Tree133 Professional Arborists

I love finding great examples of common pathogens. Pear trellis rust (Gymnosporangium sabinae) damages thousands of pear trees every year in Seattle. The pathogen is a fungus that requires two hosts. During the summer, it lives on the foliage of pear trees (Pyrus sp.) causing discoloration and unsightly growths on leaves. The fungus then over-winters on susceptible juniper plants (Juniperus sp.). In the spring, the fungus forms orange-brown gelatinous horns on juniper that are either super cool or kinda gross depending on your perspective. These are generally hard to see as they are hidden behind foliage. However, when Junipers are pruned severely, such as for sidewalk clearance, they are more easily visible.