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ISSUE 16   August 24, 2000




    There have been several reports of Dichlobenil (trade name: Casoron, Norosac) use to control weeds
around spruce trees in North Dakota. Casoron is not labeled for spruce and can cause discoloration and
possibly other injury to spruce trees.

    An experiment was conducted to determine if yellowing of spruce can be caused by Casoron. Casoron
was applied at 100 lbs./A and 150 lbs./A to Black Hills spruce in the fall of 1999. Observations during July
2000 of these treatments showed that all of the trees (12) which were treated at the 150 lbs./A rate showed
symptoms of yellowing with all but two trees being bright yellow. Only two of the trees treated at the
100 lbs./A rate did not show yellowing symptoms with most of the trees showing light or very light yellow
discoloration. None of the twelve untreated trees showed any yellowing.

    Symptoms associated with the experimental applications around Black Hills spruce included an overall
yellowing with possibly greater yellowing to the inside (older needles) of the tree. Those trees exposed to
more sunlight appeared to show more yellowing than those exposed to more shade. In addition, needles
on the tops of branches appeared to be affected more so than those on the undersides of branches. These
tests were conducted on sandy soil to determine if symptoms seen on trees around North Dakota were
caused by Casoron applications. Casoron should not be applied to light soils. The trees in the experiment
will be observed next spring for any further development of symptoms.

Marcus Jackson
Extension Forester

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