If one tree in a group of live oak trees becomes infected with oak wilt, the pathogen can spread through the common root system at an average rate of about 75 feet per year.
Spread through Roots
Infection centers among live oaks in Texas expand at an average rate of 75 ft per year, varying from no spread to 150 ft in any one direction.
Disrupting the connections between the roots of infected and healthy trees limits the spread of oak wilt and is an effective control measure. Infected trees and their roots will usually die before root grafts can be re-established.
Once an oak tree is infected with the oak wilt fungus, the tree will die and there is no treatment to save the tree. When an oak wilt infection is confirmed treatment can be applied to the surrounding trees to prevent the spread of this disease.
Oak Wilt Spread
The Oak Wilt fungus is spread via two ways: above ground and below ground. The above ground movement is facilitated by a sap-feeding beetle that carries the fungal spores to new trees. The below ground movement occurs when the fungus travels from tree to tree through interconnected roots.
Cutting down a tree with oak wilt that is still partially alive can cause the disease to spread, but removing it during the least vulnerable months can prevent this.
First, leaves turn dull green or bronze, can appear water-soaked, and wilt.
White oaks can be treated with propiconazole once they display early symptoms of oak wilt.
It may be burned up or taken to an approved yard waste facility.
If there is concern that a red oak may have been infected with oak wilt, then its wood should be immediately burned, buried, or chipped. Fire will kill oak wilt, and there is no risk of spores being transported by smoke.
Finally, I'd like to note that chainsaws do not transfer oak wilt and are self-sterilizing on account of the operating temperature exceeding the temperature that the fungus can live in.
How much does treatment cost? Fungicide treatments are based on the diameter of the tree and therefore vary quite a bit. The average tree costs about $250 to treat once. However, there are discounts available for larger projects.
Pruning between April 15 and July 15 should be strictly avoided because this is when beetle activity and viable spores are most prevalent across the landscape. If pruning an oak is absolutely necessary during this time, wounds should be immediately sealed with pruning sealer or water-based paint to prevent infection.
To stop the spread of oak wilt through the root system, trenches can be placed around a group of trees, at least 100 feet away from the dripline of infected trees and at least four feet deep, or deeper, to sever all root connections. Another common management method for oak wilt is through fungicide injection.
The Essence of the Different Laws
The oak tree law California allows individual property owners to cut down and remove oak trees within their properties but they will have to seek and obtain the permission of the local county. The underlying principle here is that if an oak tree has gone dead, it has to be removed.
(Note: Burning firewood with oak wilt spores does not spread the fungus! Fungal spores burn, too. The risk comes from transporting infected firewood and storing it where the spores can be picked up by beetles and carried out to infect nearby healthy oaks.)
Trees in the red oak group generally die rapidly, usually within weeks or months after infection. Treatment will not save an infected red oak. Trees in the White oak subgenus, such as Burr oak and Chinkapin oak, can generally be treated for oak wilt and will recover within a year of treatment.
Healthy oak trees are not susceptible to oak wilt in an infected area. Bur oak, white oak and other members of the white oak family are resistant to the fungus and can be planted in oak wilt centers. For additional prevention measures, plant tree species other than Oaks to guarantee no infection from the disease.
Live oak (Q. fusiformis) is a member of the white oak group and is the most common and abundant oak in central Texas. While live oaks succumb to oak wilt in the greatest numbers, they are intermediate in susceptibility to the disease with a small percentage (< 20%) surviving infection.
A good indicator of oak wilt infection is a carpet of leaves under an oak in mid-summer. Red oaks infected with oak wilt lose about 90 percent or more of their leaves in 1–2 months; bur and white oaks quickly lose leaves from a few branches during the summer over several years.
Pruning oak trees in winter greatly reduces the risk of an oak wilt infection as the beetles and fungal mats are not present at that time of year.
Propiconazole is the only fungicide scientifically tested and proven effective (when properly applied prior to infection) for use as a preventative treatment to protect live oaks.
Wilting symptoms are observed from June through September, most commonly in July and August. Leaves of the infected trees turn dull green or bronze, and look water-soaked. Unlike normal leaf fall, the leaves of infected trees drop to the ground when they are still partially-green.
Oak wilt typically causes red oak leaves to turn brown around the edges while the veins remain green. Leaves are rapidly shed as the tree dies. Conversely, in live oak with the sudden oak death pathogen, the veins first turn yellow and eventually turn brown. Leaves are often retained on the tree after it dies.
Oak wilt is a major problem in Illinois, Iowa, Michigan, Minnesota, Texas, and Wisconsin.