Investigation on the activities of yeasts in the post harvest spoilage of sweet potato(<i>Ipomea batatas </i>L.)

Abstract

Postharvest losses due to the activities of indigenous microorganisms occur in many crops such as sweet potato (Ipomoea batatas) which result in heavy financial losses for farmers. Correct identification of the pathogen responsible for postharvest infection is central to adopting an appropriate control strategy. Samples of sweet potato tubers were purchased from the local market and stored at three temperatures: 13°C, 21°C and 29° for four weeks. Spoilage yeasts were isolated from the tuber samples. The isolates were characterized using yeast genomic DNA extraction, polymerase chain amplification of rRNA and sequence determination. The yeasts were identified on the basis of the 26S rDNA. Six yeast species were identified as Rhodotorula mucilaginosa, R. minuta, Pichiaguilliermondii, P. anomala, Sporobolomyces marcillae and Saccharomycopsis fibuligera. The results indicated that the D1 and D2 domains of the 5’ end of the 26S rDNA showed a high degree of interspecies sequence variation for the isolates. Two of the yeasts; P. anomala and R. minuta were selected for further investigations namely pathogenicity testing and assay for extracellular enzymes. Results of the pathogenicity tests showed that P. anomala and R. minuta were clearly able to infect the sweet potato tubers. The results of the enzyme assay revealed that P. anomala and R. minuta were able to secrete varying amounts of eight extracellular enzymes: cellulose, amylase, polygalacturonase, glucanase, xylanase, xylosidase, arabinofuranosidase and ferulic acid esterases. These enzymes have the capacity to degrade plant cell walls and possibly enhanced the pathogenicity of the yeasts. Key Words: Sweet-potato, yeast, Postharvest, Pathogenicity, Ipomoea batatas