JOMAL OF AGRlClllTDRAlESEARCH 



Vol. XVII Washington, D. C, May 15, 1919 No. 2 



''^>H^ 



INFLUENCE OF SOIL ENVIRONMENT ON THE ROOT- '^IZlT^''^ 

 ROT OF TOBACCO '''^' 



By James Johnson, Assistant Professor of Horticulture, College of Agriculture, Uni- 

 versity of Wisconsin, and R. E. Hartman, Agent, Office of Tobacco Investigations, 

 Bureau of Plant Industry, United States Department of Agriculture^ 



COOPERATIVE INVESTIGATIONS OF THE OFFICE OF TOBACCO INVESTIGATIONS 

 BUREAU OF PLANT INDUSTRY, UNITED STATES DEPARTMENT OF AGRICULTURE 

 AND THE WISCONSIN AGRICULTURAL EXPERIMENT STATION 



INTRODUCTION 



The foremost considerations in connection with the study of disease 

 in plants are the pathogenicity of the parasite, the susceptibility of the 

 host, and the environmental conditions favoring the infection and prog- 

 ress of the parasite. It is well known, however, that the relative patho- 

 genicity of the parasite and susceptibility of the host are not always 

 easily distinguishable one from the other in disease, and that they are 

 largely influenced by environmental conditions. To the practical grower 

 environmental conditions have been considered as all important, to the 

 exclusion of the parasite, while, on the other hand, the tendency in the 

 past on the part of pathologists and botanists has been to devote a great 

 deal of energy to the study of the parasite, with only passing interest 

 being given to the influence of the environment on disease, as recently 

 emphasized by Jones {16).^ This is especially true of plant diseases 

 having their origin or region of attack on underground portions of 

 plants. The literature upon actual experimental data with reference to 

 the influence of soil conditions upon a soil-infesting parasite is frag- 

 mentary, and for the most part concerned with only one or two variable 

 factors, so that the conclusions can not always be relied upon because 

 of failure to give due consideration to other factors perhaps even more 

 influential in the end result obtained. The Thielavia-rootrot of tobacco 

 {Nicotiana tabacum) forms a relationship of host and parasite appar- 

 ently admirably adapted for such experimental work in that it permits 

 quantitative determination of the influence of the disease upon the host ; 



' The writers are indebted to Dr. L. R. Jones, of the Department of Plant Patholojry, Wisconsin Agri- 

 cultural Experiment Station, for helpful suggestions, and to Dr. W. W. Gamer, of the Office of Tobacco 

 Investigations, Bureau of Plant Industry, United States Department of Agriculture, for critical reading 

 of the manuscript. 



2 Reference is made by number {italic) to "Literature cited," p. 85-86. 



Journal of Agricultural Research, Vol. XVII, No. 7. 



Washington, D. C. May 15, 1919. 



rs Key No. Wis. —is 



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