Biology of Wet Bubble disease of cultivated mushrooms

Abstract

The study examined the physical and biotic factors which affect the host-parasite relationship between the necrotrophic fungus Mycogone perniciosa (Magnus) and its host Aqaricus bisporus (Lange) Sing, in vivo and in vitro and the relevance of these factors to the conditions under which A.bisporus is commercially cultivated. The effect of environmental variables on growth of M.perniciosa varied depending on its developmental stage. Mycelial growth in vitro on media developed specifically for this work was optimum at 25°C, pH 4.1 and C : N ratio 55 : 1. Growth of the fungus occurred on media containing Na-carboxymethylcellulose. sucrose, chitin or galactomannan. The environmental conditions found to be optimum for M.perniciosa in vitro closely paralleled the conditions under which A.bisporus is cultivated commercially. Germination trials showed that verticillate conidia geminate most readily at 18°C. chlamydospores at 25°C. Relative proportions and overall production of the two types of M.perniciosa spores was also temperature dependant. Conidia were air dispersed. M.perniciosa exhibits biotrophic tendencies in the presence of A.bisporus. Saprophytic ability of M.perniciosa appeared greater in the absence than in the presence of the host, with a high susceptibility to fungistasis in composts and casing. Inoculation tests in vitro and in vivo identified a range of fungi as being potential hosts of M.perniciosa. A structural study was made of infected host tissue. M.perniciosa appeared to cause highly localised host cell wall breakdown. The hypothesis that enzymic degradation of host cell walls is an important part of disease development was confirmed by ultrastructural observations and by the assay of wall hydrolase enzymes. The development of infected tissue appeared to be dependant on toxins and growth promoting substances originating from both the host and the pathogen. Disease development was also dependant on antibiosis resulting from other organisms colonising the compost and casing layers. Attempts were made to control M.perniciosa in cropping beds using such antagonistic bacteria. Isolates showing most potential were Pseudomonas bacteria of species group 3 (P.Fluorescens complex). Disease control was expressed as both (i) a reduction in the development of infected sporophore tissue, and (ii) an increase in the yield of healthy sporophores in the presence of the pathogen. Correlation between antagonism of M.perniciosa in vitro and disease control was greatest in the former case. The level of disease control achieved by inoculating antagonistic bacteria to A.bisporus casing heavily infected with M.perniciosa was significant but was highly variable.