Darkening leaves on a yam plant can signal a serious problem beneath the soil. Anthracnose, a fungal disease commonly known as yam dieback, begins with darkened leaves and can cause the plant to die early, producing undersized tubers.
Farmers in Nigeria are being warned to guard against the disease after it was recently found in Anambra and Enugu States. The states are part of the yam belt, an area of Nigeria that produces almost 70 per cent of the world’s yams. Yams are mostly grown by small-scale farmers, who often intercrop them with cereals and vegetables.
Dr. Kenneth Nwosu is Executive Director of the National Root Crops Research Institute, which investigated the incidence of yam dieback in Anambra and Enugu. He says the disease could spread quickly. And since yams are an important staple in Nigerian diets, there are concerns that this could lead to food insecurity.
The disease was likely introduced to Nigeria’s yam belt by infected cuttings or seed yams. If one farmer uses infected planting materials, yam dieback can spread quickly to neighbouring fields. Since it is carried by a splash-borne pathogen, heavy rains and humidity encourage the spread of the disease.
Dr. John Ikeorgu is the yam program coordinator for the National Root Crops Research Institute. He says that yam dieback devastated Nigeria’s yam crops in 1992. More than 90 per cent of farms surveyed at that time were affected by the disease.
The institute has several suggestions for farmers who want to avoid yam dieback. Farmers should look for fresh, new cuttings or seed yams to plant. In Nigeria, these can be purchased from Agricultural Development Projects or the National Root Crops Research Institute. The institute says its hybrid planting materials are resistant to yam dieback and can produce two crops per year.
Land fallowing – or leaving a field unseeded for a time – can also help. Dr. Ikeorgu explains that yam crops require very fertile soil. Therefore, farmers used to leave fields fallow for five to 10 years between yam crops. But increased demand for food and income have caused farmers to plant yam crops more frequently. This means that pathogens surviving in the soil may re-infect new crops.
Other tips for preventing disease and encouraging good yam yields include: planting yams at the beginning of the rainy season, before the soil becomes too wet; weeding crops four, eight, and 12 weeks after planting; and applying fertilizer at the recommended rate.