288 Disease-Producing Microorganisms 



soils on which the particular host plant has never been grown be- 

 fore. Fusarium radicicola and Rhizoctonia solani, known to be para- 

 sitic on the Irish potato, were isolated from soils never cropped 

 with potatoes and from virgin desert lands. Disease-free seed 

 planted on new lands frequently yielded a diseased product. Soils 

 in which clover or grain was previously grown are better adapted 

 to the production of disease-free potatoes than is virgin land. Some 

 plant-pathogenic fungi are able to persist in soil for many years; 



'»'yx;' A."^, 



\-.x 



"^^ 







Fig. 115. An antagonistic fungus, Tridwdcrma, attacking a plant pathogenic 

 fungus, Sclewthim roJfsii, showing one break of a septum (from WeindUng). 



flax, for example, must be grown only on new soils. Various species 

 of Phytophthora can withstand low winter temperatures without 

 much injury; they can also resist some desiccation; Ph. infestans can 

 live saprophytically in soil, growing on old, partly decomposed 

 plants. The pathogenicity of these fungi is not diminished by living 

 in the soil. Many plants are infected by fungi, the spores of which 

 may not live in the soil but may adhere to the seeds and produce a 

 mycelium, which, on the germination of the seed, is able to attack 

 the seedlings. 



Many of these fungi are facultative parasites, since they are able 

 to grow in soil in absence of the host plant. The spores of Sclerotinia 

 trifoUoriini can give rise to a mycelium which is at first saprophytic 

 and then becomes facultative parasitic. These spores germinate on 

 the vegetable residues in the soil; the mycelium spreads over the 

 soil at a rate which depends on environmental conditions. 



