Effects of Calcium Fertilization on the Susceptibility of Dioscorea Species to the Yam Storage Pathogens Aspergillus Niger van Tiegh and Botryodiplodia Theobromae Pat

The effects of soil amendment with basal dose of nitrogen (60 kg ha- 1), phosphorus (30 kg ha-1) and potassium (75 kg ha-1) for Yams Dioscorea species in South West Nigeria and calcium carbonate were assessed on yield and susceptibility to two storage rot pathogens, in Dioscorea alata TDa 92-2, D. rotundata TDr 131 and D. esculenta TDe 89-1. The study site at the Teaching and Research Farms of the Federal University of Agriculture, Abeokuta, had been monocropped to yams without fertilizer for three years before the study. A 3 by 5 factorial desgn, with the Dioscorea species as main plots and 0 kg ha-1, 2 kg ha-1, 4 kg ha-1, 6 kg ha-1 and 8 kg ha-1 calcium carbonate as subplot treatments, was used in each of two years. Aspergillus niger van Tiegh. and Botryodiplodia theobromae Pat. were the two yam storage pathogens used. Pre-planting soil analysis in the two years revealed calcium, magnesium and potassium below the critical levels required for Dioscorea species in South Western Nigeria. Calcium fertilizer enhanced resistance and reduced weight loss in yam ‘’sections;’’ of D. rotundata TDr 131 and Dioscorea alata TDa 92-2 to the pathogens after long-terms storage of over four months in both years. Response of whole tubers incubated for an additional (after the four months) period of ten weeks and six weeks in years one and two respectively, was not significantiy (P=0.05) different from the control. Futher work to determine endogenous or physiological reasons for this is necessary. D. esculenta TDe 89-1 was more susceptible to the rot pathogens than the other two species. D. esculenta TDe 89-1 was also not significantly different in infection or weight loss than the control in the two years, necessitating a need to ascertain optimum fertilizer placement methods for this species in Nigeria. There were no differences in tuber number or tuber weight per plant at the treatment levels.