ROSE CANKER AND ITS CONTROL. 39 



Disinfection by Chemicals. Laboratory Tests. 

 Some of the chemicals which have been used in the past for disin- 

 fecting soil for the control of other fungous diseases are formaldehyde, 

 sulfuric acid, copper sulfate, sulfur, lime-sulfur. The results obtained 

 by the use of these same chemicals for other fungi could not be used 

 directly in the present investigation because every fungus differs in its 

 resistance to a given chemical. It was first necessary to determine what 

 concentration and what quantity of solution per cubic foot was needed 

 to kill the fungus. These facts could be determined more accurately 

 and conveniently in the laboratory than in the greenhouse. The method 

 used in all these tests was as follows: — 



Method. — Milk bottles, each containing 33 cubic inches of soil, were steam 

 sterilized and inoculated from pure cultures of the fungus. When the soil was 

 entirely infested (requiring from twelve days to three weeks) it was stirred into 

 a loose condition with a sterile glass rod, and the proper amount of chemical in 

 solution, at the strength to be tested, poured in under aseptic conditions. Since 

 the soil did not dry out as rapidly in these bottles as it would under natural con- 

 ditions in the greenhouse, it was emptied into sterilized porous flowerpots after a 

 few hours. It was found after several trials that the pots dried out too rapidly 

 if left in the open laboratory. Thereafter they were covered with bell jars which 

 were tilted enough to allow free circulation of air beneath them, and the length of 

 the drying process could then be regulated. After eight to ten days in the pots, 

 clods of the soil were transferred from various portions of the pots to sterile agar 

 plates. If the fungus was still alive it spread to the agar; otherwise there was no 

 growth whatever from the clods. At first, the solutions were applied at the rate of 

 1 gallon to the cubic foot of earth. Afterwards, 2 gallons per cubic foot were used. 

 When dry chemicals, such as sulfur, were tested the required amount was thor- 

 oughly stirred into the infested soil of the bottles with a sterile rod and no water 

 added. 



Formaldehyde. — First tests were at the rate of 1 gallon per cubic foot 

 at the following concentrations: 1-500 (1 part of commercial formalde- 

 hyde to 500 parts of water), 1-400, 1-300, 1-200 and 1-100. None of 

 these concentrations gave complete success. On the transfers from the 

 last two, however, only a few of the clods contained living mycelium. 

 This indicated a lack of complete penetration by the solution. In the 

 next series of tests the same concentrations at the rate of 2 gallons per 

 cubic foot were used. The 1-100 and 1-200 then gave absolute control, 

 while the 1-300 usually did; but occasionally a single clod developed a 

 mycelium on the agar. The death point concentration lies somewhere 

 between 1-200 and 1-300. But to be well within the margin of safety, 

 1-200 (1 pint of commercial formaldehyde solution to 25 gallons of water) 

 was decided upon as the best strength to use in the greenhouse. 



Sxdjuric Acid. — This chemical has been successfully used in the past 

 in the control, particularly, of certain root diseases of nursery trees. At 

 the rate of 2 gallons per cubic foot, concentrations of 1, 2, 3, 4, 5 and 8 

 per cent, were used. The 5 per cent, solution killed most of the myceUum, 



