With sudden, early-season temperature changes here and there, along with excessive soil moisture across much of the state, it's been a tough environment for many trees and shrubs this year. In the last week or so, the Plant Clinic has received several yew and arborvitae samples showing dieback due to these environmental insults.
As described by Rhonda Ferree in last week's newsletter (issue no. 6) in "Waterlogged Plants", yew is extremely intolerant to even short periods (one to two weeks) of saturated soil. They begin to die back and may even die out completely in some cases. On one yew sample we received, the symptoms included fairly uniform death of the newest growth, while older leaves showed progressive dieback beginning at the leaf tip. On another sample, the dead and dying leaves showed signs of blistering (edema), which develops when the soil is saturated and transpiration is impaired. These symptoms, coupled with the presence of excessive soil moisture, often make the diagnosis pretty straightforward: it's "wet feet."
However, to complicate matters, you may also notice minute, black fruiting bodies embedded in the leaf tissue. This is not uncommon because there are many saprophytes ("weak" fungi) that rapidly colonize and break down the dead or dying tissue. However, in the latter sample we analyzed, we identified the fungus Phomopsis. There are many species of Phomopsis, ranging from aggressive pathogens (e.g., juniper tip blight) to the weak saprophytes. Although yew is considered a host of Phomopsis, the only time we see this association is when the plant is stressed by other factors. It's best to prune out the affected areas (in dry weather) and try to improve the growing conditions by diverting water and possibly changing the soil cover to help the soil air out. In some cases, serious landscape renovation or selection of more water-tolerant plant types may be necessary.
There are many other instances in which you can be tricked into believing "the fungus did it," when the real problem is actually the growing environment. For example, blackened foliage of arborvitae, tan or brown juniper branches, or branches dropped by a silver maple may, upon close examination, reveal fungal fruiting structures. Many times, the real cause of the problem is a harsh winter or suboptimal spring environment (e.g., drying winter winds, rapid temperature swings, saturated soil).
Diagnosing these problems is not easy, particularly when the damage may not become obvious until mid- or late spring. The point is, in addition to analyzing signs and symptoms, we can't forget to consider the environment.