Studies on Fungicides — II 



21 



TABLE 8. Relation of Quantity and Efficiency of Sulfur Dust to Germination 

 of Uromyces hybridii Uredospores* 



(Distilled water at 21° C.) 



* Data from results obtained by Dr. J. G. Horsfall, formerly of this laboratory. 



The results of these experiments, though exceedingly limited, would 

 seem to indicate that, provided a certain minimum of sulfur is applied, 

 increasing the amount beyond this will not appreciably increase the fun- 

 gicidal efficiency. 



RECORDING RESULTS 



As has already been mentioned, the test should not be concluded and 

 the results taken until all viable spores have germinated. There are 

 three common ways of expressing the toxicity of a fungicide : one can record 

 the percentage of germination, can record the character of the germ tubes, 

 or can attempt to grow the treated spores as was done by Lee and Martin 

 (1927) and by Young and Cooper (1917). There are objections to all 

 three. With this method of testing fungicides under asterile conditions, 

 the last-named practice is impossible. Extended measurements of germ 

 tubes involve much time and labor and are of doubtful value since the 

 tests are only relative. The method first employed by the writer was 

 merely to record the percentage of germinated spores. This was later 

 supplemented by observations on the length and the character of the 

 germ tubes. The germ tubes are provisionally classed into four types, 

 A, B, C, and D, of which A is the largest and most vigorous, normally 

 corresponding to those in the check, and D is the poorest. In general, 

 D type includes spores that have just germinated and all up to those that 

 have produced a germ tube approximately 25m long; C type includes those 

 with germ tubes 25 to 100^ long; B type, those with germ tubes 100 to 

 200 /z long; and A type, those with germ tubes 200 /x in length and longer. 

 In addition, the signs + and - may be employed in each class. From the 

 observations of Kightlinger (1925), it is probable that spores of D type, 

 and probably also of C type, would be incapable of producing infection. 

 The spores are not classified individually, but as a whole into one type 

 for each microscopic field. 



