5 Common Tree Diseases & Pests in the St. Croix Valley (and How to Fight Them)

Not everyone may be aware that the St. Croix Valley faces challenges from tree diseases in Minnesota and tree pests in Wisconsin. What looks like harmless discoloration to some may be indicative of diseases and pests that can harm these natural assets and spread. This can be concerning, as trees may decline unexpectedly, leading to removal and aesthetic loss. However, there are ways to help. With information, recognizing early signs, and taking timely action, trees can often be saved. Consult our “Ultimate Guide to Tree Care” for preventive measures. Here, we’ll look at five common local issues, providing identification tips and solutions to maintain tree health.

1. Oak Wilt

Oak wilt, a lethal fungal disease, poses a serious threat to oaks in Minnesota and Wisconsin. Key signs include sudden wilting and browning leaves in mid-summer, often dropping prematurely, and brown or black streaks beneath the bark. Preventing oak wilt involves avoiding pruning oaks between April and July, when fungus-spreading beetles are most active. If pruning or damage occurs during this period, seal wounds promptly to prevent infection. Effective treatments include fungicide injections, particularly propiconazole, administered by certified arborists to protect at-risk trees. Root trenching or severing root connections between infected and healthy trees is essential to halt the spread underground. Quick oak wilt treatment is critical, as untreated oak wilt can kill red oaks within a single season. Consulting with professional arborists early can significantly improve the chances of saving your oaks.

Oak wilt spreads rapidly through interconnected roots, and without prompt intervention, entire oak groves can be devastated. Beyond avoiding pruning, homeowners should regularly inspect trees for early symptoms during the growing season. Diseased branches must be swiftly pruned away and destroyed, ideally in winter when beetles are inactive. Sanitation practices such as removing infected wood and branches promptly from the site also help control the spread. Professional arborists can provide precise assessments, including soil treatments and ongoing monitoring plans tailored to your property’s specific needs.

2. Dutch Elm Disease

Dutch elm disease (DED), a devastating fungal infection, decimated American elms in the United States during the latter part of the 20th century. This persistent threat continues to endanger the few remaining mature elms in our area. First documented in Minnesota in 1961, DED has since spread extensively throughout the state and into neighboring Wisconsin. The fungus, which is transmitted by elm bark beetles and through root connections between elms, infiltrates the tree’s water transport system, leading to wilting and ultimately, its death.

What are the signs?

A key initial sign of infection is often “flagging,” where leaves on one or several upper branches abruptly turn yellow or brown during mid-summer, contrasting with the still-green canopy. These areas of wilted, dried leaves become strikingly visible. As the disease advances, increasing numbers of branches exhibit wilting, and these symptoms gradually spread downwards towards the tree’s trunk. If you cut into an infected branch, you’ll usually find brown streaks or discoloration in the sapwood just beneath the bark. Left unchecked, a fully susceptible elm can die within weeks to months of symptoms appearing. If an infected branch is cut, brown streaks or discoloration are typically found in the sapwood directly beneath the bark, signaling Dutch Elm Disease (DED). A fully susceptible elm, if untreated, may die within weeks or months once symptoms appear.

What can you do?

To fight Dutch elm disease, early intervention is everything. If a valuable elm on your property is not yet infected, you can proactively protect it. To prevent elm tree diseases, certified arborists can inject the tree’s trunk with preventative systemic fungicides, like propiconazole. These injections, when done before infection or at the very first hint of disease, can sometimes halt DED and save the tree. (Think of it like a vaccination – it’s most effective prior to exposure.) Keep in mind that once a tree is heavily infected, treatments are far less likely to work; quick removal of that tree may be necessary to protect other elms nearby. If an elm is confirmed with DED, removing the diseased tree (and ideally trenching roots to break connections) is often recommended to stop further spread. Homeowners who value their mature elm trees should schedule regular arborist inspections. If detected in its early stages, an expert can selectively prune infected branches and administer fungicide injections to control the fungus.

3. Emerald Ash Borer (EAB)

The emerald ash borer is an invasive insect that has devastated ash trees across the Midwest, and the St. Croix Valley is no exception. EAB is a small metallic-green beetle (about 1/2 inch long) that lays its eggs on ash tree bark; the real damage is done by its larvae, which tunnel underneath the bark and disrupt the tree’s vascular system. EAB typically kills an ash tree within 3-5 years of infestation, and has killed millions of ash trees in Minnesota and Wisconsin. Because the beetles spend most of their life hidden under the bark, an infestation can be hard to spot at first. Watch for these signs of EAB in your ash trees:

