We’ve seen a fair amount of dieback on spruce trees in the nursery and landscape over the last few years. It all seemed to start back in the fall of 2013, just after that wicked hot summer where some areas didn’t see rain for several weeks. Winter came early in 2013, we had an ice storm before Christmas and some of the coldest winter temperatures recorded in decades. The following winter (’14-’15) was also extremely cold and long and spring 2015 was very warm and DRY. These extremes in temperatures and moisture can be very stressful for our trees, especially evergreens.
When stressed, the foliage on spruce trees will often turn purplish-brown and eventually fall from the twigs. Colorado spruce trees are shameless at needlecast and will often do it in response to poor site conditions, transplant shock, root issues and aerial desiccation. To the untrained eye, it can appear as though these trees have been infected by a foliar disease.
So how can you tell if it’s a needle cast disease? We have a couple of diseases on spruce that show up commonly, especially Colorado spruce…
The key is to note where on the tree the symptoms are and have a magnifier to examine the needle stomates.
Rhizosphaera needlecast (Rhizosphaera kalkhoffii) appears as brown-purplish needles with black, smooth fruiting structures (pycnidia) pushing through the stomates. (Healthy needles have white stomates.) Older needles (2013, 2012) on lowermost branches that receive afternoon shade will be showing the most signs and symptoms of Rhizosphaera. These infected older needles will drop in spring, leaving trees looking barren.
Stigmina needle blight (Stigmina lautii) can also be found on spruce (Colorado, Norway and white) and appears as black, spikey-looking fruiting structures (sporodochia) pushing through stomates on green and reddish-brown needles. Quite often, the infected, symptomatic needles won’t drop until the following year. Pathologists have observed trees with both Stigmina and Rhizosphaera infections (on the same tree). If needles are green but stomates look black, chances are its Stigmina.
New, soft growth is especially susceptible to foliar diseases, but infection may also take place later in the season. Where there is a history of damage, treat with registered fungicides (copper oxychloride, Banner Maxx, Daconil) as buds start to open and protect new foliage. Where necessary, apply fungicides prior to precipitation events (spore dispersal).
Unfortunately, most of these blights and needlecasts can be found sporulating for much of the year so management can be difficult. Methods that improve air pore space in the root zone (e.g. soil aeration and methods such as vertical and radial mulching) can help relieve compaction issues and provide conditions more favorable to root growth of evergreen trees in the landscape.
Needlecast diseases are more common in the landscape than the nursery.