Heart Rot in Trees

What is Heart Rot?

Heart rot is a fungal disease that causes internal decay in the heartwood, the central, structural core of a tree. Unlike other diseases that may attack foliage or bark, heart rot works silently from the inside out, often going unnoticed until serious damage has occurred. The disease is caused by various types of wood-decay fungi, which enter the tree through wounds in the bark or branch stubs, especially those left unhealed after improper pruning, storm damage, or animal activity.

Once inside, the fungi begin to break down the lignin and cellulose in the wood, gradually hollowing out the trunk or major limbs. Although a tree may still appear healthy on the outside for years, the internal decay can compromise its structural integrity—making it more prone to limb drop, splitting, or complete failure during storms or high winds.

Heart rot is especially common in older, mature hardwoods such as oaks, maples, elms, and sycamores, many of which are popular across Missouri landscapes. While some trees can live for decades with heart rot, the disease reduces overall vigor and significantly increases the risk of hazardous failure, making early detection and monitoring essential.

What Does Heart Rot Look Like: The Signs Your Trees Might Have It

Heart rot often progresses silently, with few visible symptoms in the early stages. However, as the disease advances, there are several warning signs property owners in Missouri should watch for:

• Mushrooms or fungal conks growing from the trunk or large branches—these shelf-like fungi are the most obvious external sign of internal decay.
  
  
• Cavities or hollow areas inside the trunk or limbs, often visible after a branch breaks or when the bark is damaged.
  
  
• Cracks or splits in the bark, especially near old pruning wounds or storm-damaged areas.
  
  
• Dead branches high in the canopy or reduced leaf growth compared to previous seasons.
  
  
• Soft, punky wood that crumbles easily when probed near damaged areas or at the base of the tree.
  
  
• Discoloration or oozing sap around infection sites.
  

• Woodpecker activity, as these birds often drill into weakened, infested wood in search of insects.

How Heart Rot Spreads to Other Trees

Heart rot doesn’t spread in the same way as many common infectious diseases. Unlike foliar diseases or pests, heart rot doesn’t rapidly jump from tree to tree. It primarily spreads through airborne fungal spores and requires specific conditions to infect a new tree. Here’s how it works:

• Entry through wounds: Fungal spores land on exposed wood from open wounds caused by pruning, storm damage, or animal activity. Trees with bark injuries are most vulnerable.
  
  
• Infection through pruning cuts: Poor pruning practices, especially cuts made too close to the trunk or during wet seasons, can invite fungal pathogens that cause heart rot.
  
  
• Spread via root contact (rare): In some species and forested environments, decayed roots may come into contact with healthy root systems, allowing the fungus to migrate—though this is less common in urban and suburban yards.
  
  
• Persistent fungal fruiting bodies: Once a fungus colonizes a tree, it can survive for years, producing spores seasonally. These spores can travel on wind, rain, tools, or even wildlife to infect nearby trees.
  
  

H2: Common Trees in Missouri at Risk from Heart Rot

Heart rot affects a wide variety of hardwood trees, and many of Missouri’s native and ornamental species are particularly vulnerable due to age, storm damage, or improper pruning. Because heart rot is an internal decay, it often progresses unnoticed until the tree becomes a safety concern.

Here are some of the most commonly affected trees in the St. Louis region and throughout Missouri:

• Oaks (White Oak, Red Oak, Bur Oak)
  
  Oaks are highly susceptible, especially as they age or develop pruning wounds. The large size of oaks can also make internal decay especially hazardous near homes or driveways.
  
  
• Maples (Sugar Maple, Silver Maple, Red Maple)
  
  Heart rot is common in older maples or those with past storm damage or weak branch structures.
  
  
• Elms (American Elm, Slippery Elm)
  
  Elms are known for their susceptibility to a variety of tree diseases, including heart rot, especially following Dutch elm disease or limb loss.
  
  
• Hickories
  
  These strong native trees can develop heart rot in aging trunks or following injury.
  
  
• Sycamores
  
  Often found in urban and riparian areas, sycamores can be prone to heart rot when damaged or poorly maintained.
  
