DISEASES OF THE CHESTNUT AND OTHER TREES 77 



develop characters which will enable it to attack trees outside 

 the genus Castanea is not impossible, particularly if it is an 

 imported fungus. 



The Origin of the Disease. 



The origin of the disease is unknown, and until more facts 

 are determined, all views on this subject must remain mere hy- 

 potheses. My owTi working hypothesis is, that the parasite is an 

 importation from some country other than North America. The 

 resistance of the Japanese and Korean chestnuts, coupled with the 

 fact that the Japanese chestnut has been extensively imported and 

 grown in that part of the country whence the disease appears to 

 have spread, suggests that eastern x4sia may be the home of this 

 parasite, as it is of the San Jose scale. (15) On the other 

 hand, we must remember that the Japanese chestnut is also said 

 to be resistant to the Marsonia leaf -spot, and to the "inky dis- 

 ease " of Europe. That the parasite has come from Europe seems 

 less probable, in view of the fact that, according to Pantanelli (17) 

 as well as according to my own inoculations under greenhouse con- 

 ditions, the European chestnuts show no resistance to the disease. 

 Still, a European or other extra-oriental origin is entirely possible, 

 since any immigrating organism may be expected to change its 

 habits and spread rapidly in the new country. The laws which 

 have conditioned the rapid spread of imported organisms — weeds 

 such as the Canada thistle and chicory, birds such as the English 

 sparrow, insects such as the San Jose scale, gypsy and brown-tail 

 moths, fungi such as the asparagus rust and potato blight — are 

 little understood. But in view of the history of the spread and 

 increased virulence of imported fungi in all parts of the world, it 

 is not necessary to assume any recent adverse change in the envi- 

 ronmental conditions of the American chestnut, such as winter 

 injury or drought, in order to account for the spread of the chest- 

 nut bark disease. We have not found it necessary in order to 

 account for past epidemics of plant disease, to assume that the 

 American asparagus, the English gooseberries, the grapes of 

 Europe, or the potatoes of practically the whole world, were 

 suffering from winter injury, drought, or other unfavorable en- 



