by Steve Turner
Sudden oak death is as bad as it sounds. It is a disease caused by a strain of a common fungus that will often only cause minor leaf blotches on other types of plants. When introduced to oaks, however, it will cause a very quick demise of the tree. Bleeding cankers that ooze a black, resin-like substance are the calling card of this disease. The tree dies shortly thereafter.
The good news is that it has yet to be found in Michigan. So far it has been confined to the West Coast, mainly northern California and the Pacific Northwest. The bad news is that those are areas that ship many plants to the Midwest.
The odds of this disease coming here on an alternative host and then infecting our native oaks is pretty high, judging by the spread of these types of problems in the past. I’m afraid it is more of a question of when, not if, it will get here. Only by asking for and buying only Midwestern-grown stock can we help reduce the chance that this (and other invasive organisms) ends up here in the long run. At this time there is no known cure for this disease.
Next is oak wilt, also caused by a fungus. It blocks the vascular system of the trees, causing them to wilt and die, very similar to Dutch elm disease (DED). Just like DED, it is spread by an insect and by root grafts. The good news is that the picnic beetle that helps spread the fungus is not as proficient as the bark beetles that spread DED. So, following simple cultural practices, such as never pruning oaks during the growing season, and promptly repairing and sealing any wounds caused by storm damage can greatly reduce the odds of trees being infected.
However, if your neighbors or utility companies don’t follow these same guidelines and nearby trees become infected, then oak wilt can spread through root grafts to your trees. As mentioned earlier, only white oaks can be cured if they become infected. In red oaks, you can only treat as a preventative measure and just like DED, they are still vulnerable to root grafts whether they are treated or not.
The only way to prevent infection through root grafts is to trench and sever the root systems between healthy and infected trees. If you have red oaks in areas where there are known infestations and they are important landscape trees, it would be wise to take a preventative approach.
Anthracnose is a foliar disease that causes browning of leaves from the veins outward, not to be confused with leaf scorch, which causes a similar effect, but from the edges inward. Anthracnose infects trees in the spring and the damage will appear during the growing season. It seems to infect white oaks much worse than red oaks.
Severely damaged leaves will generally fall off, leaving the tree looking sparse. Often the tree will try to put out new leaves to replace the damaged ones. If the tree has been infected for several years, it has a more difficult time finding the extra energy to do so year after year.
This is the point at which a normally mild disease can start to threaten the health of these trees by weakening them enough so that other organisms such as chestnut borers can enter the tree and cause their demise. Severely infected trees can either be injected in the fall or sprayed in the spring to help control this disease.
Late season frosts are not a disease, but they can cause disease-like problems for white oaks, which tend to leaf out a little later than red oaks, making the young leaves more vulnerable to the freezing temperatures. Generally the leaves will turn black and drop off or become distorted, with dead areas surrounded by healthy ones. If it happens often enough, it can lead to the same results as anthracnose, causing the tree to weaken over the years.
Iron chlorosis (iron deficiency) is also not a disease but an environmental condition that is very common in red oaks, especially pin oaks. It is not caused by lack of iron in the soil, but by the pH of the soil being too alkaline, causing the iron to be bound up in the soil and unavailable for the tree roots to absorb.
There are several ways to address this problem. A short-term solution is a trunk injection of iron to supplement what the tree cannot get on its own. This will only last 1 to 3 years depending on the situation.
More permanent solutions include: 1) adding chelated iron to the soil; this is a special kind of iron that will allow the tree to absorb it regardless of the pH, or 2) replacing soil around the tree by blowing out the high pH soil with a special tool and replacing it with better, more acidic soil, similar to replacing the soil of an ailing houseplant. This method will last many years, but is not always feasible in all situations (if access to the root zone is limited by structures or pavement). Left untreated, a tree will slowly starve to death over many years as photosynthesis is reduced by the lack of iron.
So, if you have an ailing oak, it would be wise to have it checked out sooner rather than later to determine what might be affecting it and to select the best course of action to help it recover. We owe it to these magnificent trees that have served mankind so well for thousands of years and helped shape our evolution.
Steve Turner is a Certified Arborist from Arboricultural Services in Oakland County, Michigan.