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Physiological manipulation of endogenous reserves of entomopathogenic fungi

The infectivity and performance of fungal inoculants used for insect control tends to be inhibited in environments where water is lacking constantly or periodically at the insect surface. This is restricting the commercialisation of fungal biopesticides, and methods are needed to improve the efficacy of these products if they are to achieve their full potential. Improvements in formulation, application, mass production and shelf life are known to give better and more reliable control with fungal biopesticides. However, less attention has been given to optimising inoculum quality. A key issue is the extent to which physiologically-useful compounds affecting the ability of fungal cells to obtain water can be accumulated in fungal inoculants (Magan, 2001).

Water balance in fungal cells is controlled by the synthesis of osmotically-active solutes in the cytoplasm, and the concentrations of these compounds in inoculants can be manipulated by culturing fungi under controlled conditions of reduced water activity (aw).

In collaborative research with Prof Naresh Magan of Cranfield University, we quantified the effects of physiological manipulation on the germination of conidia of entomopathogenic fungi. Cultures of Beauveria bassiana, Lecanicillium muscarium, Lecanicillium longisporum, Metarhizium anisopliae and Paecilomyces fumosoroseus (= Isaria fumosorosea) 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 was then described using a generalized linear model. The germination of M. anisopliae, L. muscarium, L. longisporum and P. fumosoroseus was accelerated over a range of awlevels as a result of physiological manipulation. However, the relationship between the effect of physiological manipulation on germination and the osmolyte content of conidia varied according to fungal species. In bioassays with M. anisopliae, physiologically manipulated conidia germinated more rapidly on the surface of the melon cotton aphid Aphis gossypii,and fungal virulence was increased even when relative humidity was reduced after an initial high period.

The concept of physiological time used in this research has been adopted widely by plant seed physiologists, where it has been used to provide insights in a range of seed behaviour, and has proved an effective framework for practical applications such as improving crop establishment . It is also used commonly in entomology where the effects of temperature on insect development are expressed in terms of degree-days and are used for pest forecasting. It is is not used in mycology to any extent but is very relevant to improving our understanding of fungal eco-physiology in areas of study that are often thought to be problematic.

References:

ANDERSEN, M., MAGAN, N., MEAD, A., & CHANDLER, D. (2006). 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. Environmental Microbiology, 8, 1625 – 1634.

Magan, N. (2001) Physiological approaches to improving the ecological fitness of fungal biocontrol agents. In, Fungi as biocontrol agents, eds. T. M. Butt, C. Jackson and N. Magan. CAB International, Wallingford, Oxford UK. pp. 239 – 251.

Hallsworth, J.E. and Magan, N. (1994) Effect of carbohydrate type and concentration on polyhydroxy alcohol and trehalose content of conidia of three entomopathogenic fungi. Microbiology 140: 2705 –2 713.