  
• Boxelder
  
  This softwood maple species is very prone to structural defects and internal rot, especially when neglected.
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• Willows‍
  Fast-growing but weak-wooded, willows frequently suffer from heart rot, particularly at branch junctions and lower trunks.

How to Treat Heart Rot in Trees: Treatment & Prevention

Unfortunately, once heart rot has taken hold, there is no way to reverse the internal decay. However, early detection and smart tree care practices can greatly reduce the risk of infection and slow its progression. Here’s how to help protect your trees from heart rot:

Prevention Strategies

• Avoid Wounding the Tree
  
  Heart rot fungi enter through exposed wood. Prevent unnecessary bark injuries during mowing, construction, or pruning. Even small cuts can create an open invitation for decay.
  
  
• Practice Proper Pruning
  
  Always use clean, sharp tools and follow correct pruning techniques—making cuts outside the branch collar and avoiding flush cuts. Schedule pruning during dry periods when fungal spores are less active.
  
  
• Monitor Storm Damage
  
  After severe weather, inspect your trees for broken limbs or open wounds. Quick cleanup and proper trimming of damaged areas can prevent fungal intrusion.
  
  
• Improve Tree Health
  
  Keep your trees strong with mulching, deep watering during droughts, and soil care. A healthy tree can often compartmentalize decay and limit the spread.
  
  
• Remove Severely Infected Trees
  
  In cases where heart rot has compromised a tree’s structural integrity, removal may be the safest option—especially for trees near structures or walkways.
  
  

Treatment Options

• No Chemical Cure Exists
  
  Unlike some fungal diseases, heart rot has no effective fungicide or chemical treatment once it’s inside the heartwood. Prevention and early intervention are the most powerful tools.
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• Arborist Evaluation is Key
  Certified arborists can assess internal decay with specialized tools, including resistograph testing or sonic tomography. These methods help determine if the tree poses a safety risk and whether preservation is possible.
  
  

Treatment and Prevention for Oak Wilt

Oak Wilt is a serious and fast-moving disease, especially in red oaks, so proactive prevention and early intervention are critical. While there’s no guaranteed cure once a tree is severely infected, certified arborists can often help slow the spread and protect nearby trees with proper management.

Treatment Options

• Trenching to Sever Root Grafts
  
  One of the most effective ways to stop Oak Wilt from spreading underground is by installing a trench between infected and healthy trees. This severs root grafts—natural underground connections between nearby oaks that allow the fungus to move from tree to tree.
  
  
• Fungicide Injections
  
  Systemic fungicides, such as propiconazole, can be injected into the base of high-value white or bur oaks that are not yet showing symptoms. This is often used as a preventative measure or to slow progression in lightly affected trees.
  
  
• Selective Tree Removal
  
  If a red oak is already infected and declining, oak tree removal may be the best option to prevent spread. It’s important that infected wood is handled correctly—either burned, chipped, or properly sealed—so beetles don’t spread spores to other trees.
  
  

Prevention Methods

• Avoid Pruning During High-Risk Months
  
  Never prune oak trees from April through July, when sap-feeding beetles are most active. Open pruning wounds attract beetles that can carry Oak Wilt spores. Schedule pruning in the late fall or winter when beetle activity is minimal.
  
  
• Proper Wound Care
  
  If an oak tree is accidentally wounded (from storm damage, lawn equipment, etc.), immediately seal the wound with tree wound paint. While this isn’t typically recommended for most trees, it’s important for oaks at risk of Oak Wilt.
  
  
• Don’t Move Firewood
  
  Avoid transporting firewood from oak trees unless it has been properly dried and cured. Oak Wilt can survive in infected wood, and moving it may introduce the disease to new areas.
  
  
• Regular Health Inspections
  
  Annual evaluations by a certified arborist can catch early signs of Oak Wilt, allowing for quicker action and potentially saving surrounding trees.
  

Plant Resistant Dogwood Varieties:

• Consider planting Kousa dogwood (Cornus kousa) or disease-resistant hybrids like the Stellar series (‘Aurora’, ‘Ruth Ellen’, ‘Constellation’) in high-risk areas.
  
  
• These varieties offer similar visual appeal with significantly stronger resistance to anthracnose.