41 



say :'^ ^'Observations made hy tlie junior writer indicate that the 

 disease may have been present iu an orcliard in Bedford county, 

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

 tered. 



Back of the ''immnne zone" extensive areas mnst be inspected 

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

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

 and West Virginia must be covered by such inspection. There 

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

 wlicn conditions for its spread are favorable, a single diseased 

 tree overlooked may start an uncontrollable epidemic wliich will 

 necessitate establishing a new "immune zone" farther south ainl 

 starting all over. 



It is (]uite generall}' admitted that it Avill be difficult to locate 

 (ilJ 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 l)e 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 l)e 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 thorouglily, 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 rnnk 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 botli, making tliis epidemic possible. Just what this 

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

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

 tion from alu'oad. The only rational theory yet advanced re- 

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

 drouglit-in jnry dicoi-y,' l»nt <'ven (his seems insnlticient in som<! 

 respects. 



