What Are the Yellow Balls on the Jack Pine?

Peridermium harknessii

Pine-Pine Galls sporing on Jack Pine

Last summer I was asked by a friend what the yellow balls infesting his Jack Pines (Pinus banksiana) were. They were obviously some kind of gall forming rust and were quite striking even from a distance.

Peridermium harknessii

Pine-Pine Galls sporing, note spores on needles and bark below left hand gall.

The rust was Peridermium harknessii and it goes by a variety of common names such as Western Gall Rust, Pine-Pine Gall Rust, and Rust Pine Yellow Disease. It has been placed by some taxonomists into the genus Endocronartium.

Peridermium harknessii

Pine-Pine Galls are inconspicuous when not sporing.

Peridermium harknessii

The bright yellow spores are unmistakable.

It is common in the Jack Pine Plains near Grayling. The yellow spores appear in late May or June from the semi-spherical galls. A gall will produce spores for several years. The woody gall remains on the branch or trunk throughout the year and will persist after the branch dies. It is not a serious pest in Michigan and the hosts are killed only if the rust infects the main trunk of small trees. Infected mature trees often only lose a few branches.

Peridermium harknessii

The remains of a Pine-Pine Gall persist long after the branch dies.

Unlike many rusts, Pine-Pine Gall Rust has a single host cycle. Meaning that it spores and directly infects other pines. I have only seen this species, in Michigan, on Jack Pine but it has been reported on Scots Pine (Pinus sylvestris). My blog post regarding Poison Ivy Rust presents further details on the life cycles of various rusts.

Look for this colorful species in late spring.
Copyright 2017 by Donald Drife

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