28 



Harold Thurston Cook 



each temperature was examined at the end of two, three, four, five, 

 seven, and twenty-four hours, respectively, and the percentage of ger- 

 mination in each drop was noted. The percentage of germination was 

 obtained by counting 100 spores in each drop. The results, presented 

 in figure 7, are based on three replications and on counts of approxi- 

 mately 5500 spores for each temperature tested. 



According to these results the optimum temperature for germination 

 of these spores is 11° C, since at this temperature germination was the 

 most rapid and complete. Within six hours approximately all of the 

 spores had germinated. Spores kept at lower temperatures germinated 

 slightly more slowly, but the final percentage of germination was nearly 

 as high. Those kept at 13° to 14° behaved similarly. At 18° and 

 above, the higher the temperature, the lower was the final percentage 

 of germination. No germination occurred above 27°. 



Temperature also had an effect on the type of germ tube formed. 



■'■■■<*■■ " • V <u-- --<' ;/ 4'- • **&$> 







l«t> M 



)pj <& 



Figure 8. germination of conidia of peronospora destructor at the optimum 



temperature 



