Nutritional control of dimorphism in Ceratocystis ulmi

The nutritional parameters controlling the yeast-mycelial dimorphism in Ceratocystis ulmi, the causative agent of Dutch elm disease, have been established. The nitrogen source is critical. In a defined glucose-salts liquid medium the presence of proline induces the yeast morphology whereas ammonium, arginine, or asparagine induces the mycelial state. This phenomenon is not a secondary manifestation of pH changes in the respective media and it can be achieved with either a blastospore or a conidiospore inoculum. The dimorphism is, however, dependent on the inoculum size. Yeasts are only formed in the proline-containing medium with inocula giving cell concentrations of ≥10⁶ blastospores/ml. Once the spores produce visible buds or germ tubes they are "committed" to that developmental pattern in the sense that if they are now resuspended in the opposite medium, at the same cell concentration, the alternate morphology can no longer be produced.