42 MASS. EXPERIMENT STATION BULLETIN 183. 



From these tests we may conclude that soil can be disinfected by steam 

 in less than a minute if penetration is obtained. Apparently effective- 

 ness is limited only by the time required for the steam to penetrate every 

 particle of the soil. 



Greenhouse Tests of Disinfection by Heat. 



Heat may be applied to the soil by steam or by hot water. The first 

 method has been in vise in the greenhouses for the disinfection of the 

 soil used in potting since the beginning of this investigation. Perforated 

 steam pipes were laid a foot apart in a large pit. Soil a foot deep or more 

 was piled over them and the steam turned into the pipes. Burlap or other 

 coverings may be used to cover the soil and make it retain more of the 

 steam. Soil thermometers were used to determine the temperature. 

 It is only necessary to keep the temperature above 50° C. for ten min- 

 utes. A higher temperature, of course, makes for additional safety. 

 The one or two hours of heating frequently recommended for other 

 diseases is only wasted time and expense, being entirely unnecessary for 

 this fungus. Thousands of plants have been potted in soil disinfected 

 in this way during the last year, and canker has never appeared on any 

 of them. No doubt other methods of steam disinfection, such as the 

 inverted pan method, would be equally effective. Either method could 

 probably be used just as effectively on the benches, but the formalde- 

 hj'de treatment is efficient, and quicker and easier of application. 



If there is any reason to suspect the presence of the fungus in the 

 manure which is used to mulch the beds it may be disinfected in the same 

 way as the potting soil. Soil for the cutting bench may also be treated 

 in the same wa3^ 



The second method of applying heat — by the use of boiling water — 

 is now being tested. It should be just as effective as steam, and at the 

 same time much more rapid. The boiling water is forced through the 

 water pipes ordinarily used in the house, and is applied to the soil through 

 a hose with a long nozzle and a handle which will not become heated. 

 The water should be applied until a thermometer inserted into the soil 

 at any point and at any depth registers above 50° C. Higher temperatures 

 make for additional safety. This method has the disadvantage of 

 leaving the soil in poorer condition for working. The hot-water 

 method is still in the experimental stage, and is not far enough along 

 to warrant any recommendations. 



Disinfection of Pots, Tools, etc. 

 In starting new houses with clean plants and clean soil, it is very es- 

 sential that everything which is used should be free from any form of 

 inoculum. The first danger is from pots which h^ve been previously 

 used, and which are apt to contain mycelium or spores in the particles 

 of earth which still cling to them. They can be sterilized by immersing 



