I certify that I have read this study and that in my ooinion it 

 ccnforr:is to acceptable standards of scholarly presentation and is -^ully 

 adequate, in scope and quality, as a dissert3,^tion for the degree of 

 Doctor of Philosophy. 



.■v "f 



Dr. F. W. Zettler 

 Professor of Plant Pathology 



This dissertation was submitted to the Graduate Faculty of the College 

 of Agriculture and to the Graduate Council, and was accepted =s oar- 

 tial fulfillment of the requirements for the degree of Dcctor of 

 Philosophy. 



December 1977 



Dean, Graduate School 



