Table 7. Elms susceptible to Dutch elm disease.

Sanitation

Remove all elm wood or dead wood that may serve as a breeding place for bark beetles. The infected wood should be destroyed before the bark beetles emerge. The wood can be used as firewood. Intensive sanitation involving disease surveys being conducted in June, July, and August with removal of the diseased trees within 20 working days was the most effective in a Forest Survey Study. The herbicide cacodylic acid (Silvisar 510) should be applied to cuts around the trunk or injected into root flares when the tree is in leaf (Ecological Service Bulletin; no. 6)

Pruning

When pruning trees showing signs of the disease a cut should be made 5-10 feet away from the point at which visual symptoms are present. Pruning can be an effect way in which to stop the spread of the disease; 87% of the trees with greater than 10 linear feet of apparently disease-free wood between pruning cut and discolored wood were save (Ecological Service Bulletin; no. 6).

Disrupt Root Grafts

Treatment should be performed on trees less than 100 feet apart. The easiest way in which to treat the soil around trees that have been infected is to drill a series of holes 1 inch in diameter, 15 inches deep, and 6 to 9 inches apart in a line that extends from the diseased tree to any healthy elms surrounding it within the 100 foot radius. Fill these holes with 1 part SMDC (Vapam) and 3 parts water to within 2 inches of the soil surface. This treatment should be done when soil temperatures are above 50 °F and within 2 weeks before the diseased elm is removed.

Another alternative to disrupt root grafts is digging a trench 36-40 inches deep between the trees. The idea is that the majority of sizeable roots will be severed at these depths (Ecological Service Bulletin; no 6).

Insect Control

Insecticides

DDT was used after World War II as the most effective eradication method of Dutch elm disease (Wurster and Wurster, 1965), however, today there a number of other insecticides which can be used after the prohibition of DDT in 1968.The most commonly used insecticide is Methoxychlor which is applied as an emulsifiable concentrate with either a mist blower or hydraulic sprayer. The application should take place in late winter or early spring when the temperature is above 32 °F and wind speeds are less than 5 MPH. The coverage of the trees determines how much of an affect the insecticide will have on the elm trees (Ecological Service Bulletin; no. 6)

Fungicides

Two types of fungicides used are Propiconazole (Alamo) and thiabendazole (Arbotect 20S). These fungicides are injected into the tree trunk or flare roots. They can be used for preventative treatments with injections taking place every 2 to 3 years in the spring after the tree has leafed (Ascerno and Wawrynski). To re-inject drill holes that are two to three inches from the original holes and then proceed to inject with the fungicide. If the tree has already contracted Dutch elm disease it is only recommended to use these treatments if no more than 5% to 10% of the crown is wilted and only if the sysmptoms appear after July 1 st (Stennes et. Al. 1984).

Resistant Elms

There are a few hybrids that are on the market, however, the following is a list of elms that are resistant to Dutch elm disease.

Cultivars

The best way to ensure that your trees will not contract Dutch elm disease is to not plant susceptible elm trees. The following is a list of different cultivar species.