Answers to Your Questions Conifers

Answers to Your Questions: Canker on a Colorado Blue Spruce

Question

For almost 30 years, a magnificent Colorado blue spruce has graced the front of my home. It’s almost 15 metres tall. I’ve noticed for some time that the needles have been turning red and falling off, to the point where several branches are now bare. Yet every year, I apply the recommended dose of conifer fertilizer and aluminum sulfate to turn it blue. I’ve heard it’s a disease. Is there a treatment to stop it spreading?

Answer

The Colorado blue spruce (Picea pungens ‘Glauca’) has evolved in a climate as cold as ours, here in Quebec, but where the air is much less humid. As a result, it is particularly prone to certain diseases that develop in damp weather, notably Cytospora canker (Cytospora kunzei picea). In our region, the presence of this disease is even considered “normal” for the species! The disease causes browning of the needles on the lower branches, and eventually the death of these branches. White resin flows from wounds on the trunk. Occasionally, but more rarely, the whole tree may die.

Given the endemic state of the disease, it is difficult to curb, but you can reduce its scope. First of all, reduce your fertilizations: you’re only making the situation worse. One fertilization every 3 years is more than enough for your tree, or none at all. And stop using aluminum sulfate. It’s toxic to plants and does nothing to turn spruce blue (though the myth to the contrary is hard to dispel). On the other hand, watering the soil around your tree during dry spells can help reduce the extent of the disease, but avoid wetting the needles unnecessarily.

The most important thing, however, is to avoid any injury to the tree during its growth period, as the disease enters the stems through wounds, large or small. Thus, the lawnmower is probably blue spruce’s worst enemy, as just grazing the branches in an effort to mow the grass underneath causes injury. Instead, place mulch under the tree to beyond the branch line. If you have to remove dead branches, do so in winter or dry weather, and disinfect the tool between cuts.


Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil newspaper, January 15, 2006.

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, laidbackgardener.blog will continue its mission of demystifying gardening and making it more accessible to all.

1 comment on “Answers to Your Questions: Canker on a Colorado Blue Spruce

  1. Susan Tamulonis

    Colorado blue spruce color varies from darkest green to the blue likely everyone imagines … and wants. Mine, in Denver, is blue, aged, and full of cytospora canker that I don’t treat. I’ll go with Mark Antony’s idea of Cleopatra: age cannot wither her nor custom stale her infinite variety.

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