SOIL AS HABITAT FOR PATHOGENIC MICROORGANISMS 813 



above, favors potato scab, 53 all point to the conclusion that when these 

 potential parasites are present in the soil the occurrence and severity of 

 the disease must be interpreted as a resultant of several variable en- 

 vironmental factors operating simultaneously. 



The amount of organic matter present in the soil also influences 

 plant infection, since it offers a source of energy for the saprophytic 

 existence of the organisms. Thielavia basicola cannot infect the host 

 plant in pure sand, but can do so in the presence of organic matter, 

 which allows the mycelium to exist for some time. 54 Clay soils are more 

 favorable to infestation than sandy soils. 55 The decay of the under- 

 ground portion of the pea plant is largely due to four fungi : 56 Fusarium 

 martii var. pisi, Pythium debaryanum, Corticium vagum and a species of 

 Aphanomyces. The Fusarium is not disseminated by the seed, but 

 spreads through the soil and is especially favored by a high content of 

 organic matter. Ozonium omnivorum Shear, the cotton and alfalfa 

 root rot, spreads through the soil radially with a growth similar to 

 fairy rings; it is favored by heavy soils, humid weather and dense cover 

 crops. 5758 The influence of soil and manure on plant diseases has been 

 recorded elsewhere, 59 as well as the influence of soil nutrients and soil 

 structure upon plant infection. 60 



Influence of reaction upon the growth of plant pathogenic organisms in 

 the soil. Some plant pathogenic organisms are readily affected by 

 certain hydrogen-ion concentrations of the soil which are not injurious 



53 McKinney, H. H. Influence of soil temperature and moisture on infection 

 of wheat seedlings by Helminthosporium, sativum. Jour. Agr. Res., 26: 195-218. 

 1923. 



64 Massee, C. E. A disease of sweet peas, asters, and other plants. Roy. 

 Gard. Kew. Bui. Misc. Inform. No. 1. 1912, 44-52. 



56 Johnson, J., and Hardman, R. E. Influence of 'soil environment on the 

 root-rot of tobacco. Jour. Agr. Res., 17: 44-52. 1919. 



56 Jones, F. R. Stem and root rot of peas in the United States caused by species 

 of Fusarium. Jour. Agr. Res., 26: 459-476. 1923. 



57 Duggar, B. M. The Texas root rot fungus and its conidial stage. Ann. 

 Mo. Bot. Gard., 3: 11-23. 1916. 



68 King, C. J. Habits of the cotton root-rot fungus. Jour. Agr. Res., 26: 405- 

 418. 1923. 



69 Ehrenberg, P. Der Einfluss des Bodens und der Dungung auf Pflanzen- 

 krankheiten. Fuhlings Landw. Ztg. 1917, 130-132; 1919, 401-412. 



60 Levine, M. Studies on plant cancers. III. The nature of the soil as a 

 determining factor in the health of the beet, Beta vulgaris, and its relation to the 

 size and weight of the crown gall produced by inoculation with Bacterium lume- 

 faciens. Amer. Jour. Bot., 8: 507-525. 1921. 



