What’s Killing our Oak Trees?
The mighty oak is an iconic tree family that populates forests and landscapes from coast to coast. Beyond their aesthetic beauty and related benefits to humans, oak trees offer important food and shelter for many organisms from below-ground mycorrhizal fungi to insect larvae that feed on leaves, to birds and mammals that depend on acorns for winter food. But there is something wrong. Oaks are dying at a high rate from a variety of causes that can be tricky to diagnose. This article reviews some common problems, offers help in identifying cause, and advises actions to minimize harm to damaged trees and neighboring survivors.
What type of oak tree is it?
There are about 90 different types of oak trees growing across the United States. Most are classified as either red or white oaks. Because susceptibility to many diseases varies by family, it is helpful to know whether your tree is a red or white oak variety. Leaf size and shape offer a quick indication. While leaf length is similar, red oak leaves are wider. Also, red oaks have more angular, pointed lobes. White oak leaf lobes are rounded.
A Description of Common Problems
Oak decline is a widespread issue for mature oaks, of both red and white oak families. It is characterized by progressive dieback of crowns, starting from outer limbs and moving inward as the decline progresses, potentially over several years. It is typically caused by a combination of biotic and abiotic factors. Abiotic contributors include maturity, site factors, weather extremes including storm damage, extreme wet periods, and droughts. In residential settings, construction activity may cause root damage that weakens trees, causing them to become susceptible to a variety of biotic factors. For example, defoliation from gypsy moths or tent caterpillars can accelerate decline of weakened trees. Borers such as the two-lined chestnut borer and fungal diseases like hypoxalon canker are two common contributors to oak decline.
The two-lined chestnut borer is a damaging insect that is attracted to weakened or diseased trees of many varieties. The adult is a thin, black beetle that is active from May to July. The adult lay eggs in bark crevices that hatch in one to two weeks. The larvae create winding tunnels called galleries as they feed on cambial tissues, cutting off nutrient transport. They overwinter as larvae or pupae and emerge as adults through D-shaped holes in the spring.
Damage symptoms include branch dieback with leaf wilt and sudden browning. Leaves typically remain attached to branches. Borers cause death of a tree when galleries girdle the trunk, cutting off nutrient passage to branches above the feeding level. Time to death can be from one to five years.
Best prevention practices are the usual good care techniques. Keep trees healthy by providing water during droughts, avoid soil compaction and root damage from construction activity, protect bark integrity by avoiding injury by lawncare machinery, and manage defoliating insect infestations. If recognized early, professionally applied imidacloprid injections can be effective.
Hypoxalon canker is an opportunistic fungus that may attack any type of oak tree that is stressed or weakened from disease, environmental or other factors. It spreads by spores from diseased to healthy trees. Infection causes dead lesions on limbs, branches, or trunks as it develops under bark. It causes sapwood decay, damaging the structural integrity of the tree, and causing a potential safety hazard.
Symptoms include those typical of other oak tree health issues: yellow or browning leaves, small leaves and reduced twig growth, thinning canopy, dead limbs and water sprouts on trunks and large branches. In later stages, bark falls off the tree exposing the fungus and white, stringy sapwood.
Avoid the canker by maintaining good tree health, allowing the tree’s natural defenses to ward off infection. A costly but useful technique to try to save valued trees is vertical mulching. This involves drilling a grid of holes throughout the root zone, extending beyond the drip line. Holes should be a few inches in diameter and 18-24 inches deep. Fill them with a porous mixture of pea gravel or coarse sand and compost or other organic matter. This improves drainage during wet periods and conversely, infiltration and moisture retention during droughts, while maintaining aeration.
If less than 15% of the canopy is affected, remedial pruning can help. Remove dead branches 8-12 inches below the infection, sanitizing the tool between each cut with a 10% bleach solution. If more than 15% of the canopy is infected, the tree should be removed due to its likely structural damage. Because the fungus is already present in the area, destroying the wood is not likely a benefit. However stored wood should be located remotely from any remaining trees.
