Many untreated spruce trees are being affected by needle cast. Blue spruce sustains the greatest damage, but we are also noticing damage to Colorado, Norway and White spruce. Rhizosphaera needle cast is common and widespread in Michigan, and the fungus attacks needles during the growing season in spring.
This disease causes premature death and casting of the needles and tends to infect the lower branches first. Infected needles first appear mottled or speckled with small yellowish spots. Eventually, the spruce needles turn a brown or purple color and drop from the tree. Under close examination distinct black specks can be seen in rows on the discolored needles. These specks contain the spores that spread the disease throughout the spruce. The spores germinate with prolonged periods to moisture and spread by way of rain or dripping and splashing water, like sprinklers.
There is an ongoing struggle with Rhizosphaera needle cast. Repeated treatments are necessary to make meaningful improvements, and it may take several years to see the improvement. Timing of treatments is critical, two treatments in May and one in June, because it is important to protect new growth as it emerges. Previously infected needles will continue to be lost even after treatments have started.
Just remember to keep a mindful eye on your spruce trees. If you see something that doesn’t look “normal”, contact a Branch Tree & Landscape Service certified arborist.