It may not be winter injury on your Douglas fir
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EmailEditor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
For
those of you growing Douglas fir Christmas trees, if you think you are
seeing browning or bronzing of needles on your trees when looking at
them from a distance, don’t assume its winter injury. Take a closer look
because it might be the needle casting disease called Swiss needle
cast, caused by Phaeocryptopus gaeumannii.
This needle cast disease can be infecting your trees for up to three
years before you notice any symptoms; but whether it is the stress of
winter, other stresses or a combination of events, we have seen this
disease express its fruiting bodies with symptoms exploding on trees in
early spring as the trees come out of winter. This infection can limit
the trees’ market potential and you may need to apply sprays for up to
three years in order to mask the damage. So, take a closer look at those
brown or bronzing branches on your Douglas fir.
With the appearance of these fruiting bodies, a new cycle of Swiss
needle cast begins when the fruiting bodies are formed in the spring.
Look for two bands of little black specks along each side of the
needle’s bottom surface. A whitish, waxy looking plug normally sits
where the black fruit bodies erupt through the needle’s surface. Both
the white plug and the black fruit body are easily seen with a
magnifying lens. The white plugs are normal, but the black erumpent
fruit bodies are a sign of the fungus indicating needle infection.
Rainfall is essential for the microscopic spores to release from the
fruit bodies. The disease is more common on needles located on the lower
branches of the trees. Rainwater is essential for spore release and
temperature plays a minor role, if any. Therefore, cool, wet springs and
warm, wet springs will release Swiss needle cast spores in the
plantation.
To prevent these spores from continuing their infection cycle, chemical
preventatives should be applied when the new shoots are one-half to two
inches long. A second application is usually required within two to
three weeks. A third spray may even be necessary if the spring continues
to be wet.
Dr. Fulbright's work is funded in part by MSU's AgBioResearch.
