Why Are So Many Oak Trees Dying This Year?

dead oak tree

Rapid decline of an oak tree in an area with a restricted root zone. Photo: D.L. Clement, University of Maryland Extension

This season the Home and Garden Information Center has received a tremendous number of questions on rapid browning and death of many of our oak trees in urban landscapes and forest situations.  Even though it would be convenient to point to a single reason for this dieback it is most likely a combination of weather, disease, and insect factors.  

A logical starting place to look for an explanation would be the often-overlooked gradual health decline of our trees due to old age, restricted root zones, soil compaction in work zones, old trunk wounds, storm damage, poor pruning, urban stress such as reflected heat and drought, and opportunistic diseases and insects. These decline factors can extend over many years, leaving trees to try and cope with less than ideal growing conditions.

Last season these conditions were further worsened by the excessive rainfall that continued into this spring which resulted in standing water at many locations that had low spots, compacted soil, or water collection points. Flooded soils and saturated root zones further weakened trees by allowing root pathogens such as Phytophthora a chance to reduce the overall number of healthy roots. 

area of flooding near oak trees

Flooding near oak trees, Spring 2019. Photo: D.L. Clement, University of Maryland Extension

dead oak tree

Dying oaks in the same location as above, Fall 2019. Photo: D.L. Clement, University of Maryland Extension

In general, red, black, chestnut and white oaks don’t tolerate poorly drained soils. Trees can tolerate some reduction in root health, as long as temperatures remain cool, water demands aren’t high, and adequate time is allowed for root regeneration. As a root system loses the ability to support the tree’s water needs, dieback will occur especially in the upper branches.  

When the high summer temperatures began this season in mid-July and the low rainfall extended into this fall these conditions accelerated the loss of tree vigor and resulted in sudden browning of tree leaves and canopy dieback. Compromised tree health often allows pathogens such Armillaria and Hypoxylon to invade, which further accelerates dieback and death. In addition, opportunistic insects such as Ambrosia Beetles and Two-lined Chestnut Borer, will attack tree trunks and continue tree demise.  

frass on oak from ambrosia beetles

Evidence of a boring insect infestation. Photo: D.L. Clement, University of Maryland Extension

There are a few positive steps that may alleviate some tree stress. It is very difficult to reverse decline in stressed oaks so select trees that still have green foliage and irrigate near their bases during this period of high drought stress. Even minimal amounts of water can help recovery and prevent drought stress before winter dormancy. Practices that open up compacted soils to increase drainage and raise soil oxygen levels (e.g., vertical mulching) will often help as well.  

As we continue to receive information about dying oaks across the state, we still have many unanswered questions. We will continue to collect data on tree species, age, and pest occurrence, in coordination with other agencies across Maryland.

By Dr. David L. Clement, Principal Agent, University of Maryland Extension, Home & Garden Information Center and Dr. Karen Rane, Director, University of Maryland Plant Diagnostic Laboratory

8 Comments on “Why Are So Many Oak Trees Dying This Year?

  1. Great article….however no mention of Bacterial Leaf Scorch disease that is devastating oaks in NE US & New Jersey. The pin oaks trees have been hit particularly hard but red oak trees (NJ State tree) Chestnut oaks are also affected by BLS. Symptoms are especially noticeable in late summer and early fall.

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  2. If an oak tree is over 60 years old should we attempt to water it? It is at a woods edge. The actual age is unknown but it is over 80 feet tall..

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  3. You can add another oak to those lost to the weather of 2018 and 2019, as well as disease. Our approximately 50-year-old oak has grown on high ground in our suburban 1/2-acre lot for about 45 years. In the past two or three years, more and more branches have died until now only about 1/3 or less of the tree seems to be alive. Because it’s on a corner, and vehicles and children pass under its branches every day, we’ve reluctantly made the decision to have it cut down. Upside of this: More sun will come into the yard and perhaps we’ll have grass instead of moss in that area!

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  4. Should we fertilize or inject insecticide into our mature oaks to prevent problems? The tree company wants to do this, but how can I tell if its necessary?

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    • We would not recommend fertilization now because of the drought conditions. Adequate soil moisture is needed to take up the nutrients. In fact, if fertilizer is applied now, it may compromise the tree’s root system. If a soil test indicates nutrient deficiencies a fertilizer application could be made in the spring.

      We would not recommend systemic applications of insecticide now because again the tree needs adequate moisture to distribute the insecticide up the vascular system. In addition, the beetles are already deep inside the tree, will be slowing their development as temperatures cool, and will not emerge until next spring. If required an application of insecticide could be applied to the bark next spring to prevent new beetle infestations.

      -David L. Clement

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  5. Pin oaks across Cape Cod have been devastated this year, entire groves dying near Truro and Wellfleet. A shocking sight.

    There has been aerial spraying for mosquitos due to equine encephalitis outbreaks over the last few years.

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