Effect of Aspergillus niger on shoot emergence and vine development in field-sown yams (Dioscorea spp.) and rot development under long-term storage conditions

The effects of infection of seed yam by Aspergillus niger on shoot emergence and vine development were investigated in three yam species. Shoot emergence was particularly poor in D. esculenta; this was attributed to a greater susceptibility to A. niger due to the relatively smaller size of the tuber, less compact inner tissue and thinner periderm in the species. Reductions in field emergence in all three species were atributed to both A. niger infection and insect damage. An additional factor, dormancy, was suggested in D. esculenta. D. alata seed yams inoculated after bruising produced a higher number of shoots than D. rotundata and D. esculenta. The bruising may have stimulated the release of β-sitosterol, an antifungal compound known to be present in the periderm of this species. The high numbers of shoots produced may also represent an additional response to A. niger infection. The longer the period for seed yams to emerge due to infection by A. niger, the lower was shoot growth during the first six weeks after emergence, with implications for leaf formation and subsequent tuber production. The development and control of rot by A. niger on seed yam tubers was investigated under long-term storage conditions. D. alata was the most resistant with D. rotundata and D. esculenta equally susceptible, but D. esculenta lost weight more rapidly than the two other species. The relatively less compact inner tissue and thinner periderm in D. esculenta may again be responsible for high susceptibility and rapid weight loss. Benzoic acid, within the acceptable tolerance level in foods, was equally effective as the widely used fungicide Benlate for rot control. Sterilization of tubers using sodium hypochlorite solution was ineffective.