Dutch elm disease in Illinois dutchelmdiseasei53cart Year: 1967 12 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 53 by the first brood, starts emerging in mid-July and continues to emerge until late September. Peak emergence occurs in mid-August. Eggs laid by this second brood develop into larvae (Fig. 6 and 12) which overwinter in the bark. These larvae burrow into the inner bark of infested elm material, where they are well protected during the dormant season. The fungus persists from one year to another in the infested material. big. in. Tliis enlarged section of an egg gallery shows young
Dutch elm disease in Illinois dutchelmdiseasei53cart Year: 1967 12 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 53 by the first brood, starts emerging in mid-July and continues to emerge until late September. Peak emergence occurs in mid-August. Eggs laid by this second brood develop into larvae (Fig. 6 and 12) which overwinter in the bark. These larvae burrow into the inner bark of infested elm material, where they are well protected during the dormant season. The fungus persists from one year to another in the infested material. big. in. Tliis enlarged section of an egg gallery shows young larvae or grubs along the edges. Root Graft Transmission The fungus spreads through grafted roots between diseased and healthy trees. Natural grafting of roots occurs between elms growing near one another (within 35 feet in the case of large trees). The amount of annual infection through grafted roots varies in different communities depending on the spacing of the elms. SUSCEPTIBILITY OF DIFFERENT SPECIES No species or variety of elm is known to be immune to Dutch elm disease. Trees in the related genera of Planera and Zelkova have become diseased when inoculated with the Dutch elm disease fungus. It is unfortunate that the American elm, a longtime favorite for both shade and ornamental use, is the most susceptible of all elms. Al- though Chinese and Siberian elms are highly resistant to the disease, trees of these species have succumbed to natural infection in Illinois. Several hybrid elms reported as crosses between Asiatic and American species have been advertised as immune or resistant to Dutch elm disease. The hybrid elms of this type which were tested by Natural History Survey scientists were not immune, although they did show resistance somewhat comparable to that of the Si- berian elm. Other resistant elms include the Christine Buisman and Bea Schwarz seedling selections of the smooth-leaved elm.
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