ROSE CANKER AND ITS CONTROL. 41 



with steam, and which had been kept under conditions as sterile as pos- 

 sible. Three months after planting, no disease had appeared in either 

 house. Soon afterward it began to appear in the house with the ground 

 beds, and gradually increased until, almost a year after planting, it was 

 generally prevalent throughout the house. In the bench house, however, 

 no disease has as yet been found, although plant-to-plant inspections 

 have been made frequently throughout the year. The fact that a con- 

 centration of formaldehyde weaker than 1 pint to 25 gallons controlled 

 the disease in the bench house is probably due to the longer action of the 

 more concentrated fumes, and probably, also, partly to the greater 

 amount of the solution applied. The lack of control in the ground bed 

 house can be easily explained in the light of our studies on the depth of 

 penetration of the mycelium in the soil. The surface soil was disin- 

 fected, but it was not possible to disinfect it down as far as the mycelium 

 grows. After the formaldehyde had evaporated the deep mycelium 

 began to grow upward, and during that period the plants remained 

 healthy; but, after the mycelium had grown up to the surface again, 

 the cankers began to appear and the roses became as badly affected as 

 before the house was treated. Two conclusions may be drawn from 

 this experiment: (1) the soil can be disinfected effectively by the use 

 of formaldehyde, and (2) ground beds cannot be sterilized by this method. 



Disinfection by Heat. Laboratory Tests. 



The feasibility of destroying any fungus by application of heat to 

 the soil manifestly depends, first of all, on the thermal death point of 

 all stages of that fungus. As has previously been described, this point 

 for Cylindrocladium was found to be 50° C. This comparatively low death 

 point indicated that the soil could be readily disinfected by steaming, 

 because a temperature much higher than 50° C. can be easily obtained 

 by the use of steam. 



Time required to disinfect Soil by steaming. — This was further confirmed 

 by the following tests : • — ■ 



Method. — Sterile Petri dishes were filled with soil which was thoroughly in- 

 fested with mycelium. After removing the lids they were subjected to steam at 

 a temperature of 90° to 95° in an Arnold sterilizer for the desired length of time. 

 The lids were then replaced and the soil allowed to cool, when clods of it were • 

 transferred to agar plates as described above. Exposures of five, ten, fifteen, twenty 

 and thirty minutes were tried. 



No mycelium appeared on any of the transfers, even after five min- 

 utes' exposure. Shorter periods of exposure were not tried because of 

 the uncertainty of securing penetration by steam in less than five minutes. 

 But, to determine what effect shorter exposures would have on mycelium, 

 tests were made bj^ the sealed tube method described for thermal death 

 point tests. In these tests the mycelium was killed in less than one 

 minute when exposed to a temperature of 95° C. 



