96 



James G. Horsfall 



It is obvious that timothy-rust urediniospores are very resistant to cop- 

 per because they germinated approximately 10 per cent even in the presence 

 of 6.8 per cent of copper. Another fact that the data emphasize is that 

 there is no essential difference in germination whether the spores were in 

 contact with one dilution of dust or the other. Except possibly for the 

 6.8-per-cent dilution, the copper content appears to have nothing to do 

 with the results. Since the dusts were composed of inonohydrated copper 

 sulfate and hydrated lime, the lime was the only substance that could be 

 acting to equalize the dusts, because the copper content was variable. 

 The lime content varied somewhat, too, but since the least amount was 

 about 93 per cent, its relative fluctuation was not great. This conclusion 

 at the time of the tests was not without foundation because several 

 tests with lime alone showed marked reduction in germination. 



A subsequent experiment seemed to clinch the argument. In this test 

 duplicate slides on which approximately 600 spores were counted for each 

 treatment, 7.2-per-cent copper-lime dust and lime alone, showed an average 

 of 16.3 per cent germination in the copper lime and 19.3 per cent in the 

 lime alone. A reference to the drawings in figure 22 shows that germ tubes 



Figure 22. germinating urediniospores of puccinia phlei-pratensis, showing the 

 toxic effects of fungicides 



a, Check; b, in copper-lime dust; c, d, and e, in 200-mesh dusting sulfur; /, in Bank's colloidal sulfur dust 



growing in the presence of copper-lime dust appear normal. It would be 

 of interest to find an inert carrier for the inonohydrated copper sulfate 

 which would not of itself affect the spores, in order to determine just how 

 toxic copper really is. Bentonite alone in one experiment reduced the 

 germination approximately one-third. Incidentally, in a single experiment 

 the spores germinated 16.6 per cent in 4-4-50 bordeaux mixture also. 



