Pathogenicity of the Chestnut Bark Disease 61 



in the field except by the absence of the areas of fan-shaped my- 

 celium. The habitat of this fungus was carefully studied since 

 the question of its parasitism would be based largely upon these 

 observations. No case was found during these investigations 

 where the western fungus was found causing the death of a tree 

 or even growing on a live tree on an area free from other disease- 

 producing organisms. Aside from the chestnut it was found on 

 several oaks, among these the chestnut oak being its commonest 

 host. 



The Andersons (1) made a study of the "Connellsville" 

 fungus which revealed a number of striking characters by which 

 it could be readily distinguished from the so-called Diaporthe 

 parctsitica. The perithecia, asci and ascospores showed a number 

 of differences, the most marked of which was the size and shape 

 of the ascospores. Measurements of several thousand ascospores 

 from widely scattered localities in . western Pennsylvania, Vir- 

 ginia and Tennessee gave an average of 7 by 2.98 microns for the 

 "Connellsville" fungus. The measurement of an equal number 

 of ascospores of the so-called D. parasitica from western Pennsyl- 

 vania and New York showed an average size of 8.6 by 4.5 mi- 

 crons. There is also a very pronounced difference in the length 

 of the asci. The average length of all asci measured gave 34 

 microns for the "western" fungus and 51.3 microns for D. para- 

 sitica. As shown by the ascospore measurements those of D 

 parasitica are much wider in proportion to their length than those 

 of the "western" fungus. The septa in the ascospores of the true 

 blight fungus are very evident and a distinct sinus is shown on the 

 mature ascospores while the ascospores of the "western" fungus 

 lias a very indistinct septum and a very minute, if any, sinus. The 

 membrane composing the wall of the ascus is usually more evi- 

 dent in the "western" fungus. The "western" fungus has smaller 

 perithecia and much darker colored perithecial walls than the true 

 blight fungus. 



The most successful method of isolating the chestnut blight 

 fungus was to peel the outer bark with a sterile scalpel from over 

 the advancing edge of a young canker and transfer a small piece 

 of tissue just on the line between the healthy and diseased inner 

 bark to an agar slant or a Petri dish containing agar. The most 

 successful method employed in isolating the "western" fungus 

 was the conidial streak if the fungus was in the pycnidial stage. 

 If in the perithecial stage, the perithecia were induced to shoot 

 the ascospores upward onto a sterile agar plate inverted a few 

 millimeters above the ostioles. Isolations made from dead stumps, 



