I'm fortunate to work where I get a day off now and then for "continuing education" as a certified arborist. The most recent one-day class I attended included some good information on Raywood ash, an extremely popular tree variety that has fallen on hard times.
The speaker was Prof. Tom Gordon of UC Davis, and part of his presentation was about a severe dieback disease that affects Raywood ash, so much so that some municipalities and designers are no longer planting them.
Raywood ash is one of those mainstay cultivated varieties landscape architects and wholesale growers love for its rapid growth, symmetrical canopy shape, and ease of maintenance. Also known as claret ash it has wine-red fall color, and to make maintenance easier, it produces no seeds. Sounds great, doesn't it?
So, after a number of years of extreme popularity, being planted in every conceivable situation from parks and gardens to cramped cut-outs in parking lots, Raywood ash trees began to show dieback ranging from minor to severe. For quite a while extensive lab analysis came up negative, and the dieback was considered "mysterious."
Dr. Gordon related in his talk that investigation as to the cause of the dieback included 2,000 trees in 14 geographical areas around Northern California. After extensive efforts they have isolated a fungal disease, Botryosphaeria stevensii (also known as Dipolida mutila at a different stage of its life cycle). It is believed to be the organism involved in the dieback.
Botryosphaeria is well known among gardeners and arborists. One species commonly kills branches and disfigures giant Sequoias here in Napa and much of low-land California. So why the mystery? One reason is that this species produces tiny fruiting structures, an identifying feature apparently missed in some of the lab work.
Dr. Gordon reported that there was no obvious geographical correlation in the dieback i.e., no particular regional climate seemed to be involved.
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Botryosphaeria is known to begin most readily in stressed trees and plants, so they measured stem water potential, expecting to correlate water stress with the disease. But the results were not uniform. Currently Dr. Gordon favors the conclusion that affected trees in general were predisposed to infection as a result of drought stress.
I had been following the ongoing dieback problem locally for several years. In my experience I would agree, it appears to be trees that have been drought stressed that show the dieback. Good examples can be found in cramped cut outs in parking lots and curb strips around town.
I checked with Rob Hansen, the city of Napa Tree Supervisor, and found that Raywood ash was taken off the list of city-approved street trees several years ago, and is only used occasionally on city right-of-ways.
Hansen went on to explain that the tree has had several problems. About six trees by City Hall had to be removed in 2003 and 2004 because of the dieback disease. Beyond that, he noted that some Raywoods failed for lack of anchorage when planted in compacted clay soil in root barrier boxes. (The root barrier boxes are no longer used by the city.) Other problems have included whitefly infestation, (which is now generally minor because of good, biological control by established parasitoids) scale infestations and splitting.
The splitting of trunks and branches is common in Raywood ash for lack of simple pruning in the nursery and early on in the landscape, to remove crowded and weakly attached branches.
When you take a closer look at the problems association with Raywood ash it's clear that, too often, they just haven't been given a decent home, adequate water and a little training early in their lives. So, what's so surprising or mysterious about that?
A final invitation: I'll be speaking at the Napa City-County Library, Community Meeting Room, 580 Coombs Street, Napa, 7 to 8:30 p.m., Wednesday: Living With Trees in Napa Valley. Please call the library at 253-4235 to make an advanced reservation.
Bill Pramuk is a registered consulting arborist. He works for Britton Tree Services, Inc. Please e-mail questions to Pramuk@napanet.net.