• Thinning or dying upper canopy: The first clue is often the ash tree’s crown looking sparse, with lots of dead branches at the top. Due to larval damage to water channels, the trees can’t sustain healthy upper foliage when dealing with EAB.
• Woodpecker damage: Woodpecker activity often signals an initial issue. Increased feeding prompts them to strip away outer bark, creating a “blonding” effect, as they hunt for the EAB larvae below. If you notice pale patches on the trunk or lots of bark chips at the base, investigate further.
• D-shaped exit holes: Emerging adult borers leave behind characteristic D-shaped exit holes (roughly 1/8 inch in diameter) as they chew through the bark. These exit holes have a flat side and a round side (like a capital “D”). They’re often high up or on branches, so you may need to look closely. Over time, you might also see bark splitting and S-shaped tunneling under the bark where larvae have fed.
• Sprouts at the base or on trunk: Ash trees under attack sometimes try to push out new suckers on the lower trunk or roots. These shoots (and a general decline in the tree’s vigor) can hint that the upper parts are dying.

If you suspect EAB, don’t wait – early action can save the tree. Healthy ash trees can be protected with preventive insecticide treatments. The most effective option is usually a trunk injection of a systemic insecticide (such as emamectin benzoate) by a certified arborist, which can kill existing larvae and guard the tree for 1-2 years per treatment. To effectively protect trees from EAB and the resulting harm, utilizing professional trunk injections is the widely accepted and preferred method. Cities and homeowners in Minnesota have successfully saved many ash trees by starting injections before the insect arrives or in the very early stages of infestation. This is a service that Willow River Company and other tree care firms provide locally. On the other hand, if an ash is already heavily infested (e.g. most of the canopy is dead and bark is peeling), removal is usually the safest course dead ash trees become brittle and can be hazardous, so you don’t want to leave them standing.

4. Apple Scab on Trees (and Other Leaf Diseases)

Apple scab, a prevalent fungal disease, frequently affects crabapple and apple trees in the region, resulting in unsightly leaf spotting and premature leaf drop. Initial symptoms manifest as olive-green to black spots on leaves, progressively causing foliage loss that weakens trees over successive seasons. Effective management involves rigorous sanitation practices, such as regularly raking and disposing of fallen infected leaves each autumn, significantly reducing fungal spores available for spring reinfection. Applying preventive fungicide treatments in early spring as new leaves emerge is another essential strategy for managing apple scab effectively. Similarly, Rhizosphaera needle cast affects spruce trees, causing needles to discolor and shed from the lower branches upward. Treatment includes professional fungicide applications, pruning to enhance airflow, and meticulous cleanup of fallen needles. Addressing these fungal issues proactively ensures sustained tree health and appearance.

Enhancing airflow through strategic pruning and planting disease-resistant varieties can further mitigate fungal infections. Regularly inspecting trees for early disease signs allows timely intervention, reducing severity. Incorporating mulching practices around the base of trees can also limit spore splash during rains. Consulting with tree care professionals provides access to advanced diagnostic tools and tailored treatment plans, ensuring optimal management of your ornamental and fruit trees.

5. Japanese Beetles

Japanese beetles, easily identified by their metallic green bodies and bronze wing covers, pose significant threats to trees and ornamental plants in mid-summer. These beetles typically swarm, causing extensive damage by feeding voraciously on leaves, leaving behind skeletonized foliage that dramatically impacts plant aesthetics and health. For manageable infestations, manual removal by hand-picking beetles or knocking them into buckets of soapy water can reduce damage effectively. Physical barriers like fine mesh netting can protect vulnerable plants from beetle feeding. However, for severe infestations, professional Japanese beetle control through targeted insecticides may be necessary. It is crucial to employ insecticides judiciously to minimize potential harm to pollinators and beneficial insects. Consulting professionals ensures effective beetle control strategies tailored to your landscape’s specific needs.

Additionally, understanding Japanese beetle life cycles can significantly improve management effectiveness. Applying grub control treatments to lawns during early fall or late spring can help reduce beetle populations emerging from the soil. Encouraging natural predators such as birds and beneficial insects, through habitat improvements, can also provide effective biological control. Ongoing monitoring and regular inspections during peak infestation periods allow for prompt interventions, minimizing long-term damage to trees and landscape plants.

Don’t Wait to Deal with Tree Diseases and Pests
Being informed about these common tree issues enables early detection and effective action, crucial for maintaining healthy trees in the St. Croix Valley. Vigilance and proactive care, combined with timely interventions, significantly enhance the longevity and health of your landscape. Consult Willow River Company for professional evaluations, treatments, and comprehensive plant health consultations. For ongoing tree care strategies, revisit our comprehensive Ultimate Homeowner’s Guide to Tree Care and Plant Health.

Contact us for more comprehensive solutions, including effective preventative measures.