Epidemic Plant Diseases. F. T. Brooks. 2;^^ 



during the first few years until a climax of intensit}- is reached, 

 after which their attack is less serious. Thus potato blight was 

 introduced into Eiu-ope during the early part of the nineteenth 

 century and rapidly spread so that in 1840 it was responsible for 

 a complete failure of the potato crop in Ireland, thereby causing 

 a serious famine: it is doubtful whether this fungus has ever 

 been so Wrulent since. Another illustration may be taken from 

 a disease which has been introduced into this country in recent 

 years. About 1900 American gooseberry mildew became acci- 

 dentally established in the British Isles, and immediately began 

 to spread rapidly, so that in a few years all fruit areas where 

 gooseberries were gro^^"n on a large scale were attacked by the 

 disease. After some years, however, this parasite became less 

 prevalent, and it may be that its period of maximum \-irulence 

 has passed, although there has been a considerable recrudescence 

 of the disease during 1922 and 1923. 



The example of American gooseberry mildew is an illustration 

 of the danger of the introduction of a parasite to a country where 

 it is not native. Sometimes the climatic and other conditions of 

 the new country are unfavourable to the parasite, but frequently 

 it is other\n5e, when there is grave risk of the estabhshment of 

 disease in epidemic form. There are two destructive tree diseases 

 in the United States which illustrate this principle. One of these 

 is a fungus, known as Endothia parasitica which, native on cer- 

 tain species of chestnut trees in China, has in some unknown 

 manner been introduced into the eastern parts of the United 

 States, where it has caused immense damage to indigenous 

 chestnut trees. The spores of this fungus germinate on slightly 

 wounded parts of the branches and trunks of these trees, causing 

 infection of the bark, which is rapidly killed. This fungus, at first 

 only sporadic, has spread over large areas, not\\'ithstanding the 

 costly efforts that ha\"e been made to check it by felhng \nde 

 belts of trees between diseased and healthy areas. 



Another disease which has aroused consternation in the 

 United States is the blister rust of five-needled pines, including 

 the valuable We\Tnouth pine, Finns strohus. It is known exactly 

 how this disease was introduced into America, and the story is 

 not uninteresting. The We\Tnouth pine is native to the United 

 States, and being a valuable timber tree, has been used for 

 forestry purposes in Central Europe. Nurseries of this tree were 

 established in European countries where there are other closely 

 allied pines which are attacked by the rust fungus, Cronartium 

 rihicola, the disease affecting chiefly the bark of the trees. Upon 

 the introduction of the We\Tnouth pine to these countries, it 

 also was attacked by this fungus. For some extraordinary' 

 reason American foresters sent to Europe for nursery stock of 



