41 



say:® "Observations made bv the junior writer indicate that the 

 disease ma}- have been present in an orchard in IJedford county, 

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

 tered. 



Back of lilt' ''iniMiune zone"' extensive areas must be inspected 

 freciucnlly and thoroughly. Should the "innnune zone" be lo- 

 cated at or north of the Potomac, the entire States of Virj^inia 

 and West Virginia must be covered by sucli inspection. There 

 is no l\i!(t\\iiig wlicii or w licre the disease may brcaic oul, and 

 wlicii coiididoiis for its S])read are favcn-able, a single diseased 

 tree ovei'htoked may start an uncoutrolhilile epich'niic which will 

 necessitate establishing a new "immnne zone"" fjiilliei- ,s(MiIIi ;iiid 

 starl iiig ill! ovei-. 



It is (|nite generally admitted that it will l)e difficult to locate 

 (ill of the diseased trees, but there is some dilfenMice of oi)inion as 

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

 struction of 1)0 or 95 per cent, of the diseased trees the spre;id of 

 the disejise Mill be reduced to tlnit extent. This is very imi)rob- 

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

 its spread de])ends more upon weather conditions and the sus- 

 cei>tibilily of the host than upon the nund)er of spores produced. 

 When the c(»ndilions for its spread are favorable five per cent, of 

 the spores may be sulficient lo nullify any attempt to con(i-ol the 

 disense. All experience with such methods of treatment goes 

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

 eifective. 



The liisloiy of the chestnut bark disease is nn]»;ir;ille]<Mi in the 

 aniiMls of i)lant pathology. Here we have an unknown fungus, 

 none of (he relatives of which are ])arasites, suddenly becoming 

 N\ idespreiid ;iiid taking high r:ink ;is a destructive i)arasite. This 

 indicates that it may be exi)ectei] to b('lia\(' in an ci-ralic manner 

 ami be unnsnally difficult to conlrcd; also, lliat sometiiing 

 unusual has hapi)ened either to the host or to the fungus, or ju'r- 

 hai)S to both, making this epidemic possible. -Inst what this 

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

 tliat the fungus is eitlier a recent ci'ealion or a recent introduc- 

 tion from altroad. The only rational thecu-y yet advanced re- 

 ganling the origin of the e])idemic is Dr. Clinton's wint(M"-and- 

 drought-injui'V theory,' but even (his seems insufficient in s<niie 

 respects. 



