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

 opinion it conforms to acceptable standards of scholarly 

 presentation and is fully adequate, in sco^ / a'nd quality, as a 

 dissertation for the degree of Doctor of P/fjiLDsophy . 



Francis W. Zettler, Chairman 

 Professor of Plant Patholocv 



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

 opinion it conforms to acceptable standards of scholarly 

 presentation and is fully adequate, in scope and quality, as a 

 dissertation for the decree of Doctor of Philosophy. 



A 9l^M 



J^ffn R. Edwardson, Cochairman 

 'rofessor of Agronomy 



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

 opinion it conforms to acceptable standards of scholarly 

 presentation and is fully adequate, in scope and quality, as a 

 dissertation for the degree of Doctor of Philosophy. 



Ernest Hiebert 



Professor of Plant Pathology 



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

 opinion it conforms to acceptable standards of scholarly 

 presentation and is fully adequate, in scope and quality, as a 

 dissertation for the degree of Doctor of Philosophy. 



rfLt.P* 



Dan E. Purciful 

 Professor of Pi 



Pathology 



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

 opinion it conforms to acceptable standards of scholarly 

 presentation and is fully adequate, in scope and quality, as a 

 dissertation for the degree of Doctor of Philosophy. 



c 



'"V 



Henry; C. Aldrich 

 Professor of Microbiology 

 and Cell Science 



