Abstract:
Peat is a suitable casing layer most commonly used as a substrate in commercial production
of white button mushrooms. However, further exploitation of this substrate in South Africa is
restricted due the environmental status of this scarce and protected natural resource.
Therefore, finding an easily available and economically viable casing material that can either
partially or completely replace peat is important for sustainable mushroom production. In the
process of finding a replacement material, information regarding the microbiological
properties of peat and alternative casing materials is required to profile a natural stable
ecosystem since the presence of bacteria in the casing layer is important for pinning and fruit
body formation. Pseudomonad bacteria in the casing layer are known to play a role in the
fructification of mushroom mycelium. The aim of this study was to investigate the microbial
dynamics and profiles of peat, industrial by-products and peat-based mixtures with byproducts
using cultural and a culture-independent molecular technique such as denaturing
gradient gel electrophoresis (DGGE). Other aims of this study were to investigate the overall
microbial profiles of compost, casing and mushrooms in a commercial mushroom production
cycle and to evaluate effects of isolated micro-organisms on mycelial growth, mushroom
yield and in vitro disease control. Total bacterial population densities in peat-based mixtures
at pinning were comparable to that of peat alone and pseudomonads constituted part of
bacterial populations in these mixtures. Other bacteria in peat and the mixtures included
Ensifer spp., Sinorhizobium spp., Bacillus spp., Sporosarcina spp., Microbacterium spp.
Arthrobacter spp. and Sphingobacterium spp. Bacterial profiles of peat at pinning also
showed dominant bacteria other than pseudomonads including Bacteroidetes, alpha-Proteobacterium, beta-Proteobacterium, gamma-Proteobacterium, delta-Proteobacterium
and uncultured species. In viewing the overall microbial profile of compost, casing and
harvested mushrooms, bacterial profiles of mushrooms were more similar to those of casing
than to compost and were dominated by pseudomonads. Fungal profiles of compost, casing
at pinning and freshly harvested and stored mushrooms were completely dominated by
Agaricus bisporus. Bacterial and yeast populations increased significantly during mushroom
storage at 4oC while fungal populations remained low. Pseudomonas spp. and an
Arthrobactor sp. isolated from casing resulted in 100% in vitro growth stimulation of A.
bisporus. Several bacteria and yeasts isolated from compost, casing and mushrooms
inhibited in vitro growth of the fungal mushroom pathogens Verticillium fungicola var
aleophilum, V. psalliotae, Mycogone perniciosa, Cladobotryum dendroides, C. mycophilum,
Trichoderma aggressivum f. europaeum, T. aggressivum f. aggressivum and T. harizianum.
In conclusion, peat-based casing mixtures with industrial by-products harbour bacteria
important in A. bisporus fructification. Bacteria in these casings are also important in other
aspects of mushroom production such as growth stimulation. Based on results in this study,
mushroom bacterial profiles are similar to those of the casing, suggesting postharvest quality
may be controlled by manipulating casing bacteria.