Sudden Oak Death
Sudden Oak Death is caused by a fungal pathogen, actually a water mold, Phytophthora ramorum. It was transported into the Mid-Atlantic area on nursery stock from the West Coast, where the disease has caused widespread oak dieback in California and Oregon. It thrives in cool, moist environments, often infecting understory shrubbery, including rhododendron, laurel, azalea, and camellia. On the shrubs, leaves drop but the plant usually survives. Spores on shrub leaves are wind-blown or rain splashed onto the oak trunk. The fungus infects the living bark layer. The infection then spreads around the tree circumference, cutting off nutrients passing from leaves to roots, killing the roots. The upper tree dies from lack of water. Red oaks are more susceptible than white oak varieties.
Leaf spotting and twig dieback are visible. Characteristic symptoms of sudden oak death are cankers and calluses on bark and wood, often seeping a black or reddish ooze. Tree death occurs in two years or less, depending on general tree health when infected. There is currently no cure, although research at Virginia Tech has identified the pathogen’s genome and holds promise for developing a cure. Until then, it is best to remove both shrubs and trees that are infected.
Oak Wilt Disease
Oak wilt is another fungal disease that plugs water-conducting tissues with its mycelia and spores. Trees respond defensively to the invasion by plugging their own vessels and worsening the impact.
Oak wilt affects all oaks with differing rates of decline. The red oak family including black, black jack, pin, red, scarlet, shingle and shumard oaks can die in weeks. The white oak group, including bur, chinquapin, swamp white and white oaks may survive for several years, showing decline symptoms.
The disease is transmitted in two ways. Above ground, sap feeding beetles pick up the fungus by feeding on an infected tree where there is a fresh wound from pruning, storm damage or bark openings, and transport it to other newly wounded trees. Below ground, where root grafting between oak trees is common, the disease is transmitted directly from tree to tree.
Symptoms include withering of the upper canopy and browning of branches and crown portions. Red oaks show a yellowing and then browning of leaf margins or along veins, spreading outward. White oak symptoms are less distinct. Spring infections may cause mid-late summer wilting exacerbated by hot, dry weather and a resulting water deficit.
Red oaks often show dessicating bark cracked open by fungal mats disbursing the pathogen. In addition, sapwood may show brown streaks. However, conclusive diagnoses are difficult without lab work to identify the fungus.
To minimize susceptibility, avoid wounding trees during the growing season when beetles are active. If growing season wounds are unavoidable, seal the wounds with a dressing.
To prevent spread from an infected tree, remove the tree after severing root contact between neighboring trees by trenching around the root perimeter. Dig the trench prior to removing the tree to avoid a water tension imbalance that could suck fungal material into healthy trees. Leave as short a stump as possible to minimize the fungal material left behind. Because there will be fungal material beneath the bark, logs should be removed and properly disposed of. The pathogen doesn’t survive when subject to dessication and it is temperature sensitive. Chips are unlikely to spread the disease, but it is best not to use them near healthy oaks. Professional assistance is strongly advised if oak wilt is suspected. While tree removal is generally recommended for red oaks, trees in the white oak family may be saved through propiconazole injections and pruning dead branches.
Armillaria Root Rot
Armillaria root rot, sometimes called oak root rot fungus, can survive for many years in wood debris or dead stumps and root systems. It spreads to new trees of many species through root contact. It causes decay of roots and lower trunk eventually killing the tree and causing a toppling hazard.
Symptoms include poor growth, small yellowed leaves and dead branches in the upper canopy.
A distinctive sign is the growth of honey colored mushrooms at the trunk base in the fall. In addition, flat white fungal sheets grow between trunk and bark, and black fungal strands grow net-like at the trunk base and surrounding soil. Over time, the wood becomes soft and stringy from ground level to about six feet up the trunk. Young and stressed trees succumb quickly. Vigorous 15-20 year old trees are more tolerant. At this time there are no effective known chemical treatment strategies.
