracking of the cortex, a symptom associated with an S. bradys attack. To confirm the discovery, IITA conducted yam nematode inoculation tests on local potato tubers sourced from Ibadan, Nigeria.

Potato tubers potted in sterile soil were inoculated with about 5,000 S. bradys obtained from infected yam 2 weeks after being transplanted from sprouting beds. Plants inoculated with the yam nematode produced tubers with substantial cracking of the cortex and evident tuber rot.

Other symptoms observed that are typical of yam nematode infestation included a scaly appearance, surface and deep tissue cracks and distortions, and sub-surface rotting. Morphological tests of the mature nematodes recovered from the soil, roots and tubers of the inoculated plants were confirmed to be S. bradys.

IITA scientists urge further research to better understand the pathology of this nematode so that appropriate plant-quarantine measures can be formulated to stem its advance and prevent potentially severe crop losses.