Rhizosphaera needle cast is a disease which causes premature death and casting of the needles of conifers. Blue spruce sustains the greatest damage, though the fungus infects other spruces as well. The older, inner needles on lower branches is where the fungus tends to infect 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.
Rhizosphaera needle cast is common and widespread in Michigan, and even though the fungus may attack needles anytime during the growing season, spring is the most common. Repeated treatments are necessary to make meaningful improvements, and it may take several years to see the improvement. The timing of treatments is critical. 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.
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