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Suburban development in
an oak forest

When an oak forest is converted to a housing development, the large oak trees that are left standing are intended to provide beauty, shade and value. But that's not what ultimately happens. Trees left standing are exposed to higher summer temperatures and drying winds.  And we make sure they?re drier and hotter by disrupting their root systems.  Excavations for sewer, water, electricity, natural gas, basements, driveways, etc., as well as, changing soil levels around roots contribute to root damage, decay and ?smothering? of root systems.  Stumps of removed trees allow root-rotting fungi to buildup and penetrate nearby living root systems.  Then the oaks die over a period of years due to root system destruction and opportunistic pests.  The worst damaged trees are the first to go.  

These changes allow the two-lined chestnut borers to increase, cause branch dieback and oak tree mortality.  These insects derive their food from the outer sapwood and the thin film of phloem in the inner bark tissue.  Their presence is confirmed by finding elongate, white larvae and winding galleries under the bark of dead and dying oaks.  Fertilizing and watering during dry weather may help stressed oak trees avoid becoming infested by this opportunistic pest.  Two-lined chestnut borers are a symptom of the underlying root disruption and exposure problems of oaks after construction damage.

Scab and black canker of willow

Leaf and shoot blight of mature willow trees were reported in the Bemidji Area during early June on 25-30 foot tall clumps of trees.  Caused by two fungi, Venturia saliciperda and Glomerella miyabeana, this disease complex is the most destructive disease of willows.  Soon after leaves develop in spring, a few small leaves on new shoots turn black and die.  Later, all the remaining leaves wilt and become blackened as if burned by fire.  Cankers on twigs may result from the leaf infections.  After rainy periods, olive-brown fruiting structures appear on the undersides of blighted leaves, especially on the veins and midribs.  The twig infections can expand and spread, killing large branches, too.

Black, golden, heartleaf, and some varieties of white willow are especially susceptible to scab and black canker.  This fall, rake and destroy all fallen leaves and twigs.  This winter, prune and destroy dead twigs and branches tol eliminate the most important sources of spore inoculum for early season infections.  Some trees might have to be removed or replaced if they have had repeated, severe infections causing extensive topkill and mortality.