Tansley Review No. 19 Environmental and endogenous controls of developmental pathways: variation and its significance in the forest pathogen, Heterobasidion annosum

Cultural studies of the forest root pathogen Heterobasidion annosum (Fr.) Bref. have shown that the mycelium of the fungus possesses a high degree of developmental versatility, being able to grow in a variety of distinctive functional modes. Consequently the fungus can alternate between conidiogenous and meiotic reproductive pathways, rapid, sparsely branched and slow, densely branched mycelial extension, nutritive and non-nutritive modes, and'juvenile'and 'senescent'phases. This ability is probably of considerable importance in natural infection cycles and provides an interesting model for examining interrelationships between developmental regulation and host-pathogen interactions generally. It also raises important questions about the interface between environmental and endogenous controls of developmental pathways. CONTENTS Summary 245 I. Introduction 245 II. Spore germination and mycelial extension 246 III. Production of non-nutritive mycelium 249 IV. Sporophore production, senescence, somatic incompatibility and extracellular enzyme production 251 V. Discussion 254 Acknowledgements 255 References 255.