Here in Minnesota, our trees are susceptible to a range of diseases and pests that can have a range of devastating consequences. From Dutch elm disease wiping out entire neighborhoods of stately elms, to emerald ash borer decimating ash populations, to oak wilt spreading rapidly from tree to tree, these problems are real threats to property values, safety, and the overall health of your landscape.
That’s why we highly recommend investing in routine tree care. Prevention and early treatment are the best ways to protect your property’s most valuable natural assets. To help you become a more informed steward of your property, we’ve outlined the most common tree diseases and pests affecting trees in Minnesota. Keep reading to find out how to keep your landscape looking spectacular for years to come.
Common Tree Diseases in Minnesota
Fungal and bacterial diseases can weaken a tree from the inside out, so you will want to keep an eye out for the top warning signs of tree disease. In this way, you can ensure your tree gets the care it needs to beat the infection and continue to thrive.
Oak Wilt
Oak wilt is a devastating fungal disease that impacts all species of oak trees, with the red oak group being particularly vulnerable. The fungus, Bretziella fagacearum, invades the tree’s water-conducting tissues, effectively clogging its vascular system and causing it to wilt and die from a lack of water. The disease then spreads through root grafts between nearby oak trees or via sap-feeding beetles that carry fungal spores from infected trees to fresh wounds on healthy ones.
Symptoms of Oak Wilt
The first sign of oak wilt in red oaks is often a rapid wilting and browning of the leaves at the top of the tree, which progresses downwards, typically leading to complete leaf drop by late summer. White oaks tend to show a slower decline over several years.
Oak Wilt Treatment Options
To prevent oak wilt from taking root in the first place, avoid pruning or wounding oak trees from April to July, when the sap-feeding beetles are most active. If a tree becomes infected, the primary goal is to prevent its spread. This can involve trenching to sever root grafts and professionally removing the infected tree. Fungicide injections can also be used preventatively on high-value trees, but they are not a cure.
Dutch Elm Disease (DED)
Another aggressive fungal disease, Dutch Elm Disease, has been responsible for the loss of millions of American elm trees across the country. Caused by the fungus Ophiostoma ulmi and Ophiostoma novo-ulmi, it is spread primarily by elm bark beetles.
Symptoms of Dutch Elm Disease
Similar to oak wilt, DED clogs the tree’s vascular system, which leads to the most obvious sign of a fungal infection, “flagging” (in which the leaves on a single branch or section of the canopy suddenly wilt, curl, and turn yellow or brown). This can happen seemingly overnight in early summer. As the infection spreads, the entire tree succumbs.
DED Treatment Options
Preventative fungicide injections can protect healthy, high-value elms from this disease. If the infection is confined to a single branch, immediate and professional pruning may save the tree. Of course, the most effective long-term strategy involves planting DED-resistant elm cultivars.
Anthracnose
Anthracnose is a general term for a group of fungal diseases that cause discolored lesions on leaves, stems, flowers, and fruits. It is particularly common on ash, maple, oak, and sycamore trees, and its severity is often linked to cool, wet spring weather. The good news is that, while it can look alarming, anthracnose is rarely a serious threat to the long-term health of a mature tree.
Symptoms of Anthracnose
Symptoms vary by tree species but generally include dark, sunken spots or dead areas on leaves, often along the veins. Infected leaves might become curled and distorted, and they can drop prematurely. In some cases, the disease can cause cankers on twigs and small branches.
Anthracnose Treatment Options
The best defense against this common tree disease is promoting good tree health through proper watering and mulching. Raking up and disposing of fallen leaves in the fall is also a great way to reduce the amount of fungus that survives the winter, and pruning can help the foliage dry more quickly, making conditions less favorable for fungal growth. Fungicide applications are an option for high-value trees with a history of severe infection, but must be applied preventively in the spring.
Apple Scab
If you have apple or crabapple trees, you’re likely familiar with this common fungal disease. While not typically fatal to the tree, it can cause significant aesthetic damage, making beautiful trees look sickly and stressed.
Symptoms of Apple Scab
Apple scab appears as olive-green to brown spots on leaves and fruit. Infected leaves may even become twisted or puckered and can turn yellow and fall off prematurely. This repeated defoliation can weaken the tree over time, making it more susceptible to other issues.
Apple Scab Treatment Options
Rake up and destroy all fallen leaves and fruit in the autumn to reduce the amount of fungal spores that can overwinter. For persistent problems, a fungicide treatment plan can be implemented, typically involving sprays applied in the spring as new leaves emerge.
