Ash Dieback...how much of a problem is it?

Ash is a popular shade tree in North America used for parks, golf courses, large gardens, and street planting. The genus Fraxinus is very diverse with many native species and cultivars available. They handle the city environment well (pollution, glare, reflected heat) and are less likely to sun scald than, say, linden or red maple. These trees have a wide tolerance for soil extremes. White ash (F. americana) are mostly tolerant of poor soils such as bad drainage and alkalinity, and green ash (F. pennsylvanica) grows well on consistently dry soils. These trees are attractive in all seasons and are truly beautiful in fall color. Although vigorous trees do not develop that many problems, there is some concern about "ash decline" which results in shoot dieback for no apparent reason.

Dieback is observed in several cultivars of white ash, green ash and in F. oxycarpa 'Raywood'. It shows up anytime of year and can affect much of the tree canopy. Various shoot symptoms makes it difficult to diagnosis the cause of the dieback. Ash yellows, a virus-like disease, is one cause of "ash decline" but has not been found in the Northwest. Three distinct categories of ash dieback are observed in the Northwest: 1) early summer - nonuniform distortion and stunting, 2) summer/fall - nonuniform leaf scorch and dieback, and 3) winter - uniform dieback of uppermost shoots. While ash dieback is a general term, the plant damage symptoms caused by living factors and nonliving factors are well-defined.

Eriophyid Mite

Early summer symptoms first appear in June and continued through the summer. Damaged shoots first showed symptoms of leaf distortion and chlorosis (yellowing). Then, stem internode length decreased followed by stem/leaf necrosis, which eventually caused shoot dieback. Eriophyid mites are consistently found on green ash trees with this type of early summer dieback. Trees that were sprayed (bifenthrin + 1% oil, carbaryl + 1% oil, endosulfan) when mite damage was first observed had no dieback and resumed normal growth.

Verticillium Wilt

Until recently, Verticillium wilt was seldom recognized in ash. Following the first "real" heat spell of summer, symptoms of leaf scorch and premature defoliation can been found on green, white, and 'Raywood' ash. Leaves on internal branches show progressive stages of scorch, followed by some branch dieback. Tree infections can become more severe the following year, due to spread of the disease or reinfection from the soil. In some cases, the fungus may be compartmentalized after a growing season, resulting in a chronic infection or the remission of symptoms. Tree death is normally slow, and usually requires more than one growing season, especially for large trees. V. dahliae survives in the soil as small masses of thick-walled fungal strands called microsclerotia which can survive in the soil for very long time. Susceptible trees should not be planted in soils known to be infested with Verticillium. No chemical treatments are effective once a tree becomes infected, but it can respond favorably to improved irrigation practices and fertilizer applications.

Winter Dieback

Dieback that occurs during the winter dormant season effects overwintering buds and shoots of last season's growth. Usually, this type of dieback has a uniform pattern, which suggests that a nonliving factor is responsible. To help understand whether winter dieback is due to low temperature injury, freeze tests were conducted in western Oregon during the 1994-1995 and 1995-1996 winters. Results of the freeze tests indicated that ash were more freeze tolerant than the temperatures experienced in the field. 'Raywood' was slower to cold acclimate in the fall and did not become as cold hardy in midwinter as F. pennsylvanica 'Summit' and F. americana 'Autumn Purple'. Cold acclimation and midwinter hardiness of 'Summit' and 'Autumn Purple' was similar; however, 'Summit' deacclimated more rapidly. It is of interest to note the levels of hardiness developed by 'Autumn Purple' and 'Summit' trees growing in Oregon's relatively mild winter climate. They are rated to hardiness Zone 3 which has a minimum temperature of -35F. Early December freeze tests showed the stem cold hardiness levels of both these species were close to their hardiness zone rating despite being grown in Oregon. There is some evidence that irrigation practices and/or soil fertility could cause this type of shoot dieback. In many cases, nursery fields that experienced winter dieback, had irrigation withheld during the second half of the growing season. Currently, a study is underway to investigate these factors.

In conclusion, Ash has proven itself, over a long period of time, to be a worthy tree for street and city applications. Wether the problems that are being observed now are new or on the increase is difficult to say. Often, once a problem has been noticed, we become more observant and look around a little better. This can sometimes lead us to believe that the problem is increasing. What is more likely is that these symptoms have been around for some time and largely ignored. The fact that there are a number of different causes helps allay the fears of some kind of epidemic in the Ash population. Also, Ash Yellows, the virus that has caused so many problems further east, is not implicated here. Should we continue to use Ash in city plantings? Absolutely yes! Until we have more information regarding the extent of these problems, their causes and ways of counteracting them, Ash still has a lot going for it.