DUTCH ELM DISEASE

R. U. SWINGLE, R. R. WHITTEN, E. G. BREWER.

The Dutch elm disease is caused by the fungus Ceratostomella ulmi. The disease was discovered in the Netherlands 30 years ago and it spread rapidly in Europe. It was found in the United States in 1930; it had been brought here in elm burl logs imported for the veneer industry.

Native elms of the United States are dangerously susceptible to the fungus. Despite vigorous efforts to suppress it, the disease has become established in plantations and natural stands of the principal elm shade-tree areas of this country from Boston as far westward as Indiana and Kentucky and southward to Virginia. It has been found in Tennessee. An isolated outbreak was discovered in Colorado.

DUTCH ELM DISEASE produces a wilting or yellowing of leaves on one or several branches. Thereupon the leaves fall. Later in the season or in following years, the disease may spread to other parts of the tree until the entire top is affected and the tree dies. In more acute cases, the entire tree may suddenly wilt and die with or without pronounced yellowing of foliage. In all cases of Dutch elm disease, a discoloration of the sapwood occurs in affected branches, trunk, and roots. If Dutch elm disease is present, a diagonal cut through branches with wilted or yellowing leaves will show brown spots, an arc, or a complete brown circle in one or more annual rings of the wood.

Because two other common diseases of the elm produce similar symptoms, positive identification of the Dutch elm disease depends upon laboratory tests that involve identification of the fungus that may grow from the discolored wood. Without these tests, the Dutch elm disease cannot be distinguished with certainty from other wilt diseases of elm. A laboratory to which specimens may be sent for identification of Dutch elm disease is maintained by the Bureau of Entomology and Plant Quarantine of the Department of Agriculture.

Ceratostomella ulmi develops in living trees as a parasite and in dead elm wood as a saprophyte. In living trees, the fungus occurs in water-conducting vessels of the wood. It produces yeast-like spores that are carried through these vessels in the flow of sap. The toxins the fungus produces and the brown, gumlike deposits in the water-conducting vessels cause wilt and the death of the tree or its affected branches. After its host dies, the fungus, still growing on the wood as a saprophyte, produces spores under the loosened bark and in insect galleries formed between the bark and wood.

OCCASIONALLY THE FUNGUS spreads through linkage of diseased and healthy trees by natural root grafts, which frequently occur in dense elm stands and crowded street plantings. Normally, though, the fungus is borne from diseased trees to healthy trees by two kinds of bark beetles, the native elm bark beetle, Hylurgopinus rufipes (Eichh.), and the smaller European elm bark beetle, Scolytus multistriatus (Marsh.). The latter is the more important. The beetles, widespread in the eastern half of the United States, are present in many places where the Dutch elm disease is not yet known to occur an ominous warning.

The adult beetles feed in parts of living elm trees, but they breed only in recently cut, dead, or dying elms. Living elm trees are seldom injured by only the feeding of the adult, but when the beetles are contaminated with the disease organism they become of economic importance. When the Dutch elm disease fungus occurs in elm material in which these insects breed, the fungus may stick to the beetles and be carried to healthy elms or other breeding material.

The adults of the smaller European elm bark beetle emerge from infested wood and fly to nearby living elm trees, where they feed in the smaller twig crotches. The adults of the native elm bark beetle hibernate in the outer bark of living elm trees. In the spring they bore into the bark and feed on it. When the feeding injuries penetrate through the bark to the wood, the disease organism may be introduced into the vascular system of healthy elm trees. Beetles of both species may fly several miles in search of suitable breeding places, and thus may transport the disease organism from one locality to another.

All the elms commonly planted as shade trees are susceptible to Dutch elm disease. The degree of susceptibility varies both within and between species, however. The American elm, which predominates in many shade-tree plantings, is among the more susceptible species. The Chinese elm (Ulmus parvifolia) and the Siberian elm (Ulmus pumila) resist Dutch elm disease, but they have undesirable characteristics that limit their use in shade-tree plantings.

The variation in susceptibility of different species and varieties has stimulated attempts in Europe and the United States to breed and select superior types of elms that are resistant to the fungus. Scientists in Europe discovered the Christine Buisman elm, a selection of Ulmus carpinifolia, which has proved highly resistant in both Europe and America. After inoculating thousands of American elm seedlings, American scientists have found two resistant American elms. Other recent selections and hybrids seem promising, and elms that combine resistance to the disease and high quality should be available soon through commercial nurseries.

Because the Dutch elm disease organism is spread by insects, the loss of elm trees from this disease can be prevented by controlling the insect carriers. This can be done by the prevention of breeding in recently cut, dead, or dying elm trees and by the prevention of feeding on living elm trees.

Breeding may be prevented by burning or spraying all infested or likely-to-be infested elm wood. If a spray is to be used, the entire bark surface must be thoroughly covered with No. 2 fuel oil containing 1 percent of DDT. This spray is for dead material only, because it will injure living trees.

Feeding by bark beetles in living trees can be controlled by completely covering the bark surface with an emulsion-type spray containing 2 percent of DDT. Such sprays have produced residues that remained effective for more than 3 months. This method of control can be applied to individual trees. Further experimentation with these DDT sprays is necessary before we can make recommendations for their general use.

R. U. SWINGLE is a senior pathologist in the Division of Forest Pathology, Bureau of Plant Industry, Soils, and Agricultural Engineering.

R. R. WHITTEN is a senior entomologist in the Division of Forest Insect Investigations, Bureau of Entomology and Plant Quarantine.

E. G. BREWER is in charge of Dutch elm disease control, Bureau of Entomology and Plant Quarantine.