Common Tree Pests in Minnesota
Insect infestations can defoliate, weaken, and ultimately kill a tree. The good news is that, in many cases, you can control and even eliminate the infestation, effectively saving your tree and protecting nearby plants.
Emerald Ash Borer (EAB)
This invasive, metallic-green beetle is one of the most destructive forest pests ever seen in North America. EAB has killed thousands of ash trees since its discovery in the early 2000s.
While the adult beetles do minimal damage, their larvae can have devastating consequences. They feed on the inner bark of ash trees, which interferes with the tree’s ability to transport water and nutrients. If left unaddressed, the insect activity could kill the tree within 3-5 years.
Signs of an EAB Infestation
Signs of infestation include a thinning canopy, D-shaped exit holes in the bark, and increased woodpecker activity.
EAB Treatment Options
If you have an ash tree, you should plan for routine insecticide treatments. These trunk injections and soil applications are highly effective at protecting healthy trees but must be administered by an ISA-certified arborist.
Unfortunately, once a tree is heavily infested and showing significant decline, professional tree removal is often the only safe option.
Spongy Moth (Formerly Gypsy Moth)
The spongy moth is another invasive pest known for its voracious appetite during its caterpillar stage. They are notorious for causing widespread, rapid defoliation of a wide variety of hardwood trees, especially oak.
Signs of Spongy Moth Infestations
The caterpillars chew through leaves, and in a severe outbreak, they can strip a tree bare in a matter of days. While a healthy tree can often survive a single year of defoliation, repeated stress weakens it significantly, leaving it vulnerable to other diseases and pests.
Spongy Moth Treatment Options
In late summer and fall, you can search for and scrape off their fuzzy, tan-colored egg masses. During the caterpillar stage in spring, burlap bands can be wrapped around tree trunks to trap them. For larger infestations, aerial or ground-based applications of a biological pesticide are effective.
Two-Lined Chestnut Borer
This native beetle is a serious pest of oak trees in Minnesota, especially those that are already weakened by factors like drought, construction damage, or defoliation from spongy moths. A healthy tree can typically fight off an attack, but a stressed tree is a prime target.
Signs of Two-Lined Chestnut Borer Infestations
Just like emerald ash borers, the larvae tunnel just beneath the bark, creating winding galleries that interrupt the tree’s ability to transport water and nutrients. For this reason, the first sign of a two-lined chestnut borer infestation is often dieback in the upper canopy, with leaves on affected branches suddenly turning brown in mid-summer but remaining attached to the tree. Infestations typically progress from the top down, killing the tree over one to three years.
Two-Lined Chestnut Borer Treatment Options
The best defense is to keep your oak trees as healthy and stress-free as possible through proper watering during dry periods and avoiding root zone compaction. If an infestation is caught early, infested limbs should be pruned and destroyed. For high-value trees, professional systemic insecticide treatments can be effective at preventing or controlling infestations.
Pine Bark Beetles
Several species of bark beetles attack pine trees in Minnesota, most notably the pine engraver. These beetles are attracted to pines that are stressed, damaged, or recently felled, and a large-scale attack can overwhelm and kill a healthy tree.
The adult beetles bore through the outer bark to reach the nutrient-rich inner bark. Here, they create tunnels in which they lay their eggs. This activity girdles the tree and cuts off the flow of water.
Signs of Pine Bark Beetle Infestations
Evidence of an infestation includes fine, sawdust-like boring dust on bark crevices or at the tree’s base, small holes in the bark, and a fading crown where needles turn from green to reddish-brown.
Pine Bark Beetle Treatment Options
Preventative insecticide sprays can protect high-value trees, but must be applied by a professional before the beetles attack in the spring.
If a tree is infested, promptly have the affected branches or entire tree removed to prevent the next generation of beetles from emerging and attacking neighboring trees. Also, avoid stacking freshly cut pine firewood near healthy pine trees.
Protect Your Trees With Professional Care
It can feel overwhelming to consider all of the potential threats to your beloved trees. From microscopic fungi to invasive beetles, the challenges are many. But we make it simple to manage the common tree diseases and pests in Minnesota. We start with a thorough assessment of your property, identifying potential risks and current issues with a trained eye. We then walk you through all of your options, explaining the pros and cons of each so you can make an informed decision that fits your goals and budget.
Our team is led by Brandon Pankey, an ISA Board-Certified Master Arborist with over 15 years of proven experience, so you can trust us to provide the right solutions at the right time. Don’t wait until it’s too late. Be proactive in protecting your favorite trees and shrubs.
To schedule a tree health inspection, contact Blue Pine Works in Pine City, MN, today!