Denver's Dutch elm disease program
The Forestry Division of the Denver Parks and Recreation is aiming at reducing losses of elms to Dutch elm disease to a minimum. Their strategy is a three-pronged Dutch elm disease program, consisting of a sanitation and plant health care program, experimentation with fungicides and implementation of immune system augmentation. The goal of their experimentation is to establish whether or not fungicides and immune system agents can provide long term Dutch elm disease protection, without harming the trees. This program resulted in an encouraging decline in Dutch elm disease losses, up to the point where they have now reached the lowest number of losses since the onset of the disease.
Sanitation of diseased trees
The sanitation component consists of early detection of the disease and quick removal of beetle-infested and fungus infested wood. Accumulation of dead wood in the canopy is prevented with 6 year pruning cycles and diseased trees are removed within 5-10 days of sighting the infection.
The Plant Health Care program
Through the plant health care program the tree's vigour is maintained, assisting the tree to fend of beetle attacks (carrying the deadly Dutch elm disease fungus), because beetles prefer weakened trees. The health care program comprises control of the European elm bark scale by means of pesticide injections and fertilization. The combined result of the sanitation and health care program was a 40% decline in the number of Dutch elm disease infected park and parkway trees.
Dutch Trig® treatments
In Denver, the first Dutch Trig® injections took place in 1998, involving 127 elms. These were generally trees coming up for a renewal of the Arbotect injection, but instead were put on the yearly injection scheme with the biological control agent Dutch Trig®.
The injections were repeated and the injection program was expanded in 1999 and continued up to 2003. The yearly Dutch Trig® treated group comprised 645 trees, of which 609 are located in parks and along streets across Denver, under control of the Department of Parks and Recreation, and the remaining 33 trees are on privately owned lands.
The Dutch elm disease program results
The graph shows the disease incidence of both the overall group of city elms (approximately 1800 street and parkway elms) and the Dutch Trig® treated group. The initial high disease incidence in the first year of application is visible in Denver. Also the steep decline in disease incidence in the Dutch Trig® treated group in the subsequent years of treatment is clearly present. In the second year of Dutch Trig® treatments the disease incidence is only halve of that of the total group of elms under care of the city. In Denver the reduction of the overall disease incidence can be explained by the combined effect of the sanitation and plant health care program and the low disease incidence in the Dutch Trig® treated group, comprising approximately 1/3's of the elm population under care of the Forestry Division.
Unexplained wilting symptoms
In 1999, seventeen trees showed prominent wilting symptoms, described as follows: "The trunk suckers did wilt up to a height of approximately 15 feet. The leaves droop and fold inward. It appears they are not conducting water well. This happens within 14 days of injection. Then, 2 weeks later, they refoliate." The cause of this wilting is unknown at this time. The wilting could be caused by a combination of tree health, site conditions, weather, the tree's histological characteristics and the injection. To investigate this further, the injection program was continued in 2000. Again over 600 trees were treated in Denver in 2000. None of the symptoms of wilting reoccurred on any of the treated trees.
The choice of the City: to protect their elms from Dutch elm disease
The data provided above indicate that the efforts of the Forestry Division to minimize elm losses due to Dutch elm disease have lead to the fact that in the city of Denver the Dutch elm disease plague no longer "roams free" to ransack large amounts of elms. Even though these measures initially costs money and effort, the reduction of removal costs and the preservation of the city's elm's and especially Denver's parkways make up for the necessary input. The taken sanitation and preventative measures enable the city to get a grip on their Dutch elm disease problem and preserve it's green character in an environmentally friendly way. For the City of Denver it was their choice to do more than stand aside and let 'nature' take it's course, reducing their elm population and destroying the city's green image.