TREE DISORDERS, TREE DISEASE, AND PESTS

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Oak Wilt Disease

Oak Wilt is caused by a fungus called Ceratocystic fagacearum and when introduced to the vascular system of an oak tree, this fungus spreads quickly through the sapwood just beneath the bark. In attempt to protect itself, the tree produces gums and tyloses which plug the water conducting vessels. This ultimately cuts off the trees own water and nutrient supply between the roots and crown of the tree.

 INFECTION

Oak Wilt can be spread in two ways, through root graphs or by insect vectors.

 Roots graphs - Most new infections of oak wilt are spread through overlapping roots of unaffected and infected trees. Often trees as far as fifty feet apart may have roots grafted together. This can be the case with oaks of different species. Control in these cases involves severing the root connection between healthy and infected trees. This can be done by a professional by either trenching or by   applying a chemical barrier known as Vapam.

 Insect vectors - Although less frequently with oak wilt (about ten percent of the time) new infections are caused by insects, primarily sap beetles. Sap beetles are commonly in contact with fungus between the bark and wood of the oak wilt killed trees. Several species of the same sap beetles are then attracted to fresh wounds on healthy oaks from April to June, where the fungus is then transferred to the vascular system of the tree.

OAK WILT SIGNS & SYMPTOMS

 Red Oaks and Pin Oaks - are highly susceptible to infection, and this can be easily identified by rapid wilting of a tree. These trees will usually wilt completely within two to six weeks. Trees wilt from the top of the crown down and leaves wilt from tip to the base, turning bronze to brown. Fallen leaves are often green at the base. For these trees, infection is a death sentence.

 White Oaks and Bur Oaks - are less susceptible and die a slower death, usually a branch at a time and is often a process spanning many years. Treatment of these trees showing early symptoms can prevent further development.

TREATMENT AND PREVENTION 

• Avoid pruning oaks from April through July to prevent spread by insects. 
• Stop below-ground spread by cutting root connections. 
• Use fungicides on high-value trees.
• Properly disposing of logs from wilted trees.

Emerald Ash Borer

The Emerald Ash Borer is an invasive beetle that attacks and kills ash trees. The adults are small iridescent green beetles that live outside of the trees during the summer months. Adult female ash borers lay their eggs on the bark of the ash trees. When the eggs hatch the larvae burrow under the bark and eat the living tissue they find there. As they do, they cut off the life-giving channels that carry nutrients, water, and sugar to nourish the tree. After a few years, enough of the channels are cut off so the tree starves to death. Since its accidental introduction into North America, the Emerald Ash Borer has killed millions of ash trees in several eastern states. The Minnesota Department of Agriculture had announced the discovery of this destructive tree pest found in May of 2009 in St. Paul. The effect of the EAB beetle could be devasting to Minnesota as it has one of the largest concentrations of ash of any state in the country, with about 900 million ash trees, thus making it a prime target for infestation.  Minneapolis ash trees make up 20 percent of all trees on public and private land in the city. Officials urge Minnesotans to take steps to keep EAB from spreading.

EAB INFECTION SIGNS AND SYMPTOMS

The Emerald Ash Borer adult beetle is metallic-green and is a half inch long. They are active from May to September.

D-Shaped exit holes about one-eighth inch in size in ash tree bark.

S-Shaped galleries under the bark and/or segmented larva located by peeling back loose bark of an infested tree.

Crown decline with the top 1/3 dying first and then a progression down the   tree with new shoots that may be produced lower on the tree.

Woodpecker holes could be an indication of the presence of this beetle as they love Emerald Ash Borer larvae. However, other insect larvae may also attract woodpeckers as well.

A number of declining Ash in the area with similar symptoms may indicate infestation.

EAB PREVENTION

Do not purchase firewood from outside Minnesota. Always ask where the source of the wood came from before buying.

Watch for signs of infestation in your ash trees and report them if you have any concerns. If you suspect your ash tree could be infested by EAB, visit the MDA Website, and use the "Do I Have Emerald Ash Borer?" checklist.

EAB TREATMENT

There are several insecticide options available if you elect to treat your ash trees and research has shown that treatments can be effective. Studies have shown that it is best to begin using insecticides while Ash trees are still relatively healthy. This is because the insecticide must be transported within the tree and the tree must be healthy enough to carry the insecticide up the trunk and into the branches and canopy. Beetles tunneling through the water conducting vessels hinder this process. Treating trees can be costly.  If the tree is not deemed worthy of investing in long term, you should consider replacing it with an appropriate species. Left unprotected the tree will likely become a host for larvae in the future. Landscape trees typically increase property values, provide shade, and cooling, and contribute to the quality of the neighborhood. Replacing your ash now may give you a head start to a healthy tree in the future, especially if it was a less than desirable tree to begin with. These reasons should be considered in the decision-making process for tree treatment

Dutch Elm Disease

Dutch Elm disease is caused is caused by two closely related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. Ophiostoma novo-ulmi is the more aggressive species and is the most common pathogen associated with DED today. This fungus invades and grows in the water conducting vessels the elms. In an effort to protect itself, the host tree produces tylosis and gums which plug the vessels, preventing water uptake. This causes the tree to wilt and die. All species of elms native to North America are susceptible to Dutch Elm Disease. Some elms such as the Siberian Elm are less susceptible than others, but none are immune.

INFECTION

The Dutch Elm fungus is spread by one of two ways, either by elm bark beetles or through root grafts.

 Beetles - In Minnesota, the fungus is spread by either the native elm bark beetle which is more prevalent in the Northern part of the state, or by the European elm bark beetle which is the primary vector in the Southern portion of the state, including the Twin Cities. Both beetles use dead or dying elms for breeding. Beetle breeding tunnels in infected wood become filled with fungus spores which then contaminate beetles. Contaminated beetles then fly to healthy elms to feed on their branches. As they feed through the bark, they transfer the spores into the vascular system of the tree. The tree responds by plugging its vessels to stop the spread. This defense results in wilting of one or more of the upper branches. If caught in time, infected limbs can be removed to stop the spread of the disease, but this should be done by a professional who can examine the wood to be sure enough has been removed.  Fungicides when properly administrated can protect healthy elms from infection and can cure trees in an early stage of beetle infection. Dead or dying elms should be removed as soon as possible to eliminate breeding sites for other beetles.

 Root grafts - The fungus can also be spread through roots that are naturally fused together between infected and uninfected trees. This type of transmission of the fungus only occurs in about ten percent of new infections. To reduce the chance of infection through root grafts, mechanical trenching can be used to break the bond between trees. Another option is to treat the bonded area with the chemical Vapam. Death of an elm through root graft transmission is much more rapid usually lasting only a few weeks.

TREATMENT AND PREVENTION

• Preventative fungicide injections can be used to protect trees from infection, the treatment must be repeated every 1-3 years.
• Branches infected with DED should be removed the same year the infection starts.  
• Trees with many branches infected with DED should be taken down.
• Wood from DED infected elm trees should be buried, debarked, burned or chipped.