11 



Discussion 

 The relationships of inoculum density to disease incidence have 

 "been applied in the quantification of several soilborne diseases caused 

 by Fusarium spp. (l,?,ll). In this study the TDcq in nonfumigated soil 

 was 900 chlamydo spores per gram of soil, which is similar to the IDcqS 

 found in other diseases caused "by Fusarium spp. in nontreated soils 

 (7,ll). The IDcq in the fumigated soil was 300 chlamydo spores per gram 

 of soil; however the pathogen population increased from 500 chlamydo- 

 spores to ^-000 propagules per gram of soil during the experiment. Guy 

 and Baker (ll) reported a similar ID™ when the pathogen population 

 increased due to a chitin soil amendment. High TD^qs (2000 or more 

 conidia per gram of soil) were reported by Abawi and Lorbeer (l) in 

 steam- treated and nontreated soils. However, the method for determining 

 disease severity was based on percent emergence rather than on the 

 percentage of infected or diseased tissue used in other investigations (l). 

 Also, conidia were used as the inoculum source rather than chlamydo- 

 spores, as in other studies. Their data, however, still indicate that 

 the lower ID cq occurred in treated soil. The position and slope of the 

 curve derived by the log-log transformation of inoculum density to 

 disease severity affect the IDcq of a particular disease system. Guy 

 and Baker (ll) found that the addition of organic materials to the soil 

 altered the Urn by shifting the position of the curve rather than 

 changing its slope, which they attributed to the relative changes in 

 infection rates being directly correlated with inoculum density. A 

 similar shift was noted in this study in the soils that were fumigated, 



