Winter, 1997 Newsletter

Elm Trees and the Dutch Elm Disease

Dutch Elm Disease (DED) is a vascular wilt disease caused by a fungus, 2 strains of which are known. The fungus is transmitted by 2 species of bark beetle, and by natural root grafting between healthy and infected trees. DED symptoms are known as "flagging": wilting, curling and yellowing of leaves and shoot tips. Elms showing flagging may die during the season, or slowly over a year or more.

Dutch Elm Disease was first reported in France in 1919. At that time it was found in most of Belgium, the Netherlands, and northern France. Despite its devastation, only the Dutch started scientific investigation of the disease. The work of seven Dutch female scientists laid the foundation for our knowledge of DED. In the early 1920's, Dr. Marie Schmary was the first to hypothesize that the disease originated from a fungus. The cause remained controversial until 1927, when Dr. Christine Buisman reported her skillfully designed experiments confirming that the fungus, Ceracystis ulmus, transmitted by the European elm bark beetle, caused the disease. Research seeking DED resistance started in 1928 in the Netherlands and Germany, and continues today.

A DED epidemic spread throughout Europe between 1919 and 1939. By the early 1960's, selections of resistant tree strains, bark beetle control, chemical treatment and improved management practices had greatly reduced the threat of the disease. In the Netherlands, the disease had nearly disappeared and damage in Great Britain had slowed greatly.

The late 1960's, however, brought a new outbreak in Great Britain, which quickly spread throughout much of western Europe. During the 1970's, a more aggressive strain of the fungus was discovered. This new strain killed elms that had previously been resistant. This outbreak is believed to have come from fungus on logs imported from Canada and it is estimated to have killed 70% of the elms in southern England by the late '70s (over 15 million trees).

DED was introduced into the United States in the early 1930's and outbreaks were reported between 1930 and 1933 in Connecticut, Ohio, New Jersey and Maryland. By 1966 it was present in 30 states and 3 Canadian provinces, and by the late 1960's most of the elms had been destroyed. Dutch Elm Disease is assumed to have come to the United States from European logs imported for veneer. A pattern of spread from eastern ports along rail lines to the midwest has been shown. DED is now in all 50 states and provinces where elms are either native or introduced.

The U.S. first responded to the outbreak of DED with costly but futile eradication programs. Since World War II, scientific research has focused more on the disease, with more productive results. Hybrids between the American elm (which is highly susceptible to DED) and the Siberian and Chinese elms (which are fairly resistant) are readily available; these hybrids are fertile, highly DED resistant, very useful and hardy. They do, however, lack many of the desired traits of the American elm.

U.S. breeding and selection research has emphasized disease resistance and retention of the distinctive shape, rapid growth and tall crown of the American elm (Ulmus americana). The Elm Research Institute of Harrisville, New Hampshire and the United States National Arboretum are sources of information about several promising resistant cultivars of the American elm. Reports of applying genetic engineering techniques to seek DED resistance are also starting to appear in the scientific literature.

Davis has not had a serious loss to Dutch Elm Disease, since it lacks any concentration of highly susceptible elms. Sacramento, however, has one of the few remaining large populations of American elms in the United States and it supports an active DED control program.

In closing, planting many elms, even resistant cultivars, would seem unwise, as concentration invites infection and control measures are costly. It is important to remember the fickle nature of fungi -- this pathogen has “changed” once and destroyed trees many bred many years ago to be DED resistant.

For anyone who desires, a more comprehensive technical review with literature references is available from TREE Davis.

(Note from the Editor: TREE Davis will be planting a handful of DED resistant cultivars as a part of a grant from California ReLeaf and the California Department of Forestry. These trees will be planted on the school grounds of Caesar Chavez Elementary School. For those who would like to see an example of these trees, there is a young liberty elm (Ulmus americana "Liberty") growing in Central Park, which was donated to the City by Bob Laben and his wife Dorothy.)

Bob Laben is a professor emeritus at UC Davis and a TREE Davis board member.

