36 

 for up to 25 days after fumigation; conversely, when added to fumigated 



soils which had undergone recolonization for 25 days, it was unable to 



compete as a saprophyte. 



The apparent contradiction in the literature over the presence of 

 succession may be explained by examining different models of the mecha- 

 nisms of succession. Gonnell and Slatyer (6) have proposed three differ- 

 ent mechanisms of succession following a perturbation. In all three 

 models the earlier species cannot invade or grow after the site is fully 

 occupied by the same or later occurring species. In the facilitation 

 model, later species can become established only after earlier inhabi- 

 tants have suitably modified the environment. This model explains why 

 secondary invaders are detected only after a plant pathogen has invaded 

 the healthy tissues of the host. According to the tolerance model, 

 which is similar to the model proposed by Garrett (9). the later species 

 can establish themselves because they can utilize nutrients at lower 

 levels than earlier recolonizers. According to the inhibition model, 

 later species cannot grow in the presence of earlier species and their 

 establishment is dependent upon their ability to survive longer and 

 gradually replace the earlier species. The results of this study and 

 the literature indicate that succession in artificially infested sub- 

 strates follows the inhibition model, but that in nonamended substrates 

 succession follows the tolerance model. 



A basic principle of community ecology is that the success of a 

 species, such as a plant pathogen, is dependent upon its ability to 

 interact successfully with the abiotic and biotic factors of its sur- 

 rounding environment. Because fumigation practices create an environ- 

 ment conducive to the proliferation of the pathogen, a possible means of 



