44 



say: 6 "Observations made by the junior writer indicate that the 

 disease may have been present in an orchard in Bedford county, 

 Va., as early as 1903." The advance infections are widely scat- 

 tered. 



Back of the "immune zone" extensive areas must be inspected 

 frequently and thoroughly. Should the "immune zone" be lo- 

 cated at or north of the Potomac, the entire States of Virginia 

 and West Virginia must be covered by such inspection. There 

 is no knowing when or where the disease may break out, and 

 when conditions for its spread are favorable, a single diseased 

 tree overlooked may start an uncontrollable epidemic which Avill 

 necessitate establishing a new "immune zone" farther south and 

 starting all over. 



It is quite generally admitted that it will be difficult to locate 

 all of the diseased trees, but there is some difference of opinion as 

 to the importance of this fact. It may be argued that by the de- 

 struction of 90 or 95 per cent, of the diseased trees the spread of 

 the disease will be reduced to that extent. This is very improb- 

 able. If this disease behaves like fungous diseases in general, 

 its spread depends more upon weather conditions and the sus- 

 ceptibility of the host than upon the number of spores produced. 

 When the conditions for its spread are favorable five per cent, of 

 the spores may be sufficient to nullify any attempt to control the 

 disease. All experience with such methods of treatment goes 

 to show that the work must be done thoroughly, else it is not 

 effective. 



The history of the chestnut bark disease is unparalleled in the 

 annals of plant pathology. Here we have an unknown fungus, 

 none of the relatives of which are parasites, suddenly becoming 

 widespread and taking high rank as a destructive parasite. This 

 indicates that it may be expected to behave in an erratic manner 

 and be unusually difficult to control; also, that something 

 unusual has happened either to the host or to the fungus, or per- 

 haps to both, making this epidemic possible. Just what this 

 may be I am unable to say. There is no reason for believing 

 that the fungus is either a recent creation or a recent introduc- 

 tion from abroad. The only rational theory yet advanced re- 

 garding the origin of the epidemic is Dr. Clinton's winter-and- 

 drought-injury theory, 7 but even this seems insufficient in 

 respects. 



