UNITED STATES DEPARTMENT OF AGRICULTURE 



JTL^'^S'U 



BULLETIN No. 395 



Contribution from the Bureau of Plant Industry 

 WM. A. TAYLOR, Chief 



Washington, D. C. 



PROFESSIONAL PAPER. 



January 18, 1917 



PEACH SCAB AND ITS CONTROL/ 



By G. W. Keitt, 



Assistant Professor of Plant Pathology, University of Wisconsin; formerly Scientific 

 Assistant, Fruit-Disease Investigations. 



CONTENTS. 



Page. 



Introduction 1 



The disease 3 



Geographic distribution 3 



Economic importance 4 



Description 6 



Pathological histology S 



The causal organism 11 



Taxonomy 11 



Morphology 12 



Physiology 14 



Pathogenicity 21 



Life history of the causal organism in relation 



to pathogenesis 32 



Seasonal development of the disease 32 



Production of conidia 33 



Viability and longevity of conidia 35 



Life history, etc. — Continued. 



Dissemination of conidia 36 



Method of infection 40 



Period of incubation 41 



Time of natural infection 42 



Som-ces of natural infection 42 



Overwintering of the fungus 43 



Climate in relation to the disease 44 



Varieties in relation to the disease 44 



Control measures 46 



Spraying 46 



Orchard sanitation 60 



Resistant varieties 60 



Summary 61 



Literature cited 64 



INTRODUCTION. 



Peach scab (Cladosporium carpopMlum Thiim.) is a parasitic 

 disease whicli affects the fruit, twigs, and leaves of the host (Amygdalus 

 persica). It manifests itself on the fruit as small, circular, olivaceous 

 to black spots, which frequently become confluent over considerable 

 areas and seriously detract from the appearance, quality, and value 

 of the marketable product. On the twigs and leaves it occasions less 

 damage, producing small superficial injuries, which are described in 

 detail later. In the United States, east of the Rocky Mountains, it 

 occurs generally, and unless controlled it may cause serious financial 



1 The laboratory studies upon which this paper is based were conducted in field laboratories at Hart, 

 Mich., in the season of 1911, at Cornelia, Ga., in the seasons of 1912 and 1913, and in the laboratory of plant 

 pathology of the University of Wisconsin in the winters of 1911 to 1914 and the summer of 1914. The 

 field work was eaiTied on in cooperation with growers in commercial orchards at Hart, Mich., in 1911, and 

 at Cornelia, Ga., in 1910, 1912, and 1913. 

 48408°— Bull. 395—17 1 



