74 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



Control of oak wilt in localized areas has been obtained by 

 the poisoning or removing of healthy oaks adjacent to diseased 

 trees or by trenching between diseased and healthy trees. These 

 treatments interrupt an extensive underground system of inter- 

 grafted roots (Fig. 70) through which spread of the fungus from 

 tree to tree might take place. Removal of the first wilting 



Fig. 70. — The oak wilt fungus can pass from a diseased tree to a healthy 

 tree through grafted roots. Grafting occurs frequently between roots of 

 trees that are within 50 feet of each other, especially trees in the red oak 

 group. 



branches in trees of the white oak group may keep the trees 

 healthy for a few years, but these trees are subject to reinfection. 

 Effective control measures to prevent the spread of oak wilt over 

 long distances have not been developed. Squirrels and several 

 kinds of sap-feeding insects are attracted to the mats of the oak 

 wilt fungus which are produced beneath the bark of wilt-killed 

 oaks, and it has been shown experimentally that these animals 

 and insects can carry the oak wilt fungus on their bodies. Re- 

 moval and destruction of diseased oaks before fungus mats are 

 produced will eliminate this important potential source of fun- 

 gus inoculum. 



Shoestring Root Rot. — Many oak trees that show branch 

 dieback and staghead (frontispiece) are affected with Armillaria 

 or shoestring root rot caused by the fungus Armillaria mellea. 



