Gymnosporangium clavipes - Details
Young fruits of hawthorn (or other pomaceous fruits) become infected in early spring when a
spore of the rust fungus lands on and invades them. Following infection, the fungus grows
throughout the tissues of the fruits. When the fungus matures a few weeks later, it sporulates by
producing numerous tubular structures called aecia that emerge from the fruit (A). The wall of
the aecium, called the peridium, is composed of many colorless, rhomboid cells that are oriented
vertically and are united laterally (B). The outer wall of the peridial cells is ornamented with
short, coarse ridges (C), and the inner wall is thickened (D). Each aecium is filled with small
spores called aeciospores (E). The aeciospores are released when the tubular peridium splits
lengthwise, freeing the spores to be dispersed by air currents. The aeciospores are globose, with a
thick, pale yellow wall (F); in mass, they appear bright orange.
Like most rust fungi, Gymnosporangium clavipes requires two different hosts to complete its life
cycle. The aeciospores can only infect species of Juniperus; they cannot reinfect hawthorn.
Gymnosporangium globosum is a very similar rust; it differs mainly in having the aecia formed
on leaves instead of fruits, and in the nature of the spore (teliospore) formed on Juniperus.
