One good reason for inspecting your magnolias in winter is that the removal of fallen, infested leaves prior to the return of warm weather can help to reduce the severity of the problem.  While algal leaf spot is usually not a serious issue, it's best to not just ignore it.  In addition to the removal of fallen leaves, you may be able to improve sunlight penetration and air movement by selective pruning of adjacent vegetation.  You will likely notice that even on the same individual tree, the number of infected leaves will vary with more or less exposure to sunlight and air movement.  (Copper-containing fungicides can also be applied, but this should probably be limited to commercial applicators, and becomes impractical on the larger magnolias.)

Close study of an infested magnolia leaf will reveal that there's more going on than just the growth of the algae.  In the close up below (taken from the same magnolia tree), you will note an orange/yellow algal growth to the right; and to the left, a couple of larger grayish/white splotches.  These grayish/white structures are "lichenized" algae.  In other words, as explained in a Clemson University information note, "Older algal colonies may become colonized (or lichenized) by a fungus and are then classified in the genus Strigula.  This is the only lichen that is parasitic on plants.  Lichens are composite organisms that include an alga and a fungus." Visit Clemson University Cooperative Extension to review the Clemson note and to compare their images with the ones we've obtained from Craven County.