Two new spruce diseases? - Spruce Tree Stigmina Needle Cast Disease

This article was published originally on 2/6/2008
Byline:
By Christine Engelbrecht, Plant and Insect Diagnostic Clinic
Two possible new diseases of spruce have been seen in the Plant and Insect Diagnostic Clinic recently.
"Stigmina needle cast"
We have long been familiar with Rhizosphaera needle cast, a very common fungal disease that affects primarily Colorado blue spruce, and occasionally Black Hills spruce. This disease causes needles to turn purplish-brown and fall off, working from the inside of the tree out and from the bottom of the tree up. Small black fungal fruiting bodies develop in rows on infected needles, and we diagnose this disease by looking for these little black dots.
Sometimes the black fruiting bodies on symptomatic trees are not the fungus Rhizosphaera, though. Upon closer examination, diagnosticians in other states have found that sometimes the black dots are structures of a different fungus, Stigmina lautii. Under very high magnification, the Stigmina spore-producing structures appear hairy or feathery, while those of Rhizosphaera are smooth and spherical. Most magnifying glasses are not strong enough to see the difference. The spores of the two fungi also appear very different when examined under a compound microscope.
The ISU Plant and Insect Diagnostic Clinic has been carefully watching for this fungus ever since diagnosticians were first made aware of it in early 2006. In December of 2007, we confirmed the presence of Stigmina for the first time, on a Colorado blue spruce sample from Story County.
Stigmina is found on needles of trees showing symptoms very similar to those caused by Rhizosphaera needle cast (purpling and loss of older needles, working from the bottom of the tree to top). But is the Stigmina fungus really causing the problem? Several fungi closely related to S. lautii cause diseases on other plants, so it seems possible. But we don't know enough about Stigmina lautii to say if it really is a pathogen or not. Many fungi are harmless to plants but easily grow on dead or dying plant tissue. Some fungi are usually harmless, but can become harmful if the tree is especially stressed. Research into Stigmina's possible role as a pathogen is ongoing.

"Sudden needle drop"
Another problem has been noted recently on spruce in several states, including Wisconsin, Missouri, Kansas, and Minnesota. Needles on the older growth of individual branches quickly turn yellow or brown before falling off, usually leaving behind only the newest growth. Affected branches may be scattered through the canopy. The overall canopy of the tree thins, sometimes leaving bare branches. This condition has been called "sudden needle drop" or "spruce needle drop, "sneed" for short, and has been seen on Colorado blue spruce, white spruce, and Norway spruce.

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