Colorado blue spruce, Norway spruce and white spruce are the three major spruce trees in Missouri. Due to their pyramidal shape and beautiful green foliage, they have become landscape favorites over the past thirty years. No wonder that the word "spruce" as an adjective has a meaning of "neat or smart in appearance" according to Merriam-Webster dictionary. However, since none of the spruce trees are native to Missouri, they are suffering from both environmental stresses (heat, drought, compact soil, etc) and disease pressures from all types of plant pests.
The most important disease of spruce is Needle Cast disease caused by fungal pathogens such as Rhizosphaera spp. and Stigmina spp. (Figure 1). The symptoms include browning of needles, needle dropping and branch dieback. The symptoms initially show up in the inner side of the canopy as there is more humidity and shade areas. Then the disease continues to progress from inside out, and from older needles to younger needles. Although the damage and symptoms of Stigmina are less aggressive in comparison to Rhizosphaera, its fungal fruiting bodies can be found in both green and browning needles, making this disease easy to disperse. The infection of Needle Cast disease is favored by mild temperature and prolonged humidity in the spring. The symptoms may not be noticeable in the first year of infection but this disease survives on the infect needles in the winter. Disease management of these diseases includes reducing abiotic stresses by using proper irrigation, fertilization and mulching, pruning dead and dying branches or removal of dead trees, avoiding irrigation with sprinkler, active scouting of disease problems, application of broad-spectrum fungicides such as chlorothalonil or mancozeb by closely following the label and instructions. Because Colorado blue spruce is the most susceptible host of Needle Cast diseases, replacing the dying blue spruce with other conifer family trees, such as white pine, Douglas-fir and Norway spruce is ideal in controlling these diseases.
If you see patches of white to bluish resin covering the surface of a spruce branch, this tree probably was infected by Cytospora Canker caused by a fungal pathogen called Leucostoma kunzei (Cytospora kunzei var. picea) (Figure 2). This fungus normally attacks stressed trees and triggers more damage on Colorado blue spruce in comparison to Norway and white spruce. Symptoms start developing as surface lesions at the base of small branches. Old branches are more susceptible to this disease comparing to younger branches. As the disease develops, the lesions expand up the branch, creating cankers which cause the death of the portions beyond the cankers followed by needle dropping. Dieback symptoms then develop from the lower canopy to the higher canopy in a "bottom up" manner. Another symptom is the release of resin from the cankers which then drips onto lower branches or the ground and hardens into blueish-white patches. Unless the tree is severely stressed by other factors (e.g., insect damage, drought or heat), Cytospora Canker normally doesn't kill the tree. It may take years for this disease to kill a whole limb as the performance of the tree keeps declining. Since this disease survives the winter on the mycelia and fruiting structures on the dead branches, the entire diseased branches need to be cut back. Promoting good cultural practice to maintain health of the tree is vital in reducing stresses. Currently, no fungicide is available in effectively controlling this disease.
Sudden Need Drop of Spruce (SNEED)
SNEED disease can be found in all three species of spruce in Missouri. It can cause fast needle dropping on the older branches even when the needles are still green. The distribution of the affected branches seems scattered through the canopy, but it will become thinner and show some dead branches over time as the disease progresses. SNEED disease is associated with a fungus called Setomelanomma holmii although pathogenicity of this fungus is unknow. Similar with Cytospora Canker, SNEED causes more damages on stressed trees suffering from improper irrigation, fertilization and severe weather condition. This disease has been detected from two Norway spruce samples submitted to MU Plant Diagnostic Clinic (MU-PDC) in 2015. We still consider it as a new disease; therefore, little is known for the life cycle of the fungus. Disease management shall be focused on reducing environmental stresses as well as use of alternative plants.
Phomopsis Tip Blight
Although the primary host of this disease is Juniper, we have seen increasing number of spruce trees that have been infected by this disease in the past years among different states. The causal agent of this disease is Phomopsis juniperovora. This pathogen normally affects new growth early in the season as well as any time right after pruning when young tissues emerge. In contrast, older plants are more resistant to this disease than younger ones (Figure 3). The symptoms include needle dropping, tip blight and girdling of branches. To control this disease, prune the symptomatic branches and apply broad spectrum fungicide in the beginning of the year. For more information of this disease, you can review this factsheet on the webpage of disease highlights of MU-PDC.
Both armored and soft scales were detected from the samples submitted to MU-PDC previously. One common armored scale affecting spruce is pine needle scale (Chionaspis pinifoliae). The adult females of this scale are covered with a wax covering (test) that rests tent-like over the body. Eggs are produced and protected under the tests. As they are hatched in the beginning of the season (March to May), this insect enters nymph stage when the crawlers become very mobile, seeking feeding sites to settle. From July to August, they start to mate and lay eggs to overwinter for the next year. It is very difficult to control armored scales compared to controlling soft scales because: (1) there may be more than one generation of it through the year; (2) the protection of the tests affords the insect. Therefore, pesticides or neem oil are ineffective unless they are applied at the right time. Using biological control methods is a better option, especially for commercial growers. For more information of this disease, you can review this factsheet on the webpage of disease highlights of MU-PDC.
Spruce Spider mites
They could become a big headache for both homeowners and commercial growers because they affect a wide range of coniferous species and they require a lot of effort for effective control. Irregularly-shaped, small, yellow specks on the needles are the first visible signs of mite activity. As the insect population increases and weather conditions become hotter and drier, the more advanced signs and symptoms appear: premature needle drop, webbing on the twigs and branches dieback especially inside the tree. To control this insect, good scouting should begin in March and dormant oil can be applied at the same time. You can collect the mites by placing a white paper under a branch while tapping it and use a hand lens to verify those little dark specs are indeed mites. Mites can live in many species of weeds, so good weed control can also help reduce infestations. General purpose insecticides are not as effective as miticides that target both the eggs and active forms of the insect. Remember, always read and follow the label when using any insecticide. The use of biological control agents, such as lacewing and ladybugs, can be highly effective in controlling spider mites.
If you have any questions about spruce diseases or notice any symptoms developing on your trees, feel free to contact MU-PDC. Before submitted samples to us, we recommend you to read our newly published article, "Top 3 Considerations when Submitting Samples this Season".