Spruce with the Midas touch, that is, spruce needle rust
A golden cast to Colorado Blue spruce trees in northern Minnesota is due to the orange-colored spores of the spruce needle rusts (Chrysomyxa ledi and C. ledicola). The spores are exposed as the fungal blisters on current year?s needles burst. In some areas, the majority of the susceptible Colorado Blue spruce are infected. Needle rust is very common in northern counties on white spruce. In other areas the trees may show no sign of infection. This is probably due in part to the lack of alternate hosts for the fungus. Indeed this is why this rust is usually seen only in the northern areas of the Minnesota. The alternate hosts are Labrador tea and leather leaf, both native heaths and prominent members of sphagnum bog communities.
The fungi of spruce needle rusts require an alternate host to complete their life cycles. Spores released from infected spruces during late summer infect the native heaths and overwinter on them. The following spring the heaths return the favor and release spores that infect young spruce needles.
The shedding of the infected needles from the spruce occurs after sporulation, August to September. The majority of current year?s needles may be lost. And although this makes the tree unsightly, it will survive. Repeated attacks over consecutive years will slow the growth of the tree but rarely kill it.
Norway and Black Hills spruce appear to be quite resistant, white spruce is somewhat resistant, but Colorado Blue and black spruce are very susceptible. Not only is the black spruce susceptible but grows in the same habitat as the alternate hosts.
Luckily these infestations are not usually annual events and therefore control methods are rarely needed. Fungicidal treatment is usually ineffective, removal of alternative hosts is near impossible, so cultivation of resistant spruce types is encouraged.
The best thing you can do for your trees is to keep them well watered. Avoid using sprinklers though because they get the needles wet and lead to needle and twig disease problems, such as, needle rust. Keep the weeds mowed near the trees so the winds can dry the needles better and prevent infection. Mulching around yard trees keeps the soil moist and also keeps competing weeds and grasses away from the trees.