Dutch Elm Disease 451 D

Dutch Elm Disease 451 D. E. PARKER is an assistant division he worked on biological control of leader of the Division of Forest Insect forest insects y he began studying the Investigationsj Bureau of Entomology relation of insects to tree diseases, par- and Plant Quarantine. A graduate of ticularly Dutch elm disease and elm the University of Massachusetts, Mr, phloem necrosis. In this connection Parker joined the Department in 1925. Mr. Parker spent three years studying After 9 years in Massachusetts^ where the Dutch elm disease in Europe,

DUTCH ELM DISEASE R. U. SWINGLE, R. R. WHITTEN, E. G. BREWER

The Dutch elm disease is caused of the elm produce similar symptoms, by the fungus Ceratostomella ulmi. positive identification of the Dutch The disease was discovered in the elm disease depends upon laboratory Netherlands 30 years ago and it spread tests that involve identification of the rapidly in Europe. It was found in the fungus that may grow from the dis- United States in 1930; it had been colored wood. Without these tests, brought here in elm burl logs imported the Dutch elm disease cannot be dis- for the veneer industry. tinguished with certainty from other Native elms of the United States are wilt diseases of elm. A laboratory to dangerously susceptible to the fungus. which specimens may be sent for Despite vigorous efforts to suppress it, identification of Dutch elm disease is the disease has become established in maintained by the Bureau of Ento- plantations and natural stands of the mology and Plant Quarantine of the principal elm shade-tree areas of this Department of Agriculture. country from Boston as far westward as Ceratostomella ulmi develops in liv- Indiana and Kentucky and southward ing trees as a parasite and in dead elm to Virginia. It has been found in Ten- wood as a saprophyte. In living trees, nessee. An isolated outbreak was dis- the fungus occurs in water-conducting covered in Colorado. vessels of the wood. It produces yeast- like spores that are carried through DUTCH ELM DISEASE produces a wilt- these vessels in the flow of sap. The ing or yellowing of leaves on one or toxins the fungus produces and the several branches. Thereupon the leaves brow^n, gumlike deposits in the water- fall. Later in the season or in follow- conducting vessels cause wilt and the ing years, the disease may spread to death of the tree or its affected other parts of the tree until the entire branches. After its host dies, the fungus, top is affected and the tree dies. In still growing on the wood as a sapro- more acute cases, the entire tree may phyte, produces spores under the loos- suddenly wilt and die with or without ened bark and in insect galleries pronounced yellowing of foliage. In formed between the bark and wood. all cases of Dutch elm disease, a dis- coloration of the sapwood occurs in OCCASIONALLY THE FUNGUS spreads affected branches, trunk, and roots. If through linkage of diseased and Dutch elm disease is present, a diag- healthy trees by natural root grafts, onal cut through branches with wilted which frequently occur in dense elm or yellowing leaves will show brown stands and crowded street plantings. spots, an arc, or a complete brown Normally, though, the fungus is borne circle in one or more annual rings of from diseased trees to healthy trees by the wood. tw^o kinds of bark beetles, the native Because two other common diseases elm bark beetle, Hylurgopinus rufipes 452 Yearbook of Agriculture 1949 (Eichh.)j and the smaller European perior types of elms that are resistant elm bark beetle, Scolytus multistriatus to the fungus. Scientists in Europe dis- (Marsh.). The latter is the more im- covered the Christine Buisman elm, a portant. The beetles, widespread in the selection of Ulmus carpinifolia, which eastern half of the United States, are has proved highly resistant in both present in many places where the Dutch Europe and America. After inoculat- elm disease is not yet known to occur— ing thousands of American elm seed- an ominous warning. lings, American scientists have found The adult beetles feed in parts of two resistant American elms. Other living elm trees, but they breed only in recent selections and hybrids seem recently cut, dead, or dying elms. Liv- promising, and elms that combine re- ing elm trees are seldom injured by sistance to the disease and high quality only the feeding of the adult, but when should be available soon through com- the beetles arc contaminated with the mercial nurseries. disease organism they become of eco- Because the Dutch elm disease or- nomic importance. When the Dutch ganism is spread by insects, the loss of elm disease fungus occurs in elm ma- elm trees from this disease can be terial in which these insects breed, the prevented by controlling the insect car- fungus may stick to the beetles and be riers. This can be done by the preven- carried to healthy elms or other breed- tion of breeding in recently cut, dead, ing material. or dying elm trees and by the preven- The adults of the smaller European tion of feeding on living elm trees. elm bark beetle emerge from infested Breeding may be prevented by burn- wood and fly to nearby living elm trees, ing or spraying ail infested or likely- where they feed in the smaller twig to-be infested elm wood. If a spray is crotches. The adults of the native elm to be used, the entire bark surface must bark beetle hibernate in the outer bark be thoroughly covered with No. 2 fuel of living elm trees. In the spring they oil containing 1 percent of DDT. This bore into the bark and feed on it. When spray is for dead material only, because the feeding injuries penetrate through it will injure living trees. the bark to the wood, the disease organ- Feeding by bark beetles in living ism may be introduced into the vascu- trees can be controlled by completely lar system of healthy elm trees. Beetles covering the bark surface with an of both species may fly several miles emulsion-type spray containing 2 per- in search of suitable breeding places, cent of DDT. Such sprays have pro- and thus may transport the disease or- duced residues that remained effective ganism from one locality to another. for more than 3 months. This method All the elms commonly planted as of control can be applied to individual shade trees are susceptible to Dutch trees. Further experimentation with elm disease. The degree of suscepti- these DDT sprays is necessary before bility varies both within and between we can make recommendations for species, however. The American elm, their general use. which predominates in many shade- tree plantings, is among the more sus- R. U. SWINGLE is a senior patholo- ceptible species. The Chinese elm gist ill the Division of Forest Pathologyj {Ulmus parvifoHa) and the Siberian Bureau of Plant 'Industry^ Soils^ and elm {Ulmus pumila) resist Dutch elm Agricultural Engineering. disease, but they have undesirable R. R. WHITTEN is a senior entomolo- characteristics that limit their use in gist in the Division of Forest Insect shade-tree plantings. Investigations, Bureau of Entomology The variation in susceptibility of dif- and Plant Quarantiyie. ferent species and varieties has stimu- E. G. BREWER is in charge of Dutch lated attempts in Europe and the elm disease control, Bureau of Ento- United States to breed and select su- mology and Plant Quarantine.