34 THE BITTER ROT OF APPLES. 



orchards, might be the canker-forming fungus. This supposition 

 was strengthened by the fact that many of the Illinois cankers had 

 the sooty black appearance characteristic of the black-rot apple 

 cankers. 



To determine whether the bitter-rot fungus {Glomerella rufo- 

 maculans) could form cankers, a number of trees in the Missouri 

 Botanical Garden were selected. Small longitudinal slits were cut 

 into the bark, reaching the cambium layer, two slits on every branch. 

 Into the upper slit spores from pure cultures of Glomerella rufomacu- 

 lans (made from diseased apples and from cankers) were introduced. 

 The second slit, from 3 to 5 inches below the first, was used as a con- 

 trol. A large number of control slits were used, as it was possible for 

 spores flying about in the air to enter the infected slits and thereby 

 vitiate the results. It may be said at this point that in no case did 

 any of the control slits show any signs of canker formation. A num- 

 ber of inoculations were made, using pure cultures of Glomerella 

 rufomaculans obtained from apple cankers in Illinois and from 

 diseased apples. Inoculations were likewise made with ascospores 

 obtained in apple agar cultures. The first infection of apple limbs 

 was made July 16, 1902. 



The inoculations were made by inserting a needle with spores into 

 the freshly made slit, or by spraying water into the slit and then 

 placing some spores in the drop of water. Some of the slits were 

 covered with grafting wax or with cloth waxed with a cocoa-butter 

 mixture, as it was thought that the uncovered slits might be infected 

 by bitter -rot spores from the air. The results showed that this pre- 

 caution was useless, as none of the uncovered control slits showed an}' 

 signs of being infected. 



Several weeks elapsed before there was an}' evidence of develop- 

 ment on the limbs. In both the inoculated slits and the control slits 

 the bark dried somewhat along the edges of the slit, making a gaping 

 wound. After some two weeks a distinct callous layer had formed 

 under the edges of the bark of the control slits. The two callous 

 layers joined after six to eight weeks and occluded the wound. In 

 the slits where bitter-rot spores had been inserted the callous forma- 

 tion was less marked. The exposed wood turned dark, almost black, 

 and the exposed edges of the bark turned back. The living bark 

 then began to dry out gradually and became depressed (PI. IX, fig. 3), 

 and after about two months a decided sharply defined depressed area 

 had formed, with the slit in the center. Shortly thereafter small 

 black pustules broke through the dried bark in a number of instances 

 (PI. IX, figs. 2, 4, 5, and 6). By that time the infected points showed 

 all the characteristics of small cankers. 



On examination the black pustules were found to contain masses 

 of spores resembling those of the bitter-rot fungus ( Glomerella rufo- 



