A Study of Meadow-Crop Diseases in New York 



57 



uncertain. In any case, 

 conidia produced on the 

 primary lesions are splashed 

 and blown about to initiate 

 secondary infections at in- 

 tervals of a fortnight or 

 less. 



The conidia send forth 

 hyaline germ tubes from 

 one or several cells, as 

 shown in figure 12. Spore- 

 germination studies were 

 limited because conidia 

 ordinarily are not produced 



in culture. In one experiment, however, spores were washed from an 

 alsike-clover leaf and were germinated in distilled water in duplicate at 

 various temperatures. The data appear in table 18. 



Figure 12. a conidium of cercospora zebrina 

 from a leaf of alsike clover, germinating in 

 the presence of bank's colloidal sulfur dust 



No toxic effects are present except possibly a shortening of the 

 tubes 



The counts were made after twenty-three hours had elapsed, so that a 

 fairly sharp curve resulted. If more time had been allowed, the germi- 

 nation doubtless would have been better at the lower temperatures. An 

 optimum at about 25°C. is indicated. An abundant oxygen supply seems 

 necessary for germination. Some fresh spores accidentally left overnight 

 in a drop of water under a cover glass in a moist chamber were examined 

 the next morning. Spores at the edge of the mount were germinated 

 well, with long tubes, but those in the middle were ungerminated. There 

 was a marked gradient of germination toward the center of the cover 

 glass. 



Control 



Wolf (1916) and Hopkins (1921) recommend exclusion by seed treat- 

 ment with formaldehyde, 1 pint to 30 gallons of water. 



