CARTER: ILLINOIS TREES: THEIR DISEASES 51 



eases, field diagnosis of phloem necrosis is based on the color and 

 odor of the inner bark. In an affected tree, the thin layer of inner 

 bark in contact with the sapwood, especially that at the base of 

 the trunk and in the buttress roots, becomes yellow to butter- 

 scotch in color (Fig. 47). Occasionally, dark brown to black flecks 

 are evident in the butterscotch-colored bark. The butterscotch 

 color can be detected only in freshly cut samples of bark, since 

 the inner bark from both diseased and healthy trees turns brown 

 within a few minutes after being exposed to air. The odor of 

 wintergreen emanates from the butterscotch-colored bark of 

 trees affected with phloem necrosis. 



Healthy elms may be protected from infection by spraying 

 with DDT or methoxychlor (Table 1) to control the leaf hopper 

 which carries the phloem necrosis virus from diseased to healthy 

 trees. Trees should be sprayed twice during the growing season 

 to obtain maximum protection. The first spray should be applied 

 as soon as the spring leaf crop is fully mature, usually during 

 June in Illinois. The second spray should be applied immediately 

 after the midsummer or second growth of elm leaves has oc- 

 curred, usually after July 15 in Illinois. Each spray should con- 

 tain 6 per cent insecticide if applied by a mist blower (Fig. 48) 

 or 1 per cent insecticide if applied by a hydraulic sprayer (Fig. 

 49). Either methoxychlor or DDT is effective against leaf hop- 

 pers. DDT has certain disadvantages. It is hazardous to birds 

 (page 27), and its use may result in increased populations of 

 harmful mites and insects. 



The effectiveness of spraying to prevent the spread of 

 phloem necrosis will not be known for a year after the spray pro- 

 gram has been started, as trees are infected with the virus a year 

 or more before they show the disease. No tree already infected 

 with the virus when the sprays are applied will be benefited by 

 the sprays, which protect against infection only during the year 

 or years in which they are applied. For continuous protection, 

 trees must be sprayed each year. Success in preventing phloem 

 necrosis depends on spraying with such thoroughness that no 

 leafhopper bred in a diseased tree is left alive long enough to 

 feed on a healthy tree. 



In areas where both phloem necrosis and Dutch elm disease 

 occur, healthy elms may receive protection from the elm leafhop- 

 per and the European elm bark beetle by being sprayed three 

 times each year. The first spray is a dormant spray recommended 

 for Dutch elm disease control ; the second and third sprays are 



