536 SPECIAL PLANT PATHOLOGY 



the crop of peaches may be nil. The twig affection is sometimes 

 associated with gummy exudations, particularly when the enlargement 

 is terminal. It is doubtful whether the mycelium perennates in the 

 twigs, as was supposed in former years. Infection must generally 

 occur as the buds unfold. 



The mycelium of the fungus may be studied most advantageously 

 in the leaf before the fungus has appeared on the surface. At that time, 

 the hyphae show a greater protoplasmic content and sections reveal 

 the fact that the intercellular mycelium is distributed through the 

 mesophyll and cortex of the young stems. Pierce distinguishes vege- 

 tative hyphae, distributive hyphae and fruiting h}-phae. The latter 

 push up between the epidermal cells and a series of short hyphal cells 

 are formed, as ascogenous cells, which form an almost continuous layer 

 beneath the cuticle. The ascogenous cells give rise to the asci, which 

 push through the cuticle. An ascus is usually truncate at the exposed 

 end and it gives rise to four to eight ascospores, which may bud within 

 the ascus. 



Leaf curl may be controlled by the use of lime-sulphur solution 

 (1-20), Bordeaux mixture (5-5-50) and copper sulphate in water 

 (2-50), for the use of which the practical man is referred to the sprav 

 calendar given in the subsequent pages of this book. 



Pear (Pyrus communis L.) 



Fire-blight (Bacillus amylovorus (Bun.), De Trev. Toni).' — This 

 bacterial disease is found on the apple, pear and quince, but more 

 especially on the pear, so that it has been termed pear blight. It was 

 first reported from the northeastern United States, but now it is dis- 

 tributed throughout the country from the Atlantic to the Pacific 

 oceans. The disease first makes its appearance in the early part of the 

 season, when it appears in the form of a twig blight throughout the time 

 of blossoming of apples and pears, when the blossoms and tips begin 

 to wilt and show signs of blackening. This results in the complete 

 blackening and death of all the short branches, or spurs, upon which 

 flower clusters have been borne. The fire blight disease may continue 

 to extend down the twig, or the branch, the branch being entirely 

 killed, as it progresses. Under conditions more favorable to the host 



'Orton, C. R. and Adams, T. F.: Collar-blight and Related Forms of Fire- 

 blight. Bull. 136. Penna. Agricultural Experiment Station August, 191 5. 



