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. 



