Fungus or caterpillar?

Photo competition: 1st place in the category ‘Pathogen’

What at first glance appears to be an insect on a leaf is actually pear rust (Gymnosporangium sabinae), a pathogenic fungus. It is a host-switching rust fungus that requires juniper bushes as its main host year-round and pear trees as a secondary host in spring and summer. On pears, the fungus causes bright red spots on the upper side of the leaves and brown tissue growths on the lower side of the leaves, as can be seen in the photograph. The growths serve as spore stores in which the winter spores of the fungus are formed. As the spores mature, the growths rupture, releasing the fungal spores, which can now infect junipers again. The fungus weakens pear trees significantly and has a negative impact on agriculture.