Studies On The Mycoflora Of Grains Of Maize (Zea Mays L.) And The Survival Of The Contaminant Aspergillus Species

Abstract

Grains of Zea mays in storage at the Kaneshie and Tema warehouses of the Ghana Food Distribution corporation with atmospheric humidity fluctuating between 55 and 90% R. H. and those stored at 40°C in the Botany Department were attacked by a large number of fungi. Non-stackburn and stackburn grains at the Kaneshie warehouse contained 26 and 16 fungal species, respectively, and those in Tema warehouse had 32 and 19 species, respectively. Aspergillus was the dominant genus of the mycoflora and Aspergillus flavus was the predominant species occurring at very high levels. The moisture content of the grains stored in the warehouses showed, an average rise of 6.9 per cent within the six months of storage. During this period the percentage occurrence of A.flavus consistently rose to a peak at the end of the 4th month and declined. Paecilomyces puntonii and Paecilomyces variotii were consistently present in the early months of storage and then disappeared. The rest of the species taking all four batches of grains together, did not show a consistent pattern of occurrence. The air spora of the Kaneshie and Tema warehouses consisted of 26 and 27 fungal species, respectively, with the same difference in the dominant species. The most abundant species recorded in the Kaneshie warehouses in descending order were Aspergillus flavus, Ciadosporium herbarum, Aspergillus flavus-oryzae, Mucor sp., Aspergillus fumigatus, Rhodotorula sp . , Penicillium expansum, Aspergillus niger and Penicillium chrysogenum. On the other hand, the most abundant species in the Tema warehouse in descending order were Cladosporium herbarum, Aspergillus flavus, Penicillium expansum, Penicillium chrysogenum, Rhodotorula sp., Rhizoctonia solani, Aspergillus parasiticus and Paecilomyces puntonii. Grains of Abeleehi, Mixed White, Obatanpa and Yellow maize varieties stored at 40°C had 15, 16, 18 and 18 contaminant fungal species despite marked average loss of 41 .4 per cent moisture content over the storage period of 4 months. Aspergillus flavus, under those conditions, was again the predominant species. Experiments which investigated growth, sporulation, conidial germination capacity, and conidial survival in Aspergillus clavatus, Aspergillus flavus, Aspergillus niger and Aspergllus tamarii, showed significant physiological differences among the species. A. clavatus grew best at 38°C and sporulated best at 26˚C; it also grew best at 62 .4 - 7 3.4% R. H., and sporulated best at 85.2 - 100%- R.H . , conidia formed at 62 4 - 100% R. H. showed 90.0 - 99 .5 per cent germination in Potato Dextrose Broth and more than 69.6 per cent germination in 1 .0 Dextrose, 1 .0 .Sucrose and 1 .0 Peptone solutions, and more than 22.2 per cent ln exudates of grains of three maize varieties, but did not germinate ln water. The conidia of the other three species did not also germinate in water. The humidity at which the conidia were formed did not affect their rate of loss of vigour in storage. The conidia survived best at 0,60 and 100% R.H. and lost vlabl1lty quickest at 20 and 80% R.H. A flavus grew and sporulated best at 34°C; grew best at the humidities of 73 .4 - 92.8% R.H. and sporulated at 100% R.H. Conidia formed at 62.4 - 100% R.H . showed 80.7 - 94.3 per cent germination in Potato Dextrose Broth and more than 11 per cent germination in maize grain exudates but did not germinate in 1.0% Dextrose, 1.0% Sucrose and 0.1% Peptone solutions. The humidity at which the conidia were formed did not affect their rate of loss of vigour in storage. The conidia survived best at 0, 20, 40, and 100% R.H. and lost viability quickest at 80% R.H. A. niger grew and sporulated best at 34°C; grew best at the humidities of 92.8 and 100% R.H. and sporulated best at 62.4 - 92.8% R.H. Conidia formed at 62.4 - 100% R.H. showed 83.4 - 98.4 per cent germination in Potato Dextrose Broth but did not germinate in maize grain exudate, and in 1.0% Dextrose, 1.0% Sucrose and 0.1% Peptone solution. Conidia formed at 73.4 - 85.2% R.H. apparently had a greater potential for survival than those formed at the other humidities. A. niger conidia survived longest at 100% R. H. and longevity decreased with decreasing relative humidity. A· tamarii grew and sporulated best at 30°C; grew best at 62.4 - 73.4% R.H. and sporulated best at 62.4% R.H. Conidia formed at 62 . 4 - 100% R.H. showed 87.7 - 98.6 per cent germination in Potato Dextrose Broth but did not germinate in maize grain exudate nor in 1.0% Dextrose, 1.0% Sucrose and 0.1% Peptone solutions. Conidia formed at 65.0 85.2% R.H. seemingly had a greater potential for survival than those formed at the other humidities. A. tamarii survived longest at 0, 20, 40, and 60% R.H. and lost viability quickest at 100% R.H. Conidia of all the four species which were swollen prior to germ tube development and were then air-dried died within 1-6 hours. It was concluded that because of the variation in response of the fungl to temperature and humidity, warehouses held at a set temperature and relative humidity would not be able to control fungal contamination of the grains. More resistant fused pericarp and testa which could emerge from breeding programmes may be one of the more efficient ways by which invasion of the gralns could controlled.