646 



Bulletin 81. 



Number 2 in Fig. 3 shows two knots, each at the end of the 

 growth of a season. The small lines running across the twig at 

 the point of union of the parts formed in different 

 seasons represent elevations and depressions in the 

 surface. These would also afford protection and 

 moisture to any spores which might lodge there. 

 A microscopic examination shows also some 

 ver}' thinlaj^ers of bark, and it seems probable 

 that an entrance could be affected here with- 

 out much diflBculty, provided atmospheric 

 conditions are favorable. 



Number 3'of the same illustration 

 represents what may be considered 

 as an explanation of the appearance 

 of knots upon older branches (Fig. 

 4), or even upon the trunks of the 

 trees. There can still be seen the 

 remnants of small spurs which were 

 probably first attacked. From these 

 spurs the fungus entered the cam- 

 bium layers of the larger portion, 

 and thus the knot formed, and the 

 part appears to have been directly 

 attacked, although entrance in real- 

 ity was gained through the spur, 

 as is shown in No. 4 of the same 

 figure. 



These examples and many others 

 of a similar character indicate that 

 it is only comparatively young 

 wood that is affected, the covering 

 of the larger branches being so thick 

 that it can withstand the attacks 

 of the disease. 



Remedies. — Although the black- 

 knot fungus has been troublesome 

 almost from the time that the stone fruits which it attacks 

 have been grown in this countrj-, no remedy has yet been found 



4. — K)wt upon a large branch. 