Tree of the Month

Have you noticed a beautiful tree on the northwest corner of 8th and “L” Streets? At this time of year it is particularly noticeable because of dark pink pods. According to one of my very favorite plant experts, Warren Roberts, superintendent and acting director of the UC Davis Arboretum, it is a lovely example of the Formosan flame tree, or Koelreuteria elegans.

Warren was recently featured in the Cal Life section of the Sacramento Bee and particular note was made of his enthusiasm for this tree. I remembered then that I had heard him touting the fine attributes of this tree years ago -- maybe even when I worked for him as a student ... WAY back!

You may be more familiar with one of its better known relatives, the Chinese flame tree (K. bipinnata) or the goldenrain tree (K. paniculata). Sunset’s Western Garden Book certainly dismisses the Formosan flame tree as not worth more than a passing mention. And this in fact may be one reason it is not more widely known or used.

So, why does Mr. Roberts prefer K. elegans (also known as K. formosona or K. henryi) to its better known relatives? Simply stated, he told me that to compare it to the others was like comparing a $100 bill to a $1 bill. I don’t think I need to know more than that!

On the other hand, it did seem handy to know just what characteristics make it so appealing, and so I asked for specifics. Evidently, it is much showier than its relatives. In September, an abundance of yellow flowers emerge for our enjoyment. They are followed by the appearance of coral-red pods which remain until the first frost. The leaves turn yellow in the fall, which further sets off the Japanese lantern-shaped pods. And then, with the frost, the leaves and pods all drop, instead of holding on to the dead, brown pods like the goldenrain.

Warren mentioned that while being more tropical looking than the other species, the Formosan flame actually appears to be more cold-hardy. They are also well suited in size for many residential lots, reaching a mature height of about 30 feet, with a rounded or flat crown. Their large, compound leaves are more leathery than the others, which means that they do not shred or tatter as easily in high winds.

Because of its deep-rooted nature, it is possible to garden under these trees, which is definitely an advantage in a residential setting. Another positive attribute is that when the large leaves drop, lots of sunlight is let in because there is not a lot of twiggy growth left in the tree, as is the case with other types of deciduous trees.

According to Warren, the most colorful examples of these trees come from the mountains of Taiwan. However, since we are not getting too many direct shipments coming in from that part of the world (!), your best bet will be to watch for them at the annual Friends of the Arboretum plant sale that happens the first Saturday in October. Due to this year’s long, warm fall, the trees have had a chance to set seed and “the Friends” are busily out gathering. This is fortunate, because there does not seem to be much available in the nursery trade -- though, with Warren’s continued enthusiasm and that new crop of seed, maybe we will see them available in a couple of years.

Rebecca Ferrini is the vice president of JEG Enterprises and is the treasurer of TREE Davis.

“Living” Christmas Trees

This article was reprinted with permission from the November/December 1997 issue of Arbor Day, the official publication of the National Arbor Day Foundation.

Few would disagree that the beauty and spirit of Christmas are enhanced by a live Christmas tree in the home. But is cutting a live tree the best thing to do for our beleaguered environment? “Yes”, say the men and women who make a living growing Christmas trees for the annual harvest. Here is the message they are putting on tags attached to the trees in some parts of the country: “This is a real Christmas tree, intentionally grown for your enjoyment. During its growing years, this tree cleansed the air, provided you with oxygen, protected your water supply, made a home for wildlife and beautified some previously barren land. For each Christmas tree harvested, two or three are planted to continue this cycle.”

Even after Christmas, if your tree is recycled, it keeps on contributing to the environment. In many parts of the country recycled trees are chipped and used as mulch, which helps trees grow without weed competition, conserves moisture and protects the tree from mower damage. Other recycled trees plug eroding gullies, form protective piles for small wildlife or are tied to rocks and sunk into lakes and streams to improve habitat for fish.

(Note from the Editor: Davis Waste Removal will recycle your Christmas tree along with the brush clippings left curb-side. All they ask is that you cut the tree into 6 foot sections and leave it on the side of the street for collection. The trees will then be mulched and made available to the public in the form of organic compost at the Community Gardens in early spring.)