DISEASES OF SPECIAL CROPS 87 



vention must look largely toward the avoidance of wounds 

 by tools, machinery, harnesses, pickers' ladders, boots, etc. 

 It is also an excellent sanitary measure to cut out and burn 

 all infective material, and even excision of diseased tissue 

 in a limb may be practiced with profit in incipient cases. 



European canker {Nedria ditissima Tul. and Nedria 

 cinnabarina (Tode) Fr.). — The European canker was not 

 recorded upon the apple in America prior to 1899,' when 

 Paddock mentioned its presence in Nova Scotia and New 

 York. Later it was noted in New Hampshire. It con- 

 stitutes a serious disease in Europe and may spread so as 

 to be injurious here. The canker enlarges year after year, 

 but more slowly than the Sphaeropsis canker, and displays 

 when fruiting numerous minute, deep red perithecia which 

 serve to distinguish it from other cankers. 



Bark canker (Myxosporium corticolum Edg.).^This 

 canker in general aspect closely resembles the cankers 

 previously mentioned, except that the injury does not 

 penetrate the cambium zone. It is of little economic 

 importance. 



Leaf spot (see also black rot). — Aside from the leaf 

 diseases of apple already mentioned there are numerous 

 other leaf spots due to various, partly to unknown, causes. 

 These spots partake of the same general character. That 

 is, they are brown to tan colored, at first circular, later 

 irregularly circular, definitely bordered, and usually concen- 

 trically marked. If abundant, or if they enlarge rapidly 

 upon the leaves, they cause their premature fall, and 

 largely defoliate the tree. Such spots prevail to greater 

 or less extent in all apple orchards. Numerous species of 



' Paddock, W., Sci. u. ». IS, 297. 



