They do not form false heads, and can be seen to be produced singly when a culture is examined directly under the compound microscope, or under high magnification of a dissecting microscope with substage illumination. Some species, such as F. solani , produce fusiform, septate conidia in the aerial mycelium from monophialides, which do form false heads, and are not considered mesoconidia. The concept of mesoconidia is not used explicitly in this key, but the combination of fusiform conidia in the aerial mycelium and polyphialides is equivalent. The conidiophores and conidiogenous cells in the aerial mycelium can be diagnostic for some species. This character is easiest to determine in slides that contain only a very small piece of aerial mycelium, and no agar.
Most species produce monophialides, that is, phialides that produce conidia from one opening only.
Other species, particularly those in sections Sporotrichiella and Liseola , produce polyphialides in addition to monophialides. Polyphialides produce conidia from more than one opening in the same cell. This seemingly simple concept causes all kinds of practical problems, especially for novices who sometimes get the concept confused with the branching of conidiophores. The trick is to learn to recognize a conidiogenous aperture, then to recognize when more than one is present on one cell. Unfortunately, polyphialides are ephemeral in some strains of F. avenaceum and can be difficult to find in cultures more than 7 days old.