Dry bubble disease of the white button mushroom. Ecology and control of Lecanicillium fungicola

Dry bubble disease is a persistent problem in the cultivation of the white button mushroom, A. bisporus. There is a pressing need for innovative ways to control spread and development of L. fungicola in mushroom cultivation as currently disease management relies heavily on one chemical (Sporgon) for which a reduced sensitivity of the pathogen has been reported. The research described in this thesis aims to supply targets for such innovative ways. In order to develop effective control of L. fungicola, a thorough understanding of its ecology is crucial. Therefore in chapter 2, current knowledge about this mycopathogen is reviewed. The ecology of the pathogen is discussed with emphasis on host range, dispersal and primary source of infection. In addition, insights in the infection process and mushroom defense mechanisms are reviewed. In chapter 3, the ecology of L. fungicola in the casing is investigated. It was found that Lecanicillium spores remain dormant until A. bisporus colonizes the casing. It was shown that the casing microflora is involved in this dormancy and mechanisms were investigated. It appears that the casing microflora produce antifungal compounds that impose a nutrient dependency on L. fungicola spores that otherwise germinate independent of nutrients. Chapter 4 describes the search for antagonists that effectively suppress dry bubble disease. Possible mechanisms of antagonisms towards L. fungicola were investigated in vitro using well characterized Pseudomonas strains. Subsequently, a collection of bacteria that were isolated from colonized casing was screened for in vitro antagonism. In vitro, L. fungicola was inhibited by certain isolates through competition for iron and antibiosis. However, these isolates could not effectively suppress dry bubble disease. We conclude that biological control of dry bubble disease is not feasible. Control of plant pathogens often functions not only through direct antagonism of the biocontrol agent on the pathogen, but also through induction of systemic resistance in plants. Induced resistance in both plants and animals is mostly triggered upon pathogenic attack. As both plants and animals have developed functionally similar systems of induced defence, it was hypothesized that fungi too could have evolved mechanisms to respond systemically upon microbial attack. In chapter 5, however, it is shown that mushrooms of A. bisporus do not exhibit induced systemic resistance upon attack by L. fungicola. In chapter 6 application of 1-octen-3-ol to control dry bubble disease is investigated. 1-octen-3-ol is produced by A. bisporus and a principle component of the mushroom’s aroma. It was found that 1-octen-3-ol treatment reduced dry bubble disease in inoculated mushroom cultures. Under conditions that resembled the commercial production of mushrooms, 1-octen-3-ol was as effective as Sporgon. This demonstrates that 1-octen-3-ol has potential for disease management in the mushroom industry. Furthermore, preliminary results indicate that other plant pathogenic fungi are also sensitive to 1-octen-3-ol, which broadens the opportunities for 1-octen-3-ol’s application. Finally, in chapter 7, the results described in this thesis are discussed in view of possibilities to control dry bubble disease.