Tree Disease Predictions for 2008

October 16, 2007

If I had a crystal ball, I would tell Illinois arborists exactly what tree diseases would be a problem in 2008. I don’t have such a device, so I will have to rely on some educated guesses. It is not difficult to predict that we will see all of the same diseases we see every year; but the question is, which ones will be most intense? I have chosen a few of the more common problems seen at the U of I Plant Clinic, as well as a few that are just plain bothersome.

Ash decline has been a problem in Illinois for at least 20 years. And although many like to blame that decline on Verticillium, none of the 14 Plant Clinic isolations from 2007 ash samples yielded that fungus. Drought, compaction, drainage problems, and herbicides are common factors in ash decline. Decline initially appears as branch tip death and “thinning” of foliage over the entire tree. This decline occurs over many years, with temporary recovery each spring. I expect ash decline to continue to be a major tree disease problem in 2008. Although emerald ash borer threats have greatly reduced planting of new ash trees in Illinois, we still have a great number of ash trees in the state.

There are many river birch (Betula nigra) trees in Illinois landscapes. Although this tree is more disease resistant than most birches, it is sensitive to high-pH soils, often developing chlorosis and other nutrient-stress symptoms. That condition, along with moist weather, predisposes trees to anthracnose infection, resulting in loss of inner leaves. Botryosphaeria and Phomopsis cankers follow and contribute to branch decline and death. Expect problems with chlorosis and cankers in central and southern parts of the state next season, following drought experienced in those areas.

Verticillium wilt is known to be worse on stressed trees. At the Plant Clinic, we have seen most cases on catalpa, magnolia, maple, redbud, smoketree, and fragrant sumac. I don’t imagine that will change in 2008. For these tree species, you might want to consider a little extra TLC to avoid stress and predisposition to Verticillium infection or spread. For areas in drought in 2007, consider watering weekly, now until the first hard frost.

Dutch elm disease is still with us. Healthy elm trees are most likely escapes rather than resistant species. Still, don’t forget to consider other factors when elms are in decline. In 2007, only 11 of 27 elm samples submitted to the Plant Clinic for DED testing proved positive. Site stress may be involved in tree decline; or possibly elm yellows is becoming more prevalent. The Plant Clinic cannot test for elm yellows. Confirmation usually involves extracting DNA from a diseased plant, amplifying a DNA fragment by polymerase chain reaction (PCR), and identifying it. Such procedures are fairly expensive due to labor and equipment costs. Some specialty labs, such as AGDIA, Inc., offer this service. Details can be obtained at www.agdia.com.

All species of maples grown in Illinois are susceptible to Verticillium; and we see a good number of positive confirmations at the Plant Clinic each year. Still, other factors are contributing to maple decline, including deep planting, potato leafhopper stress, soil pH problems, drainage, and compaction. Watch for more maple decline in 2008, as many of these stress factors were common in 2007.

Easily the most common Illinois evergreen disease in the last few years, Diplodia (Sphaeropsis) blight is at the top of my list for 2008. Infection has become so widespread that inoculum is common in most areas of the state, and infection requires only a succulent shoot to be exposed to a wet period of 12 hours when temperatures do not go below 54°F. That happens each spring in Illinois. To make matters worse, drought-stressed trees are more susceptible.

Spruces in Illinois are commonly infected with Cyto-spora canker and Rhizosphaera needle cast. Cytospora is a stress pathogen and will be with us as long as spruces exist. Rhizosphaera infects in wet periods from spring to fall, but needle drop occurs as much as 12 to 18 months later. Because moisture was more abundant in northern Illinois in 2007, I predict Rhizosphaera will be more of a problem there in 2008.

I have left oaks for last because they are now threatened by two fatal diseases. Oak wilt is not hard to predict. It has been a fairly steady disease problem on oaks submitted to the Plant Clinic for the last two decades. Oak wilt infection does not seem to be weather dependent; however, we would not be surprised to see it more frequently on oaks in stress situations. I don’t expect the oak wilt situation to worsen, but even a steady appearance of this disease is not good news.

Bacterial leaf scorch (BLS) is the disease I suspect is more prevalent in Illinois than most imagine, especially on oaks. The symptoms look much like environmental scorch except distribution increases in a tree from year to year, symptoms don’t show until mid to late summer, and temporary recovery occurs each spring. We have found many positive cases around Champaign, but scattered cases show this is a threat elsewhere in the state.

Possibly I have been a bit conservative on the predictions for 2008, but weather and site stress will play a huge role in disease appearance; and I don’t have the aid of a crystal ball. If you would like more information on the diseases mentioned here, consult the Report on Plant Disease series of fact sheets (RPDs) available in Extension offices in Illinois or free on the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm.