Influence of culture conditions on mycelial growth and phialospore production and germination in Chalara elegans

Abstract

Mycelial growth, both linear extension and biomass accumulation, and phialospore production in three Chalara elegans strains from diverse geographical areas were greatest at 20–25 °C. Growth and sporulation were minimal at 10 °C and nil at 35 °C. The optimal pH range for growth was 4.0–6.5, and no growth was observed at pH 3.0 or 7.75. Phialospore production was most abundant at pH 5.5–6.5. A reduction in the osmotic potential of the medium from −0.2 to −0.6 MPa using PEG 8000- or CaCl2-enhanced phialospore production by twofold to threefold and increased mycelial biomass by approximately 35%. With further reductions in osmotic potential, there were corresponding decreases in growth and sporulation, both of which were nil at −1.8 MPa. Cultures grown in continuous darkness had slightly greater growth and sporulation than cultures exposed to light. Phialospores germinated on 1 % Noble water agar in the absence of exogenous nutrients, with 80–96% germination at pH 4.0–6.0, temperatures of 15–25 °C, and an osmotic potential of −0.2 to −0.4 MPa. The germ tubes either developed into extensive hyphae or terminated in short phialides within which secondary phialospores were produced without an intervening mycelial phase (microcycle conidiation). Germination to produce microcycle conidiation was strain dependent, was greater in older (8 week) than in younger (3 week) cultures, and was not affected by the density of spores plated onto the agar surface. The addition of 50 mM glucose enhanced the hyphal form of germination in all strains, whereas heat stress (40 °C for 22 h) enhanced microcycle conidiation in one strain. An osmotic stress provided by 50 mM NaCl or CaCl2 had no effect on the two forms of germination. These findings are discussed in relation to the development of root diseases caused by C. elegans. Key words: black root rot, dematiaceous hyphomycete, endoconidia, microcycle conidiation, Thielaviopsis basicola.