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 (2019) 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

14 thoughts on “Why are so many oak trees dying this year?

  1. Jim Avens October 3, 2019 / 9:10 am

    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.

  2. Liz Clark October 3, 2019 / 1:34 pm

    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..

  3. Maritta Grau October 3, 2019 / 3:08 pm

    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!

  4. Emily October 4, 2019 / 12:22 pm

    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?

    • Maryland Grows October 4, 2019 / 4:18 pm

      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

  5. jobrownbluesky October 21, 2019 / 6:45 am

    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.

  6. medoane January 4, 2020 / 1:43 pm

    I live on a 4 acre wooded lot in Jarrettsville MD and in the past three years we’ve seen a rapid decline in oak trees. Most of the forest is oak and about 10% of the old growth oaks here have fallen or turned to upright husks. First the outer branches wilt like a broken hand and the leaves brown, then within a season the entire tree goes. The bark then begins to peel and the tree eventually falls. It seems like every time this happens, the oaks in the sick ones immediate surroundings begin to show similar signs shortly after. Young oaks are also dying and turning to husks. They get mushrooms forming around the bark and black fungal and jelly fungus growths. I’ve also noticed an uptick in borers and termites on the property since these trees have begun falling. My neighbors and farmers in the area are noticing similar issues on their wooded properties. Is there any way I can help protect this forest?

    • Max Eber February 23, 2020 / 1:46 am

      There has to be something related to climate change and pathogens and this will have terrible consequences. We had a red oak die exactly in this manner. I was so distracted by a tragic ash and incredibly haggard pine trees we didn’t notice this oak tree, which always had an impressive dense canopy, suddenly just die. I’ve been going around all our remaining trees, including some vulnerable maples and hollies etc and cutting/tearing english ivy off of them and will be vertical mulching too many things are getting attacked and dying!

  7. Kim Nacrelli June 12, 2020 / 3:24 pm

    We have a large oak at the center of our back yard and it seems there are fewer and fewer leaves on it in recent years. Is that a sign of just old age/dying or is that generally a sign of something that can be treated? I would hate to lose the tree, as we’ve already lost several due to death and lightning strikes…this is the largest one on our land, and I’m not sure how to make sure it is healthy.
    Note: we’re in the Towson area, soil has a lot of clay and the tree is in full sun…trunk is about 4 1/2 feet in diameter, so it’s an oldie 🙂

  8. Julia Llewellyn January 9, 2021 / 10:25 am

    I have not seen science to support my concern, but I see a lot of trees in the path of lower flight paths of airplanes dying at the top. This has happened since the NextGen change at local airports. I have been affected greatly because of the noise of the plane’s flying so low, compared to prior flight before NextGen. I have noticed that plane positioning routinely goes in the same route, and these tall trees with dead tops are in their path. Has any research been done to look at airplanes flying closer as a factor in the decline of tall trees?

    • Maryland Grows January 11, 2021 / 3:15 pm

      We are not aware of and could not find specific research that addresses this issue specifically. Air pollution does have a variety of negative impacts on trees and different species of trees are more susceptible to certain types of air pollutants than others. This fact sheet goes into more detail about impacts of air pollutants on urban forests in general. https://extension.tennessee.edu/publications/Documents/SP657.pdf

  9. Aubin Maynard April 21, 2021 / 1:53 pm

    Over the last three years we’ve watched the two very old oaks in our neighbor’s yard die. This spring one didn’t leaf out, the second has only one branch with leaves. Worse, our oak had four branches(maybe 15% of branches) not leaf out. These trees are maybe 100 feet apart in a line. Highly suspect that these three trees are dying of age at the same time. Other thoughts?

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