STATE POMOLOGICAL SOCIETY. 169 



[Bv Prof. William Trelease, in the Report of the Agricultural Experiment Station of the 



University of Wisconsin, 1S83.] 



The scab fungus does not seem to penetrate below the epidermal 

 layer of cells. These are split open and destroyed. The cells 

 immediately below them are usuall}- somewhat shrunken and flat- 

 tened, while their contents are dead or injured, as is shown b}' their 

 brown color. Sometimes, but not alwa3's, a small mass of tissue 

 lying under the scab is quite green and intensely bitter, but the 

 injury' seldom extends far beneath the surface of the fruit, unless 

 this is deeph' cracked, and the seeds of scabb}' apples are apparently 

 as vigorous as those of sound specimens. Usually, as the spot 

 grows older and enlarges, an effort is made to throw off the parasite 

 and heal the wound bv the formation of a laver of cork iust below 

 it, and it is this brown cork which is seen through the cracks in old 

 scabs. 



From what has been said, it appears that the leaf or twig mildew 

 and the scab of the fruit are diseases caused by the same parasitic 

 fungus, which at first lives beneath the cuticle, in the one case, 

 and in the epidermal cells, in the other. I have not yet been able 

 to learn how the parasite penetrates the epidermis. It is well 

 known that the skin of many varieties of the apple is marked b}' 

 simple or areole dots which occur naturallv on the health}' fruit, and 

 are similar in their nature to the corky dots called lenticels that are 

 found on the branches of the apple tree. These dots are slightly 

 elevated portions of the skin where the epidermis is commonly 

 split, and I suspect that the spores often find a lodgement in these 

 dots, and through them reach the surrounding cells, when the}' 

 germinate. 



The age at which the fruit ceases to be infected is also uncertain. 

 Last spring Mr. Tuttle found the young apples scabbed when no 

 larger than peas, and it appears probable that the disease may 

 attack them at any time from the unfolding of the blossom to their 

 maturity ; but the scab does not seem to spread from one apple to 

 anotlier in the barrel, though, as has been said, the individual spots 

 continue to grow on the ripe fruit. 



Those conditions of the soil and atmosphere which favor the 

 development of leaf mildew are, in the main, most favorable for the 

 formation of scab on the fruit, although hotter weather may be neces- 

 sary for the excessive development of the former. This fact, 

 taken in connection with the greater prevalence of active spores 





