In issue no. 17 of this newsletter, I discussed a problem the clinic has been seeing on red oaks. I asked for reader response and eventually got quite a diversity of responses. It does not appear that there is any pattern to the red oak group problem. Most of the reports were from central and northern Illinois, but a few reports of similar oak problems came from out of state. One fact is certain: not all oak problems are easy to diagnose with accuracy. Here are some of the oak problems we have seen at the Plant Clinic in recent years.
Oak wilt is at the top of the list. This disease will kill trees in the red oak group in one season, while trees in the white oak group may survive for many years. Consult issue no. 6 for details, but look for sudden decline starting at the top of the tree, with distinct leaf scorch and bronzing. Vascular discoloration is the most diagnostic feature of this disease. Positive identification is attained by isolating the causal fungus from live, infected wood. Early summer is the best time to prepare cultures.
Bacterial scorch is an up-and-coming problem on oaks. In Kentucky there are reports of tree death by this bacterial pathogen on red oak, shingle oak, bur oak, and particularly pin oak. The disease causes marginal leaf burn, early defoliation, and reduced growth of branches. Initial symptoms show in summer on one branch. Each year more branches are affected, and each year new leaves emerge normally only to drop early. Dieback and decline follow until the tree dies. Details about bacterial scorch are found in issue no. 6. We cannot test for this bacterium at the Plant Clinic, so we send our samples to a private lab in Indiana called AGDIA. The lab has a serological test for the bacterium that can be done on young twigs and leaves. There is a fee. Call ahead to be certain you have prepared the correct sample and avoid resampling at your expense. Consult AGDIA at http://www.agdia.com or call them at (219)264-2014 or (800)62-AGDIA. Only one of the oaks that I have sent from the Plant Clinic tested positive for this bacterium. The disease is present in Illinois but has not been confirmed as a common problem at this time.
Oak tatters is another problem we have on oaks and one for which we do not have a known cause. You can learn more about this problem by referring to issue no. 5 and the article on oak skeletonizing. The symptoms are leaves that appear to have been eaten by some voracious pest that did not like to eat the veins. This problem is difficult to miss. A very good fact sheet on the problem can be found by going to the forest service web site http://willow.ncfes.umn.edu/fth_pub.htm. There you will find forest insect and disease leaflets as well as pest alerts. Tatters is found under the pest alerts.
The final problem we see on oaks is our catchall: oak decline. This is quite real as you can read in Sinclair, Lyon, and Johnsonís book, Diseases of Trees and Shrubs, but once again we cannot pinpoint one particular cause for the decline. Look for slow growth; sparse, undersized, discolored leaves; death of small branches throughout the year; branch dieback; and sometimes growth of sprouts along the trunk or major branches. This is the category in which our current red oak problem falls. Drainage, temperature, site stress, root rot, wood rot, and even a few pathogens can be associated with this problem. We have seen this on pin oaks, red oaks, and some shingle oaks. What is the treatment? We donít really know. It is helpful to remove dead wood but only if you do so when sap movement is slow. It may help to water the trees. Fertilization has been suggested but at low levels and in a slow release form. I think you will find a wide range of recommendations in that regard.
Maybe you have read about sudden oak death. This is a decline problem currently raging in California on tanoak, coast live oak, and black oak. It appears that a Phytophthora fungus is associated with that decline. You can find out more at http://phytosphere.com/tanoak.html. There is no evidence of this problem on oaks in Illinois.
Besides all the problems listed, many of you have seen cupping of new growth on white oaks each season. What is the cause of this symptom? Chemicals have been blamed and may be the true cause. Affected trees usually develop healthy, unaffected leaves in the second flush of growth. The problem does not always return but may recur annually.
The point of this article is not to confuse but to list some of the major problems we have seen on oaks the past two decades. The distinction between problems is not always clear, and diagnosis can be very uncertain. Keep an eye on problems with oaks, and take notes of the facts. List species, time of symptom expression, change over time, and diagnosis (if that is certain). It is much easier to help with a diagnosis when we have the facts as opposed to generalities. I hope this helps with your oak problems, and I look forward to looking at your oak samples (and facts) next May when the clinic opens for the year 2001.