64 DISEASES OF TREES 



gonidia may be most quickly prevented by heaping on earth. If 

 the diseased plants occur singly they should be carefully pulled 

 out and buried in a firmly trodden trench, in order to guard 

 against the dissemination of the sporangia. In traversing the 

 bed the spread of this disease ought to be obviated as much as 

 possible by the workman not allowing his boots subsequently to 

 come into contact with healthy plants. The seed-bed should 

 also be inspected daily. 



PHYTOPHTHORA INFESTANS. THE POTATO DISEASE. 



Although the fungus which produces the potato disease had 

 been introduced from North America before 1845, it is only 

 since that year that it has assumed the dimensions of a plague in 

 Europe, where it always causes great loss in wet years. In the 

 mode of its distribution and in its dependence on wet weather it 

 very closely resembles P. omnivora. A characteristic feature is 

 the occurrence on the leaves of black blotches, which, con- 

 stantly increasing, in circumference, and finally embracing the 

 stems, may bring about the premature death of the parts of the 

 plant above ground. Although the tubers of diseased plants are 

 generally more or less affected, still this is sometimes the case 

 only to a small extent, being recognizable merely by a few brown 

 specks on cutting into the tuber. During wet years the tubers 

 often rot for the most part in the field, those that are less 

 attacked decaying in the cellar or pit during winter. These 

 changes (wet- rot) are to a large extent due to the action of 

 bacteria.* 



The mycelium of P. infestans passes the winter in the tubers, 

 and when these are planted out it grows into the sprouting shoots 

 invading the tissues of both stem and leaf. On examining the 

 neighbourhood of the black blotches, one recognizes, even with 

 the naked eye, a zone which is distinguished by its mouldy 

 appearance. Here are to be found the numerous sporangiophores, 

 for the most part projecting from the stomata. They agree 

 in shape with those of P. omnivora, and bear similar but more 

 numerous sporangia, which convey the disease to sound plants, 

 and are even carried by the wind to adjoining fields. There is 



* [See footnote on p. 38. ED.] 



