Needlecast disease on 2 Colorado blue spruce killed both trees in less than a year. Trees have been removed and stumps ground. Question: Do I need to discard the soil to prevent contamination of replacement plants? No more spruces will be planted in the area.
Colorado blue spruce, Picea pungens, needlecast is a fungal infection caused by Rhizosphaera kalkhoffii. It is spread by spore dispersal from infected needles primarily during wet weather periods and is difficult to control once it is present.
This fungus can also infect White spruce, Picea glauca and Norway spruce, Picea abies. They may be a bit more resistant to fungal infection but that is not always the case.
As you have already removed your infected trees completely, replanting something that is not susceptible to the fungus is paramount. Spores can remain present in the old tree locations as they can survive winter on living and recently killed needles.
I would suggest cleaning up as much debris as possible on these sites (needles, branches, twigs, bark and sawdust etc…), and leaving these spaces vacant for at least a year. This way the soil and surrounding vegetation have a chance to settle and disease pathogens lessen.
Going forward, you have the opportunity and time to do some research into disease resistant trees that might bring these two sites to life again – maybe it’s time for something completely different. If you are having trouble deciding on new plants, consultation with a certified arborist may be wise before any new purchase.
Hope this information is helpful.