This year there have been a few observations from bonsai enthusiasts of an orange fungus appearing on their Juniper bonsai trees. It is likely that this is a species of Gymnosporangium which is host shifting rust that causes systemic infections. It is widespread and common and visible from March to August occurring on different species of Juniperus.

There are various species, one is Gymnosporangium sabinae (Pear rust fungi) and is associated with causing infections on Pear trees and Junipers.

Gymnosporangium juniperi-virginianae known as Cedar Apple Rust is more widespread in eastern North America and affects Junipers and Fruit trees.

Gymnosporangium clavariiforme appears on Juniperus communis and is a jelly like orange fungus in the shape of horns or tongues.

Control is problematic on bonsai due to the size of the trees but the use of fungicides sprays (Bayleton, Funginex, Rubigan EC) can be highly effective against rust diseases and may help to inhibit the transmission cycle if applied correctly with an approved chemical product following the instructions.

Reference: Laess√łe, Thomas & Petersen, Jens H (2019) Fungi of Temperate Europe Volume 2 Princeton University Press