296 Disease-Producing Microorganisms 



2. Special physical methods of soil treatment, such as soil cultiva- 

 tion, change of soil reaction, use of organic matter, use of specific 

 fertilizers. 



3. Partial sterilization of soil. 



4. Use of chemicals for the destruction of specific disease-produc- 

 ing organisms. 



5. Biological control or introduction of organisms destructive to 

 the parasites. 



Crop Rotation 



In practicing crop rotation, one should remember that many of 

 the disease-producing organisms can persist in the soil for a number 

 of years and some are capable of leading there a normal saprophytic 

 existence. A rotation of at least 5-6 years should be practiced 

 against the club root of cruciferous plants and the sugar-beet 

 nematode. 



Physical and Chemical Methods of Soil Treatment 



Among the most efficient methods of control of soil-borne infec- 

 tions is the adjustment of the soil reaction, by the use of either 

 alkali-forming (lime) or acid-forming (sulfur, ammonium sulfate) 

 materials. Addition of sulfur or inorganic acids to soils having a 

 reaction of pH 5.9 or above is recommended; the amount of sulfur 

 or acid to be applied depends, of course, on the initial reaction and 

 buffer content of the soil. Hov^ever, the action of sulfur in control- 

 ling the wart disease of potatoes depends not alone upon the acidity 

 produced, but also upon some other mode of action of the sulfur, 

 probably thiosulfuric acid produced at an early stage of oxidation 

 of the sulfur. 



For the control of potato scab, sulfur is effective. Sweet-potato 

 scurf and pox can also be checked by the application of sulfur. 

 Lime, which reduces the acidity of the soil, and stable manure favor 

 the development of scab. The addition of fertilizers (acid phos- 

 phate) to make the soil reaction acid tends to decrease the develop- 

 ment of scab. According to Millard, sufficiently liberal dressings of 

 green manure added to the soil will inhibit the disease. This is 

 probably due to the temporary increase in soil acidity, as a result of 

 the decomposition of the organic matter by the soil fungi, and to an 

 increase in soil moisture. Scab is much more prevalent in dry 

 seasons, since actinomycetes are much less active in very moist soils. 

 Sanford suggested that the soil reaction may not be the important 



