FUNGICIDES 371 



spore germination tests of fimgicides-^^- ". 26, 29 j^ analysis of 718 indi- 

 vidual toxicity or dosage-response curves was undertaken.^^ Six different 

 fungi and 20 different chemicals, both soluble and insoluble, were used. 

 Four different types of slopes were observed on logarithmic probability 

 paper: (a) orthodox simple straight lines, (6) double slope with left hand 

 "break" in lower values giving a curve concave upwards, (c) double slope 

 A^th right hand "break" in upper values or curve convex upwards, and 

 (d) triple slope or sigmoid curve. The first three types are illustrated in 



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R P. M. METALLIC ION 



Figure 145. Toxicity curves of heavy metals on Mocrosporium sarcinaeforme. Note 

 that relative toxicity of the metals varies at the different inhibition levels due to differ- 

 ences in slope of the curves. Each point mean of six repUcate tests. 



Fig. 145. The type and steepness of slope are determined more by the 

 chemical than by the fungus. Differences in the slope of the dosage-response 

 curve for two fungicides ^vill cause changes in their relative effectiveness 

 at different levels of inhibition or control; this is especially noticeable if 

 the curves cross each other. Since fungi vary in their sensitivity from day 

 to day or season to season, not only in the laboratory but also in the field, 

 conmionly observed differences in the relative rating of two fungicides at 

 different times can be expected and explained. 



Error of replicate tests. It has long been observed that spores germinated 

 at different times, though produced under seemingly standard conditions, 

 vary -^ddely in their response. A portion of this variation was found to be 

 due to the presence of variable amounts of water-soluble nutrient derived 

 from the cultures when procuring the spores.^^ If the spores are obtained 

 by a vacuum technique or are washed and centrifuged, they may be rid of 

 the contaminating stimulant. However, in many cases it will be necessary 



