20 DISEASES OF CULTIVATED PLANTS 



bearing such spores is shipped as fodder, or used for any 

 other purpose, and on its arrival in another country is eaten by 

 some animal, the probable result is a crop of fungi on the dung. 



Respecting diseases that can be dispersed by vegetative 

 methods of fungus reproduction, mycelium hibernating in 

 seeds, bulbs, tubers, etc., I have already shown that in the 

 case of the potato, the potato blight, also potato * leaf-curl ' can 

 be introduced from one district or from one country to 

 another without the possibility of detection. Unfortunately, 

 two other potato diseases come under the same category. 

 ' Black scab ' is one of these. In its most pronounced form 

 this disease is very conspicuous and unmistakable, but as I 

 have proved by many experiments, the spores may be present 

 in the ' eyes ' of a potato tuber without betraying their pre- 

 sence until ihe potato begins to grow, when the young shoots 

 are at once attacked. The fourth disease, known as ' black 

 leg' is caused by a bacterium (Bacillus phytophthorus\ and 

 hence is outside the realm of fungi. 



Some members of the family forming * smut ' have mycelium 

 perennial in the host-plant. In Scilla bifolia the ' smut ' in 

 the anthers originates from mycelium present in the bulb. 

 This mycelium persists in the bulb from year to year, hence 

 the anthers are always ' smutted.' Furthermore the perennial 

 mycelium present in those bulbs which produce young bulbs 

 passes into the latter, consequently every bulb originating 

 from a diseased parent inherits the disease, in fact it is what 

 we call in other instances * constitutional.' Smut in the anthers 

 of Lychnis diurna also originates from hibernating mycelium 

 present in the root. 



Carrots are often much injured by a minute fungus called 

 Phoma sanguinolenta ; when the disease is slight and escapes 

 attention, the roots may be planted the following year for 

 seed. If this occurs, the mycelium of the fungus hibernating 

 in the root passes up into the stem and prevents the formation 

 of seed. The hibernating mycelium of Peronospora schachtii 

 in the crown of beet and mangolds acts in a similar manner. 



The very destructive disease known as peach ' leaf-curl ' 

 has perennial mycelium in the branches, which grows along 

 with the shoot and enters the leaves each year. When the 

 leaves have fallen it is often impossible to detect the presence of 

 the fungus, and such infected trees could be sent to any part 

 of the world, and thus distribute the disease. 



In cases of true parasitism the relation between parasite 



