Many oak problems have been reported in Illinois. Phone calls, plant samples, and e-mail messages report problems on oaks, especially white oaks. I would like to provide one concrete diagnosis, but the situations vary almost as much as the callers. Oddly, most inquiries are about white oaks. If you think you know the cause of the problem, feel free to voice your concern to me at email@example.com, with “oaks” in the subject line. Here are a few of the problems that we have been able to confirm on oaks so far this season.
Flooding has caused injury in some areas. Such injury can occur from actual flooding or from less obvious situations, such as water-saturated soils or poor drainage of clay sites. Although you would expect all the oaks in a flooded area to be affected equally, species differ in their tolerance to flooded soils. According to Sinclair, Lyon, and Johnson in Diseases of Trees and Shrubs, bur and pin oak have intermediate tolerance to flooding, while red and white oaks are intolerant. Injury symptoms might include downward bending of the petioles, wilting, chlorosis, leaf margin necrosis, leaf drop, twig dieback, root death, and possibly death of the tree. Any sort of root injury could mimic these symptoms.
Phytophthora collar rot is a disease that is closely associated with flooding. The fungal pathogen (Phytophthora) lives in the soil and invades the tree at the root collar. A wound increases chances of infection, but the fungus can enter unwounded, young roots to get to the collar. Wet conditions are a necessity for infection. The disease causes the collar and some roots to rot, inhibiting movement of water and nutrients. Wood at the collar is brown and watersoaked. Aboveground symptoms include stunted, sparse, chlorotic foliage, much like flooding damage.
Oak wilt, discussed in issue no. 9 of this newsletter, may account for some of the oak decline in Illinois. The Plant Clinic has confirmed this disease on only five oaks in 2004. Species information is not always available, but at least one is known to be a white oak. Three samples were in DuPage County, one in Madison County, and one in Winnebago County. Others are still incubating. Oaks in the red oak group die suddenly from this disease, usually in one growing season. The white oak group shows a slower decline from oak wilt, sometimes taking several years for tree death to occur. Oak wilt causes a discoloration of the vascular tissue in affected branches. This diagnostic clue should help distinguish it from other problems.
Bacterial leaf scorch (BLS) is another possible player in oak decline. That disease was discussed in issue no. 12 of this newsletter. The bacterium (Xylella fastidiosa) that causes BLS causes slow decline and death of the tree. It has been confirmed on red, pin, bur, shingle, and white oaks in Illinois. Usually this disease causes marginal leaf necrosis (browning), often bordered by a band separating the necrotic and green tissues. These symptoms recur each year, spreading a bit further in the tree with each passing year. BLS usually takes 4 to 6 years to kill a tree.
We have also had reports of oaks infected with Armillaria root rot. The disease is caused by a fungus that invades below ground. Armillaria can continue to live as long as it has organic matter on which to survive. Often it remains in the on roots of a tree that has died and has been cut down. Interestingly, this fungus attacks plants predisposed by drought, flood-ing, poor drainage, frost, repeated insect attacks, mechanical injury, and the like. You can see that the oak decline problem may involve many factors. For more on Armillaria root rot, consult Report on Plant Disease, no. 602, “Armillaria Root Rot of Trees and Shrubs.” This report can be found on the Web at http://www.ag.uiuc.edu/%7Evista/horticul.htm. The diagnostic signs provided by this disease are mushrooms at the base of the tree or shoestringlike rhizomorphs growing just under the bark at the base of the tree.
The final problem we have seen on oaks is oak tatters, a problem discussed in issue no. 5 of this newsletter. Leaves retain only major veins and a bit of blade tissue around those veins when this condition occurs. Leaves look like something has eaten them and left the major veins. Insects are not involved. White oak is the common oak species affected. Many of the trees with oak tatters are affected early in the season but produce normal leaves later in the season. There has been some concern that a tree repeatedly attached by oak tatters might decline and even die. No evidence exists to confirm this theory, but the question merits investigation.
It is likely that other problems could be involved in oak decline. The frequency of oak concerns of 2003 and 2004 suggests that environmental stress may be a large part of the problem. The possibilities listed here show that the decline may be caused by different reasons in different locales.