AMERICAN CHESTNUT BLIGHT 699 



tisstie. To illustrate: Wood destroying fungi although, in case of a 

 living tree, apparently attack a living organism, do not do so, for they 

 w^ork in the lifeless wood of the central portions and are, therefore, 

 actually operating on inert or dead material. Obviously, immunity 

 can not develop here as no struggle takes place. In the blight the living 

 tissue is attacked (as the cambium and adjacent areas) and the life 

 forces are brought into action. If the sprouting continues long enough 

 to give time it would seem that all essential requisites are present for 

 immunity to develop or evolve. It is no more unusual or unreasonable 

 for this to happen than for parasitic fungi to become saprophytic or 

 the reverse. 



Recovery from Blight 



It is important to note the reaction of the host to the blight when 

 attacked at different points. Young sprouts are the best material to 

 observe. In large trees the blight works downward from the point of 

 attack beneath the bark and will eventually reach the base and kill the 

 entire tree if it is not killed by a reinfection below the original point 

 of attack. Now, many cases of healed cankers were observed by the 

 writer when small young sprouts were attacked near the top. The 

 blight was arrested in its downward movement and if the sprout was 

 killed later it was through reinfection from below. For convenience 

 the points of attack may be grouped as near (1) top of sprout, (2) 

 midstem, and (3) basal. In young sprouts, as before stated, recovery 

 was noted frequently in (1) occasionally in (2) and not at all in (3), 

 although much hypertrophy was noted in basal attack. A recent article 

 by Collins" records an apparently complete recovery (T years' dura- 

 tion) from a basal attack on a young sprout. It is interesting to note 

 his general statement on, and classification of, resistance: "In general 

 it may be stated that resistance has been more pronounced in the last 

 few years (four or five) than in earlier years," and further: "(a) the 

 disease is progressing more slowly, or (&) the host is rnore successfully 

 resisting the disease, or (c) the more susceptible trees have all been 

 killed, or (d) some other factors are influencing results." These ob- 

 servations certainly support the hypothesis that immunity is being de- 

 veloped in American chestnut following a period of struggle. 



CONTROL OF RESULTS 



It seems more than likely that the blight must nm its course in a 

 given locality and the chestnut develop immunity by natural processes 