In addition to the oak killers listed, there are a number of lesser diseases that may cause concern but are not generally lethal to oaks.
Anthracnose is a fungal disease that causes areas of browning on leaf margins and smaller necrotic spots on the leaf surface. It forms fruiting structures on the underside of leaves in necrotic areas, usually next to veins. It can infect twigs causing dieback prior to bud opening. The fungus likes cool, wet springs and tends to subside during hotter, drier summer weather. While unsightly, anthracnose is not usually seriously harmful to otherwise healthy oaks. Manage it with good hygiene. Remove dropped leaves, but don’t prune trees until the dormant season to avoid subjecting the tree to other potentially more serious diseases.
Oak leaf blister is another fungal disease that commonly affects the red oak family during cool wet springs. It causes circular raised brown sections up to about 2 inches in diameter over the leaf surface. It may cause leaves to fall to the ground prematurely. If leaf drop happens early, second leafout may occur. If defoliation occurs late in the season, a single occurrence normally has little health effect on the tree. Consecutive infections can impact tree health.
Treatment includes maintaining good hygiene and watering during drought periods. It is possible to have an arborist apply a fungicide in early spring to protect at-risk trees.
Good Hygiene Minimizes Risk
Reviewing the disease summaries above shows a clear pattern. Good hygiene promotes oak tree health and will help them fight off disease. Key advice includes:
- Provide good drainage to avoid oxygen deprivation during wet periods
- Water during dry spells and droughts
- Mulch under the canopy to promote soil health. Leave a gap around the trunk base.
- Take care not to wound bark with lawn or other equipment.
- Remove damaged limbs and use a wound sealant during the growing season.
- Perform normal pruning during dormancy
- Avoid compaction and take care to prevent root damage during construction projects.
- Remove diseased debris when trees are removed to prevent spread to healthy neighbors.
Whether you are trying to protect valued oaks or are concerned about specimens showing signs of weakness, I hope this information is helpful in keeping your trees healthy.
https://extension.psu.edu/oak-diseases (Oak Diseases)
https://naswc.org/docs/oak_tree_diseases.pdf (Oak Tree Diseases)
https://marylandgrows.umd.edu/2019/10/01/why-are-so-many-oak-trees-dying-this-year/ (Why are so many oak trees dying this year?)
https://climate-woodlands.extension.org/oak-decline/ (Oak Decline)
http://www.pilotonline.com/news/environment/vp-nw-oak-mortality-20191220-kkkafwo5trgqzbyv24bs6fjvhu-story.html (Virginia Seeing Surge in Deaths of Oak Trees)
https://extension.psu.edu/stay-alert-for-oak-wilt (Stay Alert for Oak Wilt)
https://forestry.ca.uky.edu/oak_wilt (Oak Wilt)
https://vtnews.vt.edu/articles/2004/06/2004-264.html (Virginia Tech Advances Fight Against Sudden Oak Death)
https://www.fs.usda.gov/naspf/sites/default/files/publications/pest_alert_sudden_oak_death_west.pdf (Sudden Oak Death caused by a new Pest)
http://www.suddenoakdeath.org/diagnosis-and-management/hosts-and-symptoms/ (Sudden Oak Death)
Tex.A&M AgriLifeExtension/hypoxylon-canker-of-oaks (Hypoxylon Canker of Oaks)
MissouriBotanicalGarden.org/advice-tips-resources/pests-and-problems/armillaria (Armillaria Root Rot)
http://hyg.ipm.illinois.edu/article.php?id=648 (Red Oak Borers)
https://hort.extension.wisc.edu/articles/two-lined-chestnut-borer/ (Two lined Chestnut Borer)
https://plantpathology.ca.uky.edu/files/ppfs-or-w-12.pdf (Bacterial leaf scorch)
https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/3001/3001-1433/SPES-83NP.pdf (Bacterial Leaf Scorch)
https://www.pubs.ext.vt.edu/430/430-456/430-456.html (Guide to Successful Pruning)
Featured photo: White oak tree by Pat Chadwick