Evaluation of a new method for testing the pathogenicity of molds to yam tubers

Abstract

Aims: The Objective of this study was to compare methods involving cutting of yam, with a method developed in our lab, which exposes the yams to the pathogens but without cutting the protective skin as well as to estimate the weight loss and percentage severity of rots caused by rot-causing molds of yam tubers. Methods: A total of thirteen test molds viz., Aspergillus sp., Aspergillus tamarii, Aspergillus niger, Aspergillus flavus, Fusarium solani, Lasiodiplodia theobromae, Mucor circinelloides, Paecilomyces sp., Fonsecaea sp., Phialophora sp., Graphium sp., Saccharomyces sp. and Exophiala sp., as identified using the partial ITS rDNA sequencing analysis and a BLAST search using the GenBank sequence database, isolated from previous works from five yam varieties namely: Dioscorea dumetorum, two varieties each of D. alata and D. rotundata, were used for this study. Comparative pathogenicity tests were carried out with fresh, healthy yam tubers by the methods described by Okigbo and Nmeka, (2005) and a method developed in our laboratory. The percentage weight loss over a period of four weeks, symptoms observed and percentage severity of Frank Chukwunwike Ogbo1, Kingsley Chukwuebuka Agu2 Affiliations: 1PhD, Ogbo Frank CProfessor, Applied Microbiology and Brewing Department, Nnamdi Azikiwe University, PMB 5025, Awka, Anambra State, Nigeria; 2MSc, Agu, Kingsley CLecturer II, Applied Microbiology and Brewing Department, Nnamdi Azikiwe University, PMB 5025, Awka, Anambra State, Nigeria. Corresponding Author: Frank Chukwunwike Ogbo Awka, Anambra, Nigeria, 234; Ph: +234-8035642014; Email: frankogbo@yahoo.com Received: 04 August 2015 Accepted: 27 October Published: 07 December 2015 rots produced by the pathogenic molds were all studied. Results: The ability of both tests to detect severity of spoilage were studied and recorded as + (symptoms noticed), – (No symptom noticed) and +/C (symptoms noticed and consistent). Only seven isolates were pathogenic by the method of Ogbo and Agu, whereas all thirteen isolates were pathogenic by the method of Okigbo and Nmeka (2005). There was no significant difference p>0.05 between the weight losses and percentage severity of rots amongst the test molds with both methods except in the case of D. alata var. abana mmee tested with Fusarium solani which displayed high percentage rot severity of 450.00 by the method of Ogbo and Agu as against 153.85 displayed by the method of Okigbo and Nmeka (2005). Results obtained helped to ascertain the state of the molds (opportunistic pathogens or pathogens). Conclusion: Only molds capable of breaching the protective cover of yam tubers and eliciting original symptoms consistent with those seen in previous studies and reported in other literatures were actual yam pathogens.