Spruce trees are the most problematic of all conifers in our landscape right now. There are so many factors contributing to their decline that it has become nearly impossible to tease them apart. New pathogens seem to be discovered annually, and the unusual weather conditions in recent years have further complicated everything. I will try to break down our understanding of spruce decline into environmental and biotic factors.
Our environment is not good for spruces, and it is getting worse: Essentially there are no spruces native to southeast Michigan. Most of the species in the landscape come from mountainous habitats where the air is dry and the soils are cool and reliably moist.Two severe droughts in the last three summers have been hard on landscape spruces. Muggy spring weather contributes to the rapid spread of fungal diseases. Our clay soils are good for sugar maples, but difficult for the roots of spruces to penetrate and thrive in.
Diseases old and new: insects are (in general) easy on spruces, but the fungal diseases are numerous and their numbers are growing. Some of the diseases are familiar: Rhizosphaera needle-cast causes older needles (i.e., those not at the ends of the twigs) to turn purplish and subsequently fall off. Sometimes the purple discoloration, which usually shows up in late summer, is shocking.What remains after the infected needles drop are thin, unhealthy-looking specimens with needles only at the ends of the branches. Norway spruce is usually not badly affected by this fungus.
Cytospora canker refers to a fungal disease in which the pathogen resides under the bark of twigs and branches, and causes entire branches to die back to the trunk. Usually the lower branches are first to die, but sometimes the disease begins in the upper crown or at multiple points. Often a white resin will drip from the infected branches. Trimming off the infected branches back to the trunk will reduce inoculum and slow the spread of the disease.
Whereas spruce decline is widely recognized to be occurring throughout the Midwest, there is uncertainty and debate about the extent to which other pathogens might be involved. A species of Phomopsis fungus once know only from nurseries has now been found in the landscape. Some pathologists believe it is a key causal factor in spruce decline in Michigan. Spraying specimen trees in spring may help contain this pathogen, but no studies have yet been conducted to test this. This Phomopsis disease may be the one most aggressively attacking Norway spruce.
Additional suspects have been identified. One was given the charming name SNEED (spruce needle drop), but last I heard the consensus was that it is not a disease-causing organism. There are others, and it gets very messy.
The challenge of disease control: Fungicide is a misnomer. A more precise term would be fungistat. Those products called fungicides do not kill fungi, but rather prevent spores from germinating or retard the growth of mycelia (the fungal mass).
With insects, control is relatively easy: see a bug, spray it, and it rolls on its back, kicks up its six legs and dies. Most commonly fungicides used in the landscape only act as prophylactics. Given that the fungal spores need water for germination, disease control usually involves spraying the fungicide in between rainy periods, so that when the environment is conducive to disease spread, the fungicide is in place to act as a preventative. As long as the environmental conditions favor disease spread, fungicides need to be reapplied approximately every couple of weeks in order to guarantee control.
In the real world of landscape plant management, it is only practical to spray two or three times a season, and those applications are timed to coincide when disease spread is most likely. With crab apples, a couple sprays in early spring are sufficient since the time of disease spread is very predictable. It is much more of a challenge in the case of spruce trees: for example, needle-cast diseases are most likely to spread in the spring, but weather conditions can allow their spread to occur in summer or even fall. Also different diseases might spread at different times of the year. The plain truth is that the more often the trees are sprayed with fungicide, the more likely it is to achieve disease control.