DUTCH ELM DISEASE IN ILLINOIS 



17 



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Diseased 

 Elm 



Heolthy 

 Elm 



STREET 



Heolthy 

 Elm 



Fig. 15. — The rows of black dots show two patterns of applying a soil sterilant 

 to prevent spread of the Dutch elm disease fungus. The "T" pattern is used to 

 avoid injuring the nearby hedge. When the diseased tree shows advanced wilt 

 symptoms, it is advisable to establish a second barrier between the healthy trees. 



Additional Control Treatments 



Potassium iodide. This treatment is sometimes used as a supple- 

 ment to sanitation. In woodlands and other areas where it is not 

 feasible to burn trees, or in municipal areas where elms cannot be 

 removed promptly, bark beetles can be prevented from colonizing 

 in diseased trees by impregnating the outer sapwood and bark with 

 potassium iodide. Elms are killed by this treatment (Fig. 16). For 

 most effective distribution of this chemical, diseased elms must be 

 treated when earliest symptoms of foliage wilt are visible. 



The potassium iodide solution (4 pounds of the chemical per 

 gallon of water) is applied to each tree in an ax frill. The ax frill is 

 made by chopping a continuous angling cut around the trunk (Fig. 

 17), cutting through the bark and at least three wood rings. The 

 potassium iodide solution, approximately 1 pint per tree, is slowly 

 poured into the frill as the operator circles the tree two or three times. 

 This procedure allows time for the chemical to penetrate the sap- 

 wood. 



In woodland areas where elms are of little value, or in other 

 areas where sanitation and spraying are not carried out, potassium 

 iodide can be used to advantage to poison all healthy elms before 

 they become diseased and colonized by bark beetles. 



