Studies on Fungicides — II 13 



racks must be thoroughly washed with water. The use of dirty or used 

 moist chambers markedly affects the results of experiments. 



The necessity for sterile conditions has not been felt in the use of this 

 method. Interest is only in germinating the spores, and if ordinary pre- 

 cautions are taken to prevent undue contamination it has been the writer's 

 experience that satisfactory results can be obtained. It is, of course, 

 entirely possible that with spores of certain fungi sterile conditions will 

 be necessary. 



Other than the fungicide, no foreign substance should be placed in 

 the chamber. Clark (1899), Duggar (1901), and Melhus and Durrell 

 (1919) have noted that the presence of such foreign substances as carbon, 

 paraffin, and vaseline has stimulated spore germination. Glue and balsam 

 used to cement certain types of slide racks have greatly influenced spore 

 germination in this laboratory. The glass-tube type of rack now used 

 carries no foreign material and can readily be washed. 



STANDARDIZATION OF THE FUNGUS AND OF THE CONDITIONS FOR SPORE 



GERMINATION 



These factors are of paramount importance, though unfortunately often 

 disregarded. It is very essential, if one plans to work with any fungus 

 on a large scale, that it should be " standardized "; that is, the optimum 

 factors for spore germination should be determined, and all conditions 

 controlled, in so far as possible. Unless the control spores germinate 

 consistently, no significant comparisons of toxicity can be made. This 

 is because spores of lowered vitality are greatly influenced by any adverse 

 condition, such, for example, as treatment with a toxic agent. The result 

 of this is a low germination out of all proportion to that of the control 

 when compared with treated and control spores of greater viability. 



Source of spores 



Ideally, perhaps, the spores should be obtained from nature, growing 

 on their natural substrates. This is not always possible, however, 

 especially during the winter months, when the tests are most likely 

 to be made. Furthermore, the writer has found that spores obtained 

 directly from nature generally vary more in their germinating ability than 

 do those grown on artificial media. The spores from culture are thus 

 more satisfactory to work with. Where the fungus is to be grown in 

 culture, the choice of a suitable medium must be made. A suitable 

 medium is one on which the fungus will produce an abundance of spores, 

 and spores that will give maximum germination. 



The maintenance of a high percentage of germination by spores from 

 artificial culture following isolation from nature may present a problem. 

 It was found, for example, that conidia of Sclerotinia americana obtained 



