Spruce (Picea spp.) is commonly planted in landscapes throughout Minnesota, and may sometimes suffer needle loss. Factors like improper planting, environmental stress, pests, and disease can all play a role in needle loss.
The two most common types of needle cast are Rhizosphaera needle cast and Stigmina needle cast.
Both the Rhizosphaera needle cast and Stigmina needle cast infect Colorado blue spruce (Picea pungens), white spruce (Picea glauca), Black Hills spruce (Picea glauca var. densata) and Norway spruce (Picea abies). While both diseases show similar symptoms, they require somewhat different management strategies.
Signs and Symptoms
A faint yellow band on needles are the first symptom to appear. The cluster gradually expands and changes from yellow to brown, or purple. Discoloured needles either fall off the tree or stay attached for a couple of years
Common signs of this disease include rows of small dark brown or black smooth round fruiting bodies (pycnidia) on infected needles. A standard magnifying glass is required to see these mature growths. Once heavily infected needles, the pycnidia can be seen with the naked eye as continuous fine black lines up and down the length of the needle.
Rhizosphaera Needle Cast
The fungus Rhizosphaera kalkhoffii causes Rhizosphaera needle cast.
Rhizosphaera needle cast has a one-year life cycle. In Minnesota, pycnidia produce spores in late March through May, in periods of wet weather. These spores spread by rain and will infect all needles, regardless of age. The first symptoms of the disease on recently contaminated needles will appear in the spring of the following year.
Different species of spruce have different levels of resistance to Rhizosphaera needle cast.
The most susceptible spruce is the Colorado blue, the white spruce is moderately vulnerable, and the Norway spruce is resistant. For this reason, Birch Tree Care recommends planting Norway or white spruce.
Stigmina Needle Cast
The fungus stigmina lautii causes Stigmina needle cast.
Stigmina lautii has a two-year life cycle. In most cases, a year after infection the sporodochia develop just before the growth of new shoots. In the twin cities, the stigmina lautii spores are discharged during the spring.
Susceptible varieties of spruce trees include the Colorado blue spruce and white spruce. The extent Norway and black spruce are susceptible to the fungus remains unclear. Needle loss that causes sustained growth reduction threatens the long-term health of a tree.
Preventative & Chemical Treatment
Allowing for enough spacing between trees can also help prevent the fungus spread. And, when planting new spruce trees opt for sunny locations, so needles dry quickly.
In yards, one easy way to avoid needle cast disease is to diversify the trees planted. If you want year-round green foliage, Birch Tree Care recommends planting at least two types of evergreen. Each of these different types of evergreen trees should be planted beside each other. For separate yard spruce, water them with a drip irrigation hose during a drought to reduce stress.
Fungicides can treat Rhizosphaera needle cast. For landscape trees, needles should be protected for two months after bud break. And spraying should occur every year. The timing of fungicide applications is crucial, so it’s best to hire a professional. Reach out to Birch Tree Care today.