SOURCE OF INFECTION. 11 



spore find their way thi'outih the t;kin of the apple. The most coninion 

 belief expressed by writers upon this siil)iect is that the fundus enters 

 through insect punctures or some other al)rasion of the skin, and it has 

 also been suj^crested that the f unj^nis couJd prol)ably enter through the 

 unbroken skin. The writers obser\ations would indicate that a wound 

 is not at all necessary for successful infection and that ti)e fungus most 

 connnonly penetrates the skin. Several hundred points of infection 

 were examined without finding any indication of a previous puncture. 

 A large percentage of the apph^s on the untreated trees used as checks 

 in the spraying experiment had from one hundred to a thousand points 

 of infection, and in many cases the spots were so thick that when only 

 one-sixteenth of an incli in diameter they overlapped. In the labora- 

 tory, infections were easily made by dropping water containing spores 

 on the unbroken skin of an apple in a moist chamber. 



BITTEK-HOT CANKKRS ON THE BRANCHES. 



In 1902 it was discovered by Mr. R. II. Simpson, of Illinois, that 

 iiml) cankers were associated with outbreaks of bitter-rot. Messrs. 

 Burrill and Blair", of the Illinois Agricultural Experiment Station, and 

 Messrs. von Schrenk and Spaidding'', of the Mississippi Valley Labora- 

 tory of the Bureau of Plant Industry, working independently, soon 

 established the relationship of these cankers to the disease on the fruit 

 and proved by inoculation tests that these cankers were caused by the 

 same fungus that attacks the fruit. 



In describing this form, von Schrenk and Spaulding' state that 

 ''The cankers found on apple trees in Illinois appear as blackened 

 depressions on apple limbs of various sizes, from last year's fruit 

 spurs to limbs 3 to 4 inches in diameter. Thus far the cankers have 

 not been found on the main trunk. On these limbs rounded or oblong 

 sooty-black sunken spots occur from one to several inches long, which 

 have more or less ragged edges." Limb cankers occur abundantly in 

 the Virginia orchards, but the writer has so far been unable to find 

 the bitter-rot fungus associated with any of them. However, limbs 

 of young apple trees on the grounds of the United States Department 

 of Agriculture inoculated with bitter-rot spores rapidly develqped 

 these cankers. 



SOURCE OF INFECTION AND SPREAD OF THE DISEASE. 



The question as to the chief source of the first infection each year 

 has not been satisfactorily settled, nor is it definitely known how the 



"Circ. 58, 111. Agr. Exp. Sta., July, 1902, and Bui. 77, 111. Agr. Exp. Sta., 1902, pp. 



355-357. 



'> Bui. 44, Bureau of Plant Industry, U. S. Dept. of Agriculture, 1903, pp. 29-36. 



cL. c, p. 31. 



