THE SPREAD OF DISEASE 213 



The following examples of the method of perpetuating 

 a disease by means of hybernating mycelium serve to 

 illustrate the gradual obliteration of spores as a means of 

 infection : 



(1) Both spores and hybernating mycelium in the seed 

 are necessary. 



This illustrates the least differentiated attempt to per- 

 petuate a disease by means of hybernating mycelium. 



Examples. ' Smut,' caused by species of Ustilago, in 

 some cereals and other grasses. 



(2) Hybernating mycelium alone is capable of per- 

 petuating the disease from one generation of the host-plant 

 to another; spores, however, are as yet produced, which 

 by their diffusion inoculate other plants, and thus serve to 

 extend the area of the fungus. 



Examples. ' Potato blight,' caused by Phytophthora 

 infestanS) and potato 'leaf-curl,' caused by Macrosporium 

 solani. 



(3) Hybernating mycelium located in the seed is alone 

 present. 



This phase illustrates the perfection of the modern 

 method of infection and dispersion of a fungus by means 

 of hybernating mycelium. The ancient method of infection 

 by means of spores has been completely obliterated. 



It has been assumed up to the present that the infection 

 of cereals by 'smut' (Ustilago) spores could only be 

 effected during the earliest seedling stage of the plant. 

 -Such infection was considered to be effected by spores 

 present in the soil, or more generally by smut spores 

 adhering to the ' seed ' and sown along with it. As a 

 preventive against such infection, it is the common practice 

 to treat ' seed ' before sowing with a solution of formalin, 



