A generalized life cycle of AM fungi begins in the spring with the germination of a spore produced the previous autumn. Spores are induced to germinate when (1) any necessary dormancy period has been met (Tommerup, 1981), and (2) the soil reaches the proper temperature (e.g., 18-25°C; Daniels and Trappe, 1980) and is further stimulated (3) by increased CO2 levels in the soil due to microbial or root respiration (Bécard and Piché, 1989a). The germ tube hyphae grow through the soil in “search” of a host root to colonize. Spores are capable of regerminating a number of times if initial attempts to Ÿnd a root to colonize are unsuccessful. Growth during this “asymbiotic” or presymbiotic phase is supported by carbon reserves-mainly lipid, trehalose, and glycogen-in the spore (Bago et al., 1999). Other forms of inocula of AM fungi that can overwinter in agricultural soil are colonized root pieces containing AM fungus vesicles or spores and the extraradical mycelium (ERM) of the previous crop host.