Strange growth on juniper bonsai

Credit: Lynne DeCew


Q: My Chinese juniper bonsai recently developed a strange growth after being left in the rain. Over a dozen fingernail-sized, gelatinous, odorless, orangey-brown blobs were stuck to the branches. A few days later, in drier weather, the blobs shriveled and turned almost invisible. After a few more wet days, they returned. Now there’s a brownish sticky residue. Do you have any idea what this is and if it’s harmful?

This disease, called “pear trellis rust” (Gymnosporangium fuscum), is an air-borne fungus which appears on junipers as orange, gelatinous spore masses in spring.

During the summer this disease is common on pear trees, usually as orange to red blotches on the leaves. The fungus can travel at least 30 meters in the air. This disease is believed to be only to occur in the Lower Mainland and Vancouver Island south of Duncan. The fungus requires at least two different host plants to reproduce.

This disease is not detrimental to junipers. It’s more annoying to home gardeners growing edible pear trees. So it’s a good idea to keep junipers far away from ornamental and edible pear trees – at least 30 meters. Fungicide sprays are only available to commercial nursery growers and are applied at
least three times during the fall (late August to late Oct.).