43 



failure of tlie disease to spread to nearby trees was due to the 

 removal of the diseased trees? Is it not more likely that its 

 spread was prevented by the conditions being unfavorable for in- 

 fection ? 



Returning now to the main question : No such method of 

 controlling a fungous disease has ever been attempted. Our 

 knowledge of fungous diseases in general indicates that it is im- 

 practicable. It will be extremely difficult to locate all of the 

 diseased trees and absolutely impossible to remove all of the 

 fungus after the diseased trees are found. The fungus spores, 

 whidi are produced quickly and in enormous numbers may be 

 widely disseminated in several different ways, some of which 

 cannot be prevented. The work will be exceedingly expensive 

 and must be continued indefinitely. Taking all. these things 

 into consideration, the chances of success are much too small to 

 warrant the expense. 



It is true that some fungous diseases, notably the plum black 

 knot, are more or less successfully controlled by the prompt re- 

 moval of diseased plants or parts of plants; but it should be 

 noted that the diseases successfully controlled in this way have 

 two characteristics which make this method of control possible: 

 (1) The diseased plants mixj be readily detected in the early 

 stages of the disease; (2) the causal fungus requires a long time 

 to ripen its spores. Plum black knot may be readily detected 

 from one to several months before the ripening of the spores of 

 the causal fungus. Hence, the knots may be removed before 

 they have had a chance to spread the infection. Not so with the 

 chestnut disease. It possesses neither of tliese characteristics. 

 It is difficult to detect in the early stages, and multitudes of 

 spores may be produced within a month after infection. 



Undoubtedly, the spores are carried long distances by birds, 

 especially woodpeckers, which visit the diseased trees, seeking 

 borers, in the tunnels of which most of the infections occur.^ It 

 naturally follows that the "Immune zone*' must be many miles 

 wide, — Dr. Metcalf sngges^ts ten or twenty miles wide. In this 

 connection, please note that while the main line of infection is 

 now somewhere north of the Potomac river, advance infections 

 already occur in southern Virginia and West Virginia, 150 miles 

 or more soutliwest of Washington. In fact, Metcalf and Collins 



