ROSE CANKER AND ITS CONTROL. 19 



the final percentage of germination. In this respect there is a rather 

 regular curve. The optimum is at about 25°, where germination begins in 

 two to three and one-half hours. At 12° it required five hours, and at 8° 

 no germination was apparent until after twenty-four hours. The fact that 

 spores do not germinate at a certain temperature does not mean that 

 they are dead. Spores kept for two days at 5° showed not the least indi- 

 cation of germination, but when brought back to ordinary room tem- 

 peratures they quickly germinated to over 95 per cent. Experiments 

 to be described later show that spores may be kept for long periods at 

 temperatures both lower and higher than indicated in this table and 

 still retain their viability. 



Apparently there is little opportunity for retarding the progress of the 

 disease by maintaining temperatures in the house unfavorable to the 

 fungus, because the optimum temperature for spore germination is ap- 

 proximately the same as the optimum for growing roses. The latitude 

 of the germination optimum is also unfavorable to such a method of 

 control. 



Effect oj freezing the Spores. 



It is a well-known fact that the spores — especially the conidia — of 

 many fungi are quickly killed by freezing, and this weakness may be 

 utilized in checking disease. The purpose of the present investigation 

 was to determine whether the spores of Cylindrocladium can be killed by 

 freezing, and if so, how much exposure is required. Two methods were 

 used. 



First Method. — Petri dishes containing young cultures with abundance of spores 

 were exposed to out-of-door temperatures of — 3° to — 10°C. Checks were first 

 made at room temperatures to test the viability of the spores. Spores were re- 

 moved from the frozen plates at regular intervals and put to germinate in moist 

 chambers at ordinary room temperatures, as described above in spore germination 

 tests. Bj^ this method the spores were dry when frozen. 



After about two hours the percentage of germination began to decline; 

 in eight hours it had fallen to 10 per cent.; in twelve hours, to less than 

 1 per cent.; and at the end of fourteen hours there was no germination 

 whatever. All checks germinated 95 per cent. 



Second Method. — Spores were transferred from plates along with a portion of 

 the agar to drops of water on slides. All was macerated until the spores were well 

 distributed through the water. They were immediately put outside to freeze and 

 one slide brought into the laboratory at the end of each hour and tested for ger- 

 mination. 



The results were very similar to those obtained by the first method. 

 Freezing for one hour seemed not to affect them at all; in two hours the 

 percentage dropped to from 75 to 80 per cent.; in three hours, to 30 per 

 cent.; in six and one half hours, to 25 per cent.; in ten hours, to 1 per 

 cent. From 1 to 2 per cent, germinated even after exposures of twenty- 



