22 



SOIL TREATMENT WITH CHLORIDE OF LIME FOR FUNGOUS 



DISEASES 



J. W. Eastham, Assistant Botanist, Central Experimental Farm, Ottawa. 



In the case of many of the diseases of plants due to parasitic fungi it is known 

 that the parasite has the power of existing in the soil for a considerable period, 

 often extending over several years, and during this time any susceptible crop planted 

 in the soil is likely to succumb to disease. In what condition the organism exists 

 during this time is in many cases not known, but it is probable that many fungi 

 which are capable of causing serious diseases in certain plants are also able, in 

 the absence of suitable hosts, to exist on the dead organic matter in the soil. 

 In other cases it is possible that the resting spores of the fungus remain dormant 

 for varying periods of time so that successive germinations occur in successive 

 years, while in yet other instances the disease-producing organism is kept alive in 

 the land through the agency of weeds allied to the crop host-plant. Examples of 

 plant diseases which after once getting established in the soil are capable of 

 attacking crops planted therein for several years are potato scab, potato canker, 

 club-root (or finger-and-toe) , of crucifers and the root-rots due to species of 

 Rhizoctonia. 



In such cases, it is, of course, highly important to avoid the infection of land at 

 present free from such diseases. To this end only seed or tubers apparently quite 

 sound should be used, and if possible selected from healthy stock. Even then, 

 treatment of seed or tubers with such substances as formalin or corrosive subli- 

 mate to destroy any adherent disease germs is often a wise precaution. Care 

 •should also be taken that refuse from a diseased crop does not find its way to 

 the manure-heap and thus demonstrate one of the most potent means of dissemin- 

 ating disease. Where, however, the soil is already infected it becomes necessary 

 either to " starve out " the parasite by growing, for a series of years, only those 

 crops which are not attacked by the particular parasite in question, or to adopt 

 some method of treating the soil whereby the parasitic organism is killed. To 

 obtain such a complete or partial sterilization of the soil several means have been 

 employed, as for example, the " firing " or dry heating of the surface soil, the 

 injection into the soil of steam at high pressure, and the application of certain 

 chemicals. These methods, however, are on the whole too costly to find employ- 

 ment for any but very special field crops, though it is often worth while to make 

 use of one of them for greenhouse or seed beds, or where very intensive cultivation 

 is practised. Sterilization by suitable chemicals has the advantage of not requir- 

 ing the fitting up of special apparatus, and the application of a solution of formalin 

 is perhaps the commonest method in use. It is, however, rather expensive and it 

 is often a considerable time before the formalin has evaporated sufl&ciently to 

 make sowing or planting safe. 



