Crop & Pest Report


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Winter Injury on Conifers (04/29/21)

Over the last month, pines and spruce trees throughout much of North Dakota have shown symptoms of decline. The causes are varied, and treatment recommendations are equally diverse.

Over the last month, pines and spruce trees throughout much of North Dakota have shown symptoms of decline.  The causes are varied, and treatment recommendations are equally diverse.

In 2020, many individual ponderosa pine trees across the state died in shelterbelts or yards.  We believe that the extremely wet fall of 2019 is to blame.  Soils were saturated in many areas, especially with the big snowstorm on October 10, 2019.  Even areas that were not necessarily low spots accumulated a large amount of moisture.  The saturated soil either killed the pines’ root systems directly, or else kept the trees from developing full hardiness before the winter.

Symptoms on pines were dramatic.  Entire trees died, showing symptoms throughout the whole tree, seemingly all-at-once.  In most cases, there was no progression of symptoms from bottom-to-top, or interior-to-exterior.  Instead, trees began to grow in the spring of 2020 then simply stopped with the needles turning brown.

Additional death of individual ponderosa pine trees is expected again in 2020.

More recently, spruce trees have been showing a variety of symptoms that have been diagnosed with the broad term of ‘winter injury’.  Winter injury generally shows up as early as February but is more commonly seen in March or April.  Needles often turn a purplish-brown as they die.  There is often no pattern to this dieback – the needles may be on one side of the tree, or perhaps the bottoms of branches, but may also be scattered throughout the tree crowns.  Sometimes, only needle tips are dead while in other cases, entire needles die outright.  Affected trees can be right next to trees that are showing no symptoms at all.  On occasion, entire trees may turn brown and die.

Wildly fluctuating temperatures in February and March are often to blame for winter injury.  This year, the drought of 2020 is likely a major factor in the damage.  Additionally, other pests such as spider mites or pine needle scale may play a part, especially in those trees where the damage doesn’t follow a pattern.

Treating trees is difficult, since most of the damage is due to environmental problems.  Where trees lost most of their needles, they’re likely to not recover no matter what we do.  For those trees that retain most of their needles, we need to make sure that we keep the trees otherwise healthy.  Specifically, water those trees that are suffering from drought.  A long, slow soaking is better than multiple, brief additions of water.  Move the hose around, as the tree’s roots extend in all directions from the stem.  The roots go out far beyond the drip line of the tree crown, extending to as far as four or five times the height of the tree.  The ground should be moist, not saturated, after watering.

Scout for pests such as insects and diseases, and be prepared to treat them as appropriate.  Scouting is important because treatment timing is critical to be effective.  Spraying for a pest at the wrong time of year is ineffective and is a waste of both time and money.



Joe Zeleznik

NDSU Extension Forestry Specialist


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