CARTER: ILLINOIS TREES: THEIR DISEASES 



49 



Phloem Necrosis. — Phloem necrosis, caused by a virus, is a 

 widespread and destructive vascular disease of American elm, in- 

 cluding the Augustine ascending, Moline, vase, and holly-leaf 

 varieties. The virus can infect the winged elm. In 1963 phloem 

 necrosis was known to occur in 15 states; it was present in the 

 southern two-thirds of Illinois and in a few isolated places in 

 the northern third of the state. 



Earliest visible symptoms of phloem necrosis appear usually 

 as drooping or curling of leaves, followed by yellowing and 

 browning of leaves, and finally by defoliation of the affected 

 tree. Most elms that show these leaf symptoms in June and 

 July die in a single growing season. On elms which die within 

 2 or 3 weeks, the leaves do not droop and turn yellow, but wilt 

 rapidly and turn brown ; many remain attached to the branches 



Fig. 47. — The butterscotch color produced in elms affected with phloem 

 necrosis is present in the thin layer of inner bark which is in contact with 

 the wood. When a chip of bark and wood is removed from a diseased elm 

 stem, the butterscotch color appears as a ring where the inner bark is in 

 contact with the underlying wood. 



