NDSU Extension - Griggs County

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May 28

Rhizosphaera and Stigmina on Spruce

Weekly column by Jeff Stachler

 

Rhizosphaera and Stigmina on Spruce

 

Good day!  I hope all is well.   Temperures have been at or above normal with some good rains is parts of the county!

 

As of Monday, all of the wheat, barley, and corn has been planted in the county.  About 80% of soybeans have been planted as of Monday. 

 

Spruce trees are having a hard time surviving these days.  Some environmental stresses are outright killing trees, but other trees are dying slowly from two needle diseases. 

 

The needle diseases attacking spruce trees are Rhizosphaera and Stigmina.  Rhizosphaera is caused by the pathogen Rhizosphaera kalkhoffi and Stigmina is caused by Stigmina lautii.  Both diseases attack the needles of Colorado blue, white, and Norway spruce, although Norway spruce is considered to have the highest level of resistance.  The diseases usually attack older trees, but not always.

 

Spruce trees generally keep three to four years worth of needles.  Both diseases attack the oldest needles causing the needles to turn purple or brown and eventually fall off the tree.  The diseases usually start at the bottom of the tree and work its way upward.

 

Stigmina lautii has a two-year life cycle.  Sporodochia (fruiting body) develop in late spring the year after infection, mature that fall and produce spores that can cause infections the next spring.  Spores typically develop in the spring just prior to new shoot growth.  The spores can infect any age class of needles throughout the season when the temperatures are above 50°F.

 

Rhizosphaera kalkoffii has a one-year life cycle.  Pycnidia produce spores in late May through July during periods of wet weather.  The spores are spread by rain splash and will infect all age classes of needles.  The symptoms of the disease on newly infected needles will not appear until spring of the following year.  Spores are produced shortly after the appearance of the pycnidia.  Infected needles on the tree and those that have fallen from the tree produce spores.


To identify the diseases’ you need to use a hand lens to observe the white-colored stomata (openings for gas exchange) in the needle.  The needles should have three or four rows of white spots indicating the presence of the stomata.  With diseased needles the pycnidia of Rhizosphaera emerge from the stomata forming a smooth-edged black circular pattern.  The sporodochia of Stigmina emerge from the stomata forming discontinuous black fuzzy lines.  The fruiting bodies are visible on green and discolored needles.  Proper identification of these fruiting bodies is important to know how to manage the diseases.

 

For cultural control of both diseases, obtain maximum airflow around the tree and maximum sunlight to reduce the time the needles are wet.  Keep trees spread apart as much as possible to reduce tree to tree spread.  Remove infected branches and severely infected trees.  Clean up the needles from under the tree and burn the needles and remove infected branches. Plant the most resistant spruce which is Norway, although both diseases may still affect the Norway spruce.

 

Chlorothalonil and copper containing fungicides can be used to control RhizosphaeraThese fungicides may control Stigmina, but limited research is available to prove this.  For Rhizosphaera, apply the fungicide when buds have broken and new needles are half their normal length.  We are approaching that time.  It is critical to make a timely application.  Apply the highest labeled rate.  Apply the fungicide again three to four weeks later.  Apply fungicides for the next two years. After this scout the trees to determine when or if future treatments will be necessary.  The best information for controlling Stigmina is to apply a fungicide at bud break and then three to four weeks later.  Some suggest a third application three to four weeks after the second application.  Preliminary research shows these treatments may need to be applied yearly for an indefinite period to control Stigmina.  Applying these fungicides this often will become quite expensive, so you will need to decide how much money you want to spend to protect these trees.  Removing the tree may be less expensive.  If choosing to cut the tree, cut it down early to reduce the spread of the disease to nearby trees.                                                                                                                                                                                                                                                                                                                       

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