UNITED STATES DEPARTMENT OF AGRICULTURE 
jj| BULLETIN No. 395 Mm/Ik 
^Ht^ Contribution from the Bureau of Plant Industry ^^r^P^ fj£? 
\»0>t WM. A. TAYLOR, Chief ^^P&WS^T 
ssjv&su 
J&F^U 
Washington, D. C. 
PROFESSIONAL PAPER. 
January 18, 1917 
PEACH SCAB AND ITS CONTROL. 1 
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 8 
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 
Page. 
Life history, etc.— Continued. 
Dissemination of conidia 36 
Method of infection 40 
Period of incubation 41 
Time of natural infection 42 
Sources 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 (Cladosporiwn carpopliilum, Thum.) is a parasitic 
disease which, 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 hi 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 carried 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 
