Yams (Dioscorea sp., family Dioscoreacea) constitute a major carbohydrate food source in West Africa (Osunde and Yisa, 2003). The tubers have organoleptic qualities which make them the preferred carbohydrate staple and can contribute up to 350 dietary calories per person each day (Asiedu et al., 2001). The plant has a tremendous sink capacity to store food reserves and individual tubers may weigh as much as 20-30 kg (Fuccillo et al., 2007).

 Yam therefore has immense potential as an “insurance crop” in Africa where it is heavily depended upon, because it plays a vital role in food security both at household and national levels. Yams are utilized in different ways in West Africa, a known centre of yam diversity and production. In Ghana for instance, yams may be boiled, mashed, fried or roasted.  Pounded yam or “fufu” eaten with soup is a very popular form of yam consumption. Ghana is the leading exporter of yam in Africa with yam contributing to 16% of the Agricultural GDP (FAOSTAT, 2006). Besides, tubers are processed into flour, noodles, chips, and dry slices (Zheng, 2011). 

Being a popular food product, yam has a surprising number of alternative uses. In the pharmaceutical industry, diosgenin, a steroidal saponin is extracted from the root of wild yam (D. villosa L.) with the potential to minimize post-menopausal symptoms (Benghuzzi et al., 2003) while dioscorin is extracted from the tuber of Chinese yam (Dioscorea batatas Decne) with sufficient antioxidant potential which confers on it tremendous health benefits (Wen-Chi et al., 2001). 

In the hedonistic world, (Dioscorea opposita Thunb.) yam has been successfully used for the preparation of beer (Xu, 2007). The general significance of yams in the lives of a people cannot be over emphasized. However, the production of this tuber crop is constrained by both biotic and abiotic factors. Plant parasitic nematodes constitute a major biotic factor that constrains the production of yams. Those known to cause serious damage are the yam nematode (Scutellonema bradys Steiner and LeHew), the lesion nematode (Pratylenchus coffeae Zimmermann) and the root knot nematodes (Meloidogyne Goeldi spp). The yam nematode, S. bradys causes a decay of tubers known as “dry rot disease”. P. coffeae cause deep cracks on tubers leading to a corky appearance and diseased tubers become spongy (Bridge et al., 2005). Meloidogyne species cause galling of roots and tubers which appear warty. Nematode infestation reduces the market value of yam tubers which negatively affects the farmers’ profit margin.

For sustainable yam production therefore, the nematode menace should be managed. Synthetic agro-pesticides (nematicides) application is the single most effective management strategy against nematodes (Noling, 2012). It is universally acknowledged that synthetic agro-pesticides usage presents environmental problems (Bell, 2000). However, recommended agro-pesticides applied at the recom-mended dosages could be successfully used for food production without endangering man and the environ-ment. Planting on ridges has been shown to increase root and tuber yields (Ennin et al., 2009). The seed bed type has also been reported to interact with fertilizer application with planting on ridges resulting in higher yam tuber yield response to fertilizer than the traditional method of planting on mounds  (Ennin et al., 2013).  The two-fold objectives of this  study  therefore, were to use a recommended nematicide, Fulan (a non-fumigant with 90 days waiting period and dimethoxytetrahydrofuran) in managing plant parasitic nematodes population, and to investigate nematode control interactions with seed bed type and fertilizer application on yam production.

Data analysis

The two years data were pooled and analyzed once using the mixed model (REML) approach. Nematode count and indices based data were log (ln (x +1)) and square root √(1+0.5) transformed to improve homogeneity of variance before analysis using GenStat 8.1 (Lawes Agricultural Trust, VSN International). Significant mean separation was determined with Fisher’s Protected Least Significant Difference (LSD) Test at a ≤ 0.05. 

Significant (p < 0.05) differences were observed amongst treatments in yield of yam. Ridged bed type + the highest level of fertilizer applied (60-60-80 kg/ha) N-P₂0₅-K₂0 + Fulan application interacted positively (p < 0.05) resulting in the highest yields which were 32% and 30% over the control treatments on mound and ridged bed type respectively (Figure 1).

The same treatment resulted in the highest total yield of yam which was 61% over ridged bed type + fertilizer applied at 60-60-60 kg/ha N-P₂0₅-K₂0 without Fulan application (Figure 1). However, no differences were observed in both the number of plants and tubers harvested amongst treatments (Figure 2). All treatments were equally affected by anthracnose and virus infections(Table 1). Treatments did not either inhibit or increase the severity of anthracnose and virus infections. However, Fulan application resulted in reduced tuber cracks. Mound + F4 + Fulan recorded 92% less cracks compared with mound bed type only (control). Fulan application suppressed nematode population compared with untreated plots in the three nematode genera encoun-tered; however, nematode suppression was significant only in the root-knot nematode, M. incognita and the lesion nematode, P. coffeae. The lowest M. incognita and P. coffeae population was recorded in ridged bed type + fertilizer applied at 45-45-60 kg/ha N-P₂0₅-K₂0 + Fulan treatment which was 93% over mound bed type without Fulan and fertilizer application and 97% over mound bed type + fertilizer applied at 60-60-80 kg/ha N-P₂0₅-K₂0 without Fulan application respectively (Table 2). However, population of the reniform nematode, R. reniformis was not affected by treatments.  Similarly, there was no difference amongst treatments in tuber galling index (Table 2).

Reductions of more than 90% were significant. Additionally, the potential of Fulan manifested in the development of insignificant cracks in Fulan treated tubers while untreated tubers suffered major cracks. Though, Fulan suppressed R. reniformis populations, such reductions when compared with untreated plots were not significant. Similarly, no difference existed between tuber galling from Fulan treated samples and untreated samples. The highest yield and nematode suppression were recorded in ridged method of planting. Ridging was effective in reducing nematode density because in ridging, nematodes and their eggs are more exposed to sunshine which kills them compared with mounding, and also resulted in a more efficient use of Fulan and fertilizer. Therefore, farmers’ incentive in adopting ridging over mounding in addition to the foregoing is the fact that ridging represents drudgery saving intervention.

This study has revealed that the recommended nematicide Fulan, applied at 10 g/stand effectively managed plant parasitic nematodes irrespective of the other components of the factorial regime. Application of Fulan increased the response of yam tuber yield to fertilizer rate especially on ridges, with 60-60-80 kg/ha N-P₂0₅-K₂0 resulting in highest yields. Finally, ridging was a better alternative to mounding as yield was not compromised and the method also lends itself to mechanization. Fulan, the non-fumigant with a 90 day resting period is safe for yam production since yam matures at 210 days on the average.

The efficacy of synthetic agro products would continue to attract farmers, their implications on the environment notwithstanding. Pragmatic policies should therefore be put in place by governments to ensure their sustainable use by farmers.

The authors have not declared any conflict of interest.

Authors gratefully acknowledge the financial support of the West Africa Agricultural Productivity Programme (WAAPP), which made the study possible. We are also indebted to John Sackey-Asante and Sampson Obeng, Technicians of the Nematology laboratory and the Crop Resource and Management Division of the Crops Research Institute, respectively.