374 GROWTH OF PLANTS 



tact with the chemical. For fungicidal tests the fungus spores were allowed 

 to remain in contact with the chemical for different periods of time, after 

 which the chemical was removed by centrifuging and decanting and the 

 spores were washed and allowed to germinate in water. As fungicidal 

 activity cannot exceed fungistatic activity, three kinds of response are 

 possible. Thus certain chemicals such as silver nitrate and copper sulfate 

 show both high fungistatic and fungicidal activity; others exhibit high 

 fungistatic but low fungicidal, such as formaldehyde and phenol; and stUl 

 others are low in both properties. Hence the correlation between fungicidal 

 and fungistatic properties is high except for the second group. The fungi- 

 cidal dosage-response curves, like the fungistatic, tend to plot as straight 

 lines on logarithmic probability paper. However, in many cases the fungi- 

 cidal curves are decidedly flatter. 



These fundamental studies on spore germination tests of fungicides, espe- 

 cially the dosage-response curve, have not only encouraged and advanced 

 laboratory testing but they are also stimulating comparable studies under 

 actual field conditions and are resulting in new concepts regarding the com- 

 parison of fungicides in the field.® 



The standard method. The slide-germination method as now developed 

 has been accepted as a standard by the American Phytopathological 

 Society.^ The method is designed to evaluate the fungistatic properties of 

 protectant fungicides. Spores of certain fungi are placed on glass shdes in 

 water in the presence of the chemical to be tested. All conditions are rigidly 

 controlled and, except for the chemical, are favorable for spore germination. 

 The chemical may be applied either by the test-tube dilution teclmique or 

 by the horizontal sprayer or settling-tower technique.'® The results are 

 expressed as concentration of chemical necessary to inhibit the germination 

 of 50 per cent of the spores, i.e., the LD50, or as the LD95; the former is 

 more precise. 



Settling tower. Prior to the development of the standard method, a special 

 study was made of the method of applying the fungicide, and the settling 

 tower as finally developed constitutes the most precise method of applying 

 insoluble fungicides to glass slides. The tower is illustrated in Fig. 147. 



Greenhouse Methods of Evaluating Fungicides 



'Recently, attention has been devoted to developing greenhouse methods 

 of testing as an intermediate between the laboratory and field tests. This 

 has seemed particularly desirable because of the specificity of the newer 

 fungicides, especially among the organic chemicals. 



Tomato foliage diseases. The tomato foliage diseases,^® early and late 

 blights, caused respectively by Alternaria solani and Phytophthora infestans, 

 have been found particularly suitable for this purpose, in addition to repre- 

 senting important economic diseases caused by members of the Moniliales 

 and Phycomycetes. The leaf spot (Septoria lycopersici) has also been used 

 but has not been found as desirable. The tomato plants are sprayed with 



