378 



GROWTH OF PLANTS 



cides are applied to the foliage of plants they act in a fungicidal rather 

 than a fungistatic manner. For the first time an absolute and direct com- 

 parison was made of the action of the same fungicides on the same organ- 

 ism, Alternaria solani, at the same spore concentration on glass slides and 

 on the leaves of a growing plant. ^'^ The results of this interesting compari- 

 son are shown in Fig. 150. It will be seen that the LD50 levels are about 

 equal, but as noted above, the greenhouse or leaf curves are much flatter; 

 however, the same fungicide Thiosan (tetramethylthiuram disulfide) was 

 the more toxic in both methods. 



is; 



0.1 



.0? 



/<r LABORATORY V 



/» 



/ 

 / 



■ 1111 I I 1 1 



J l_J L-l-L 



' ■ ■ I 



I I r I I I 



0.1 



100 



03 13 10 30 



MICROGRAMS FUNGICIDE DEPOSITED PER SQUARE CM. 



Figure 150. Absolute comparison of laboratory and greenhouse dosage-response 

 curves; identical fungus and fungicides. Tennessee Copper "34" and Thiosan against 

 early blight {Alternaria solani Maine strain). Spore concentration constant, 70 per 

 sq cm. 



It was found with these greenhouse toxicity curves that the most precise 

 point of comparison was not at the LD50. From a study ^ of 431 pairs of 

 replicate tomato plants infected with early or late blight lesions, a linear 

 regression was obtained between the logarithm of the weight of per cent 

 disease in replicate plants and the logarithm of the mean per cent disease. 

 By means of the z'^ transformation, empirical probit weights were ob- 

 tained. The maximum range of precision for comparison was thus found 

 to be within the range LD80 to 95; accordingly the LD95 or 95 per cent 

 disease control is recommended for comparison of fungicides by this 

 method. 



