Issue 5, May 22, 2009

Stressed Yews

A few samples of stressed yews have arrived at the Plant Clinic this season. Callers also have been concerned about their yews. Clients complain that they are seeing branch tip dieback, often scattered throughout the established plants.

There are a few disease problems that affect yews in Illinois, but in most cases I would have to put yews in the category of "not bothered by serious disease problems." The three fungi that you may find reported on stems of this host include Botryosphaeria, Pestalotiopsis, and Phomopsis. In all cases, however, the fungi are reported as opportunistic on dead or dying tissue. They do not cause the tissue to decline. They follow the decline. One of our cases this year was infected with Phomopsis dieback. Sprays are available, but only as preventives as the new growth emerges. Phytophthora root rot can be a problem too. It will appear on plants that are growing in very wet or poorly drained locations. On the other hand, yews are known to decline in wet situations. As the literature states, "they will not tolerate wet feet." Obviously this means wet roots. Except for one wet nursery situation, I have not seen Phytophthora root rot on yews.

It is very likely that tip dieback on yews at this time of year is the result of cold injury or very wet roots. New growth is most susceptible to cold injury as seen in the first image. Look for this problem where warm temperatures allowed plants to come out of winter dormancy and then a cold snap followed. Usually such injury affects new growth and not older tissues. Removing dead growth is the most you can do to help the plant.

If older growth seems to be affected as well, then wet soils are more likely. The second image shows injury on a plant in very wet soil. Noninfectious problems such as these are often difficult to prove. Eliminating other factors is helpful. I suggest that you pull soil away from the trunk (using gloved hands only) and look for girdling roots or other obstructions just below the soil. Many times deep planting is also a factor.

Do not be quick to use fungicides to remedy these situations. It is very likely that a fungal pathogen is not to blame.--Nancy Pataky

Nancy Pataky

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