SOIL AS HABITAT FOR PATHOGENIC MICROORGANISMS 807 



with potatoes as well as from virgin desert lands. 19 Disease-free seed 

 planted on new lands yielded a diseased product. Land previously- 

 planted to alfalfa, clover or grain is better adapted to the production of 

 disease-free potatoes than virgin land. Various species of Phytoph- 

 thora may persist in the soil for considerable periods of time and can 

 stand the low winter temperatures without much injury; they can 

 also resist some desiccation. Ph. infestans can live saprophytically in 

 the soil, growing on old, partially decomposed plants. Their patho- 

 genicity is not diminished by living in the soil. 20 A number of these 

 and other organisms are facultative parasites, in other words, they are 

 capable of growing in the soil in the absence of the host plant. The 

 spores of Sclerotinia trifoliorum, for example, were found 21 to give rise 

 to a mycelium which is at first saprophytic and then becomes faculta- 

 tively parasitic. The spores appear to germinate on vegetable residues 

 in the soil; the mycelium spreads over the soil at a rate which depends 

 on the environmental conditions. These plant pathogenic fungi com- 

 prise several distinct groups: 



1. Various damping off fungi, including Pythium debaryanum, Sclerotinia 

 libertiana, Phoma solani, Sclerotium rolfsii, Rhizoctonia solani (Corticium vagum), 

 species of Fusarium and Colletotrichum. 22 



2. Root rots as well as other root infections comprising a number of fungi, 

 such as Rh. solani and other species of Rhizoctonia. 23 The constant culture of 

 wheat on the same soil will bring about a condition of "wheat sickness." This 

 is not a question of soil infertility or thu formation of toxins detrimental to 

 wheat, but is due to the introduction of fungi which cause various wheat dis- 

 eases, by blighting, rotting and destroying the roots. These fungi are capable of 

 persisting in the soil, living on the decomposing straw. In this group are species 

 of Macrosporium, Alternaria, Helminthosporium, Fusarium and Colletotrichum. 



3. Wilts. Fusarium oxysporum has been isolated as a soil saprophyte. 24 



19 Pratt, O. A. Soil fungi in relation to diseases of the Irish potato in Southern 

 Idaho. Jour. Agr. Res., 13: 73-100. 1918. 



20 Bruyn, H. L. G. de. The saprophytic life of Phylophthora in the soil. Medd. 

 Landbou. Wageningen., 24. 1922; The overwintering of Phytophthora in- 

 festans (Mont.) DeBy. Phytopath., 16: 121-146. 1926. 



21 Wadham, S. M. Observations on clover rot (Sclerotinia trifoliorum Eriks). 

 New Phytol., 24: 50-56. 1925. 



22 Hartley, C. Damping-off in forest nurseries. U. S. Dept. Agr. Bui. 934. 

 1921. 



23 Bolley, H. L. Wheat: soil troubles and seed deterioration. N. D. Agr. 

 Exp. Sta. Bui. 107. 1913; Beckwith, T. D. Root and culm infections of wheat 

 by soil fungi in North Dakota. Phytopathol., 1: 169. 1911. 



24 Goss, R. W. Relation of environment and other factors to potato wilt 

 caused by Fusarium oxysporum. Nebraska Agr. Exp. Sta. Res. Bui. 23. 1923. 



