UNITED STATES DEPARTMENT OF AGRICULTURE 



| BULLETIN No. 934 J 



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Contribution from the Bureau of Plant Industry 

 WM. A. TAYLOR, Chief 



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Washington, D. C. 



PROFESSIONAL PAPER 



June 16, 1921 



DAMPING-OFF IN FOREST NURSERIES. 



By Carl Hartley, formerly Pathologist, Office of Investigations in Forest 



Pathology. 



CONTENTS. 



Page. 



Damping-off in general 1 



Damping-off of conifers 7 



Causal fungi 27 



Cortieium vagum 27 



Fusarium spp 34 



Pythium debaryanum 35 



Rheosporangium aphanider- 



matus 55 



Phytophthora spp 59 



Miscellaneous phycomycetes 61 



Other fungi 64 



Relative importance of the damping- 

 off fungi on conifers 65 



Page. 

 Damping-off fungi as causes of root- 

 rot and late damping-off 70 



Relation of environmental factors to 



damping-off 73 



Density of sowing 74 



Moisture and temperature factors 75 



Chemical factors 79 



Biologic factors 82 



Acknowledgments S6 



Summary 86 



Literature cited 91 



DAMPING-OFF IN GENERAL. 



Damping-off is the commonest English name for a symptomatic 

 group of diseases affecting great numbers of plant species of widely 

 separated phylogenetic groups. It is commonly used for any disease 

 which results in the rapid decay of young succulent seedlings or soft 

 cuttings. Young shoots from underground rootstocks may also be 

 damped-off before they break through the soil (66). 1 The same term 

 is even used for diseases affecting the prothallia of vascular crypto- 

 gams (2). The name apparently originated in the fact that the dis- 

 ease is usually most prevalent under excessively moist conditions. 

 In those cases in which the disease becomes serious without the pres- 

 ence of unusual amounts of moisture the term is a misnomer. It is, 

 however, so thoroughly established in practical use that it would be 

 impossible, even if desirable, to establish any other name. 



1 The serial numbers in parentheses refer to " Literature cited," at the end of this 

 bulletin. 



19651°— Bull. 934—21 1 



