810 PRINCIPLES OF SOIL MICROBIOLOGY 



and sugar-beet scab, are true soil organisms. Application of lime, 

 which produces a favorable reaction, and of barnyard manure favor the 

 development of scab. The addition of acid fertilizers (acid phosphate) 

 or fertilizers which make the soil reaction acid (sulfur, ammonium salts) 

 tends to decrease the development of scab, as shown elsewhere (p. 301). 

 According to Millard, 38 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; actinomyces are much 

 less active in very moist soils) . 



Sanford 39 suggested that the soil reaction may not be the import- 

 ant factor in controlling the development of potato scab in the soil. 

 Moisture was found to be directly or indirectly the main factor, a high 

 moisture content controlling the disease, while abundant scab is formed 

 in dry soils. The development of scab is influenced also by the tem- 

 perature of the soil, the optimum for scab being 22°C. 40 



Plant and animal diseases caused by invertebrate animals found in the 

 soil. Among the animal pests present in the soil, we find protozoa, 

 nematodes and other worms, crustaceans, myriapods and insects. 

 Among the nematodes, we find Heterodera schachtii causing the disease 

 of mangels, Tylenchus tritici 41 of wheat, Heterodera radicicola causing 

 swellings or knots on roots of tomatoes, cucumbers, etc. 42 Tylenchus 

 dipsae (syn. devastatrix) causing the root knot on oats, tulip root, 

 clover (one form of clover sickness), and Aphelenchus olesistus causing 

 leaf blight. 



Relation of soil environment to plant infection. The soil environment, 

 including temperature, moisture, reaction and composition, has an 

 important controlling influence upon all plant parasites found in the 

 soil, whether they are obligate parasitic or can also exist in the soil 

 facultatively. These soil environmental factors may determine not 



38 Millard, W. A. Common scab of potatoes. Ann. Appl. Biol., 9: 156-164. 

 1922; 10: 70-88. 1923. 



19 Sanford, G. B. The relation of soil moisture to the development of common 

 scab of potato. Phytopathol., 13: 231-236. 1923. 



40 Jones, L. R., McKinney, H. H., and Fellows, H. The influence of soil tem- 

 perature on potato scab. Wis. Agr. Exp. Sta. Res. Bui. 53. 1922. 



41 Guenaud, C. Zoologie agricole et Entomologie et Parasitologic agricole. 

 Bailliere Ed. 



42 Bessey, E. Root-knot and its control. U. S. Dept. Agr., Bur. PI. Ind. Bui. 

 217. 1911. 



