32 



THE BITTEE EOT OF APPLES. 



the humus compounds. It is probably one of the decomposition prod- 

 ucts which forms when the bark and cambium are killed and which 

 infiltrates the wood. One finds numerous fungous hyphae in the 

 medullary ray cells and the larger vessels, but at this stage it is not 

 possible to say whether these are hyphee of the bitter-rot fungus. 

 Further studies in this direction are being made. 



The formation of the cankered spot probably starts at some small 

 wound (or branch, as stated above). The fungus begins to grow in 

 the living bark and kills the bark and the cambium. As a result no 

 new wood is formed at the point where the cambium is killed (see PI. 

 VIII. fig. 1). and a small depression forms as the wood at the edges of 

 the dead cambium increases in thickness. As the fungus grows out 

 from the original point of infection, more and more bark and cambium 

 are killed, until at the end of the growing season a large spot on the 

 limb is dead. 



Since there is always a small series of wood cells formed at the 



beginning of the year during which 

 the attack takes place, the fungus 

 probably starts to grow in the bark 

 earh T in June. (See fig. 6.) 



The majority of the cankers found 

 during the last summer probably were 

 started two years ago. During the 

 first year the fungus made very little 

 headway. A very small central area 

 was killed, generally around and in- 

 cluding a small branch. The following 

 year the larger part of the canker was 

 formed. Whether the cankers will con- 

 tinue to increase in size is as yet undetermined, but it does not seem 

 probable, for if such were the case cankers three or more years old 

 ought to have been secured in the orchards where the bitter rot has 

 been common for manv years. 



Fig 



-Diagrammatic cross section of 

 apple canker. 



RELATION OF THE CANKERS TO THE BITTER ROT. 



The discovery of the cankers was brought about directly by tracing 

 groups of diseased apples to these sunken areas on apple limbs. The 

 numerous observations made by Mr. Simpson and by those who fol- 

 lowed him seemed to prove beyond question that the cankers were in 

 some way responsible for the infection of the apples. Instances were 

 frequent where two or more apples hung just below a canker. These 

 were generally badly diseased, while all other apples in their immedi- 

 ate vicinity were perfectly healthy. 



Although it seemed extremely probable from Mr. Simpson's obser- 

 vations, confirmed and extended by the writers, in his orchard and 



