Abstract:
Fungi shown to cause soft rot symptoms in yam consisted of Aspergillus niger (A1), Aspergillus niger (A9), Botryodiplodia theobromae, Candida SP. (A5, Candida Sp. (A7), Cladosporium Sp. and Sclerotium Sp. No soft rot could be induced with Aspergillus flavus and Rhizopus Sp. Viscometric assays indicated high cellulase, pectin hydrolase and pectin lyase activities for Aspergillus niger (A9). Botryodiploidis theobromae showed high activities for cellulase and pectin lyase while Candida spp. demonstrated equally high activity for pectin hydrolase and pectin lyase. These three genera of fungi were markedly phytopathogenic in yam inoculation test. Aspergillus flavus with only pectin hydrolase activity was non-pathogenic suggesting that hydrolase activity alone may not be decisive for soft rot development.
The pectin hydrolase and lyase of Aspergillus niger (A9) following partial purification by ethanol precipitation, ammonium sulphate precipitation, DEAE-cellulase (DE52) column chromatography and sephadex G150 gel filtration showed the following properties: PH optima 5-5.5; PH stability range, 3 - 6; temperature optimum, 450 C; temperature stability range, 35 - 550 C and molecular weight, 30, 900 for pectin lyase; PH optima, 4 - 4.5; PH stability range, 3 - 7; temperature optimum 350 C; temperature stability range, 20 - 500 C and molecular weight, 38, 020 for pectin hydrolase. Purified pectin lyase macerated yam tissue at PH 3 - 8 with PH 4 as optimum; for the pectin hydrolase the PH range was 3 -6 with optimum at PH 4 - 4.5.
At PH 4.5, total inhibition of pectin hydrolase activity was independently accomplished with potassium sorbate (0.6 mg/m1). At PH 5.0, only potassium sorbate and benzoic acid, (each at a concentration of 0.9 mg/ml) caused the total inhibition of the lyase enzyme. Total inhibition of this enzyme by sodium benzoate could not be achieved even at the maximal concentration of 1mg/m1.
