THE APPLE TWIG BLIGHT 39 



away by the wind. Those which light upon red cedar, 

 under favorable conditions, may develop, and by so 

 doing carry the disease back to the original host plant. 

 Thus there is a continual alternation of the generations 

 of the fungus. 



The form of the fungus which occurs upon apples is 

 commonly called the Rcestelia form. Besides the dis- 

 colored areas on the ujoper surface of the leaf, the pecu- 

 liar ^^cluster-cups" of the fungus are developed on the 

 Tinder surface. A group of these is represented in Fig. 

 20. The ^^cups" are usually about a quarter of an inch 

 in diameter. In them the chocolate brown spores are 

 developed ; some of these are seen in 2, where the thin 

 places in the wall through which the germinating tubes 

 are to develop are seen at a, while in 3 some germinating 

 spores are represented. 



Treatment. — The knowledge of the fact that one 

 •essential stage of the fungus is passed upon red cedar, 

 leads to the suggestion that, in a given region, the injury 

 to apples may be prevented by destroying the red cedars. 

 It is probable that spraying orchards with fungicides for 

 scab, or other diseases, will also assist in preventing 

 the rust. 



Literature. — Dr. B. D. Halstead has published an 

 -excellent general discussion of the relations of apple 

 rusts and cedar ap23les, in the 1888 Report of the De- 

 partment of Agriculture (pp. 370-381). Professor A. B. 

 Seymour has also treated of them in the Transactions of 

 the American Horticultural Society (v. IV, p. 152), and 

 Dr. E. Thaxter, in the 1891 Re2:>ort of the Connecticut 

 Experiment Station. 



The Apple Twig Blight 



Micrococcus amylovorus 



The twigs of apple trees, together with the leaves, 

 flowers or fruit which thev bear, often turn brown or 



