Thursday, June 17, 2010

Oak Wilt Watch: So Far So Good

Quick update on the red oaks across the street that I have been concerned about: So far they look good, and it appears that the preventative fungicide injection done by S&S Tree Care is working.

Regular readers will remember that on May 22, 2009 - smack dab in the middle of the "danger zone" period for the spread of oak wilt - a hack fly-by-night "tree care" company pruned my neighbor's (a.k.a. nicest old lady in the world) gorgeous red oaks. I happened to be away at work that day and arrive home too late to stop the carnage.

As I feared, later in the summer one of those amazing trees, a 36" diameter beauty, succumbed to oak wilt. Unlike many fungal diseases oak wilt has a very inefficient method of spread from one area to another. In April, May and June spore mats - the reproductive phase of the disease - form on red oak trees that had been infected the previous year. These spore mats exude a sweet, fermenting smell that attracts an insect called the picnic beetle. The picnic beetles feed on the sweet exudate of the spore mat, and in the process fungal spores adhere to the insect. The the bug flies off in search of the next sweet, fermenting smell. It could be your potato salad at a family reunion, or it could be the sap flowing from a freshly wounded oak tree. If it's the latter, while feeding on the sap the fungal spores are spread to the new tree.

Like I said, very inefficient, very hit-or-miss. Were it not for unwitting homeowners and unscrupulous "tree care" companies the overall effect of oak wilt would be very limited.

However, once that one tree is infected with the fungus the disease now become deadly efficient in the way it spreads. First, it actually causes the tree to kill itself. Trees naturally for gums called tyloses to plug vascular (water and nutrient conducting) tissues containing a pathogen in an attempt to compartmentalize that pathogen and prevent its spread to other parts of tree. However, the oak wilt fungus spreads so quickly within the tree that very soon the tree plugs the vascular tissue around its entire circumference and literally starves or strangles itself to death in a futile attempt to block the spread of the fungus.

While spore mats form only on red oaks (red, black and northern pin), both red oaks and white oaks (including white, bur and swamp white) are susceptible. Once infected red oaks, as in the case of my neighbor's tree, can die in a matter of weeks. White oaks can survive for some years before finally succumbing to the disease.

Second, the fungus then exploits an amazing adaption of oaks: Oaks of like species for root grafts or unions with neighboring trees. Whether this is an amazing display or boreal socialism and an attempt to share the water and nutrients of a site to give its own species to spread and fill spaces that otherwise would not support oak growth, or an act of boreal parasitism enabling stronger individuals to rob the resources of surrounding trees, I'm not sure.

From the standpoint of the oak wilt fungus, however, what this adaptation does is provide a superhighway to spread to surrounding oak trees... in a hurry (I believe the rate of spread through the roots is about 20 feet per year).

So from that one hit-or-miss infection an entire stand of red oaks or northern pin oaks or bur oaks can be wiped out.

There are two ways to stop the root zone spread of oak wilt to adjacent trees: Use of a deep vibratory plow to mechanically sever the root grafts, and injection of adjacent trees. In my neighbor's case the proximity of sidewalks, driveway and underground cables made trenching impossible.

My neighbor's red oaks are part of a line of 12 red oaks that span 5 front yards; they must have been planted by the homeowners at the same time and in perfect alignment. They stand about 23 feet apart on average.

S&S Tree injected a total of 4 trees, the 2 adjacent trees on either side of the infected tree. Some of these were in neighbor's yards, and so their approval was needed. Thankfully the neighbors have been incredibly cooperative.

What a hassle! All because one company that happened to own a truck and a chain saw decided to make a few extra bucks (actually I saw the invoice and it was a lot more than a few extra bucks - what a rip off!) at the expense of a sweet old lady.

Thank goodness for highly qualified tree care companies (with no quotation marks around tree care) like S&S Tree Care. And thank goodness the adjacent trees look good and it appears the damage was limited to that single tree. As bad as that is, it could have been a whole lot worse.

Click to enlarge. One of the trees adjacent to the dead and removed tree: so far so good.

The ID tag left on an injected tree by S&S Tree Care. Great work guys! Thanks for your expertise, and thanks for your fast service late last summer!

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