SECTION I 

 EFFECTS OF FUMIGATION AND ANTAGONISTIC SOIL FUNGI ON THE 

 RELATIONSHIPS OF INOCULUM DENSITY TO INFECTION INCIDENCE 

 AND DISEASE SEVERITY IN FUSARIUM GROWN ROT OF TOMATO 



Introduction 



Fusarium crown rot of tomato ( Lycopersicon esculentum Mill . ) , 

 caused by Fusarium oxysporum Schlecht f . sp. radicis-lycopersici Jarvis 

 and Shoemaker, is a disease which is severe when tomatoes are grown in 

 soil treated with biocides (l^,15i 28) . This phenomenon fits Kreutzer's 

 (l?) concept of disease trading in which dominant pathogens are con- 

 trolled by soil treatments but minor pathogens are elevated to major 

 importance because they can recolonize soil in which their competitors 

 and antagonists have been eliminated. The traditional methods of 

 applying fungicides directly to plants growing in previously treated 

 soil have proved ineffective in the control of Fusarium crown rot (27). 

 Rowe and Farley (27), however, controlled the disease with the applica- 

 tion of captafol to freshly steamed soil before planting. The success- 

 ful results were attributed to the selective action of the fungicide 

 which inhibited reinvasion by the pathogen but did not adversely affect 

 the recolonization of the soil by other airborne microorganisms. 



Thompson (36), as early as 1929, realized that chemical agents 

 would be useful in disease control mainly where conditions are rela- 

 tively unfavorable for the pathogen and that biological agents would be 

 more important when the environment is conducive to activity of the 



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