3l6 GKNERAL PLANT PATHOLOGY 



very strong hold upon the orchards in the Delaware and Chesapeake and 

 region, the north portion of the peninsula, and has destroyed thousands 

 and thousands of trees, rendering a great industry unprofitable and 

 precarious." The recent spread and virulency of the chestnut-blight 

 fungus, Endothia parasitica, from the neighborhood of New York City, 

 where it was probably first introduced, is so recent and fresh in the 

 minds of the public, that an extended account of the epiphytotism 

 (epidemic) need hardly be made here. The disease has practically 

 destroyed the native chestnut trees of the forested areas of the east- 

 ern states east of a line running northeast and southwest through 

 the central part of Pennsylvania . There have been a few sporadic cases 

 west of that line removed through the heroic efforts of the men em- 

 ployed by the Pennsylvania Chestnut Blight Commission, who with a 

 big appropriation of state money tried to find a way of heading off the 

 disease and finally controlling it but without success. Introduced in all 

 probability from China, where it has been found recently, the ravages 

 of this disease have been without precedent. 



As to the epiphytotic diseases of plants due to animals, we have a 

 number of instructive illustrations. The accoimt of the introduction, 

 spread and final control of the cottony cushion scale forms one of the 

 most interesting chapters in the history of American phytopathology. 

 Having been introduced from Australia to California in 1868, it 

 spread so rapidly during the next twenty years that its ravages proved 

 a very serious menace to the citrus industry of the southern part of 

 California. The Australian ladybird beetle, which was introduced 

 into California from Australia in i88g for the purpose of controlling 

 this scale, was so successful, that except for occasional outbreaks it 

 ceased to be considered a serious citrus pest. 



All of these epiphytotisms (epidemics) and others that might be 

 cited have been possible in all probability because the climatic condi- 

 tions of temperature, moisture, rainfall, wind and soil conditions have, 

 been favorable during the period of most active virulency, when the 

 diseases became firmly established. As an important contributing 

 cause may be considered the unhealthy, abnormal, or susceptible condi- 

 tion of the host plant owing to the methods of cultivation which have 

 reduced the disease-resisting capacity of the plant. In the case of 

 the chestnut, the restoration of the trees by sprouting from the stump 

 was undoubtedly one of the contributing causes of the rapid spread of 



