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

 of Philosophy. 



C 



.^-n^^-^.^^ L 



"JamesFTPreston 

 Professor of Microbiology and Leii 

 Science 



> This dissertation was submitted to the Graduate Faculty of the 



Collegr^f Agriculfu^e and to the Gradut School and -s acce s 



oartial fulfillment of the requirements for the aegree ot uu 

 ^ Philosophy. I A :f ^ 



k' December 1991 ^ ^ ;.. ■ ' - pean, /Oill^ of A^lture 





Dean, Graduate School 



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