Differential Rate of Dry Rot in <i>Dioscorea rotundata</i> (White Yam) along the Tuber Length Due to Rot Causing Fungi

Abstract

The susceptibility of different regions in Dioscorea rotundata (white yam) tuber to rot infecting fungi was investigated. Isolation was made from the periphery of the rotted tuber tissues, followed by a pathogenicity test and identification of isolates. Three fungi associated with D. rotundata (white yam) were isolated; they include Penicillium oxalicum, Aspergillus niger and Rhizopus stolonifer. Each of the isolates from pure cultures were inoculated on the head, middle and tail regions of healthy yam tubers. The three fungi were found to be pathogenic at different rates at the head, middle and tail ends of the yam tuber respectively. The head region was less susceptible to the three fungi with the following rot depths (P. oxalicum 25 mm a , A. niger 18.2 mm a and R. stolenifer 12.7 mm b ). Rot depth in the middle region was (P. oxalicum 15 mm a , A. niger 10.6 mm a and R. stolenifer 8.8 mm b ). While the tail region of the yam tuber recorded rot depth of (P. oxalicum 32.0 mm a , A. niger 26.4 mm a and R. stolenifer 20.8 mm b ) respectively. Generally, the tail region of D. rotundatawas more susceptible to fungal attack and the rot recorded in the tail region was significantly different from rot at the middle and the head. It was recommended that yam tubers should be stacked with their head on the ground to reduce incidence of rotting in stock-piled yam tubers.

Isolation of Six Microorganisms from Rotten <i> Dioscorea alata </i> (Water Yam), and Antimicrobial Sensitivity Test with Nine Plant Extracts

“…In the case of Dioscoreaceae, inhibition of microorganisms is best achieved and prolonged during the dormancy period [23]. Pathogenicity test carried out on yams shows that different sections of yam react differently to microorganisms [17]; it is opined that, this is possibly due to variation of free sugar concentration along the length, which is carbon source to the microorganisms.…”

“…alata like D. rotundata tubers have outer cover, whose microorganisms cannot easily penetrate, but it is easily wounded by rodents, nematodes and man during weeding, harvesting and postharvest handling. Such wounds facilitate the penetration and development of rot microorganisms [17] [36].…”

“…In the dry rot, the infected tissues become hard and dry with varying coloration depending on the microorganism involved. It is caused by the Fusarium species, Aspergillus niger, Aspergillus flavus, and Pseudomonas aeruginosa [17] [34]- [36]. Soft rot causes the infected tissues to become soft ramified by the fungal mycelium and turn brown and sometimes wet due to a rapid collapse of the cell walls inducing pinkish with yellowish border on the affected tissues.…”

“…In storage, heat or high temperature causes rot in the stored tubers [14]- [17]. Water yam tubers respire continuously and need adequate aeration; this is often achieved by providing adequate ventilation or putting on fans to enhance air circulation in the store house or storage area [14] [18] [19].…”

“…The wet or watery rot of yam tuber is characterized by the oozing of whitish fluid from the tissues when pressed. It is usually associated with Erwinia carotovora, a bacterium [17] [34] [38].…”