ACTION OF FUNGICIDES 261 



tested on Fusarium graminearum, Penicillium digitatum, and Botrytis 

 aim (McGowan et al., 1948). The fungicidal action of substituted 

 pyrazoles was tested on spores of Alter naria oleracea and Sclerotinia 

 americana in the laboratory, and for the control of apple scab, cedar-apple 

 rust, and late blight of potato and tomato. Some of these compounds 

 show promise, although the mechanism of action is not known (McNew 

 and Sandholm, (1949). For a survey of the newer fungicides see Well- 

 man (1948). 



EVALUATING FUNGICIDES 



The preliminary tests of fungicidal activity are made in the laboratory 

 in order to eliminate inactive compounds or to compare the activities 

 of different compounds under identical conditions. Evaluation in the 

 greenhouse and field is the final test of a new fungicide. This discus- 

 sion will be limited to a general consideration of laboratory testing of 

 fungicides. 



Fungus spores rather than mycelium are used in most laboratory tests 

 because it is the function of a protectant fungicide to kill or inhibit spore 

 germination. Three basic types of procedures may be used in laboratory 

 tests (McCallan, 1947): (1) Spores are suspended in solutions or suspen- 

 sions of the fungicide under test, and the inhibition of germination is 

 noted as a function of time of exposure and concentration of the fungicide. 

 This is a modification of the Rideal- Walker method of evaluating anti- 

 septics. (2) The compound to be tested is incorporated in a suitable 

 solid or liquid medium, which is then inoculated with spores of the test 

 fungi. The amount of inhibition of germination or growth is determined. 

 (3) Glass slides are covered uniformly with the fungicide, and after dry- 

 ing, the spores are sown on the treated slides. The inoculated plates 

 are then placed in constant-humidity chambers and the percentage of 

 germination determined after 20 to 24 hr. ; or the effectiveness of a fungi- 

 cide may be studied as a function of time of exposure. 



The second and third methods appear to be the most useful. Fleury 

 (1948) studied the fungistatic action of thiourea on Aspergillus niger 

 by adding this substance to a liquid basal medium. Thiourea was a 

 much more potent inhibitor w^hen nitrate nitrogen was used than when 

 ammonium or organic nitrogen was present in the medium. Agar 

 medium has been used by Leben and Keitt (1949) to assay the amount of 

 toxicant on leaf surfaces. A suspension of spores of Glomerella cingulata 

 was prepared in warm (38 to 40°C.) agar medium. Five milliliters of 

 this seeded medium was added to Petri dishes which contained 15 ml. of 

 solidified agar medium. After the seeded agar had solidified, leaf disks 

 of uniform size were cut from sprayed leaves and placed on the agar. The 

 amount of toxicant present on the leaf surface was determined by measur- 



