Blackwell Publishing LtdOxford, UKEMIEnvironmental Microbiology 1462-2912© 2006 The Authors; Journal compilation © 2006 Society for Applied Microbiology and Blackwell Publishing Ltd ? 20068916251634Original ArticlePhys- iological manipulation of entomopathogenic fungiM. Andersen, N. Magan, A. Mead and D. Chandler Received 13 December, 2005; accepted 4 April, 2006. *For correspondence. E-mail n.magan@cranfield.ac.uk; Tel. (+44) 1525 863539; Fax (+44) 1525 863540. Development of a population-based threshold model of conidial germination for analysing the effects of physiological manipulation on the stress tolerance and infectivity of insect pathogenic fungi M. Andersen, 1 N. Magan, 2 * A. Mead 1 and D. Chandler 1 1 Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF, UK. 2 Applied Mycology Group, Institute of BioScience and Technology, Cranfield University, Silsoe, Bedford, MK45 4DT, UK. Summary Entomopathogenic fungi are being used as biocontrol agents of insect pests, but their efficacy can be poor in environments where water availability is reduced. In this study, the potential to improve biocontrol by physiologically manipulating fungal inoculum was investigated. Cultures of Beauveria bassiana , Lecani- cillium muscarium , Lecanicillium longisporum , Metar- hizium anisopliae and Paecilomyces fumosoroseus were manipulated by growing them under conditions of water stress, which produced conidia with increased concentrations of erythritol. The time- course of germination of conidia at different water activities (water activity, a w ) was described using a generalized linear model, and in most cases reducing the water activity of the germination medium delayed the onset of germination without affecting the distribution of germination times. The germination of M. anisopliae , L. muscarium , L. longisporum and P. fumosoroseus was accelerated over a range of a w levels as a result of physiological manipulation. How- ever, the relationship between the effect of physiolog- ical manipulation on germination and the osmolyte content of conidia varied according to fungal species. There was a linear relationship between germination rate, expressed as the reciprocal of germination time, and a w of the germination medium, but there was no significant effect of fungal species or physiological manipulation on the a w threshold for germination. In bioassays with M. anisopliae, physiologically manip- ulated conidia germinated more rapidly on the sur- face of an insect host, the melon cotton aphid Aphis gossypii, and fungal virulence was increased even when relative humidity was reduced after an initial high period. It is concluded that physiological manip- ulation may lead to improvements in biocontrol in the field, but choice of fungal species/isolate will be crit- ical. In addition, the population-based threshold model used in this study, which considered germina- tion in terms of physiological time, also called hydro- time, could have general application in mycology and environmental microbiology. Introduction Entomopathogenic fungi are natural enemies of a wide range of insects, and a number of fungal species are being used as microbial biopesticides for pest manage- ment (Tanada and Kaya, 1993). Important taxa include the anamorphic ascomycetous genera Beauveria , Metarhi- zium , Lecanicillium and Paecilomyces . Like many other fungal pathogens, the germination of conidia on the sur- face of the host and the prepenetration growth of germ tubes are critical stages in the establishment of infection. Both processes require high water activities (a w ) (equiva- lent to equilibrium relative humidity ERH = 100 a w ) at the site of infection and therefore are vulnerable to unfavour- able environmental conditions (Milner and Lutton, 1986; Charnley, 1989; Goettel and Inglis, 1997). Poor establish- ment of infection in environments where water is lacking constantly or periodically at the insect surface is limiting the use of fungal biopesticides, and methods are needed to improve the performance of these products if they are to achieve their full potential (Magan, 2001). One approach to improving fungal biopesticides is to enhance the ecological fitness of conidia by physiological manipulation (Magan, 2001). Fungi respond to water stress by accumulating osmolytes that reduce the intrac- ellular water activity and enable water uptake by osmosis (Jennings, 1995). The compounds that are principally associated with adjustment of cytoplasmic osmotic pres- sure are polyhydric sugar alcohols (polyols) and the sugar trehalose, which can be accumulated at high concentra- tions under water stress conditions without disrupting