32 MASS, EXPERIMENT STATION BULLETIN 183. 



The last test was made May 10, after the bottles had experienced the 

 coldest winter on record in jSIassachusetts. The fungus was still living 

 in the soil. Apparently, then, soil cannot be made safe by exposing it 

 during the winter out of doors. 



Thermal Death Point of Mycelmm. 

 Anticipating soil sterilization by heat, the thermal death point for 

 the mycelium was determined. 



Method. — The same method was used as for determination of the thermal 

 death point of spores, except that bits of agar containing mycelium were inserted 

 into the sealed tubes, and after exposure for ten minutes to the desired temperature 

 were transferred to sterile agar plates. If the mycelium was still alive it quickly 

 began to spread to the agar. Temperatures between 42° and 55° C at intervals of 

 1° were tested. 



Up to and including 48° the treatment seemed to have no effect on the 

 mycelium. At 49° it was sometimes killed and sometimes not. It never 

 grew after ten minutes' exposure to 50°. We may therefore consider 50° 

 the thermal death point. It will be noticed that the thermal death points 

 of mycelium and spores differ by only 1 degree. The mycelium tested 

 contained, besides the ordinary white mycelium, also the dark bodies 

 with thick walls which we have called chlamydospores and sclerotia. 

 As was the case with spores, so also the mycelium may be killed by a 

 longer exposure to a lower temperature. Based on an exposure during 

 one hour, the thermal death point was found to be 48°. 



DISSEMINATION. 



In deciding on a method of controlling a disease it is of prime impor- 

 tance to find out how the pathogene is spread about, where it comes 

 from, how it reaches the host. In the present case a threefold question 

 is involved: (1) How did the fungus get into rose houses in the first 

 place? (2) How is it spread from the houses of one rose grower to those 

 of another? (3) On the premises of a single grower, how does it pass 

 from house to house, bench to bench, or plant to plant? In the light 

 of what has been learned concerning the life history and habits of the 

 pathogene, we may undertake to answer these three questions. 



1. Original Source of the Pathogene. 



The fungus, from all that is known of its past history, is a native of 

 America. Since it has been reported but a few times, it probably is not 

 very common out of doors. As greenhouse roses are grown in the section 

 of the country where it has been reported, it would not be far-fetched 

 to imagine the fungus being carried into rose houses with rotted leaves, 

 where it was able to adapt itself to parasitic life on the rose. It is not 

 necessary to assume, then, that this is an imported pathogene. Early 



