A PRELIMINARY NOTICE OF GENETICAL STUDIES 
OF RESISTANCE TO MILDEW IN OENOTHERA ^ 
Peter J. Klaphaak and H. H. Bartlett 
(Received for publication December 24, 1921) 
Introduction 
Ever since the senior writer of this paper commenced the growing of 
Oenothera cultures for experimental purposes, it has been noticed every 
year that nothing is more characteristic of the various elementary species 
and hybrids than the great differences that they show in susceptibility to 
infection by mildew. Thus, among the recently described species, such 
ones as Oenothera stenomeres and Oe. pratincola have been uniformly, year 
after year, heavily infected. Others, such as Oe. Reynoldsii, Oe. numis- 
matica, and Oe. scitula, have been quite as uniformly immune. Similar 
facts have come to the attention of Professor de Vries, who, in a recent 
letter, writes that certain of the types grown by him would have been 
admirably adapted to a study of the inheritance of immunity. 
For several years prior to 1919, more or less adequate notes had been 
kept in the garden as a whole as to the prevalence of mildew, but it became 
obvious that the solution of the problem would demand special cultures of 
forms particularly marked in their resistance or susceptibility, as the case 
might be, which might be handled with the question of disease resistance 
paramount. Furthermore, since the differences shown by certain pairs of 
reciprocal hybrids were so astonishingly definite, the one being white with 
mildew and the other absolutely free, although both were grown in ad- 
joining rows, under identical conditions, and often with interlocking 
branches, it seemed that the material offered an excellent opportunity for 
biochemical studies, to be conducted parallel with the genetical work, and 
designed to trace, if possible, the relationship of immunity and suscepti- 
bility to chemical characters of the forms. Consequently, a biochemical 
study of carefully selected material from these cultures has been under- 
taken by Mr. Joaquin Mejorada Marafion. His results cannot be reported 
in this preliminary notice, which, even on the genetical side, aims to present 
only part of the results, typical of those which are being obtained. 
Most of the previous work on the problem of varietal and specific re- 
sistance to mildews has been done by Salmon (8, 10). He has published 
several short papers on varietal susceptibility to the powdery mildew of 
^ From the Bureau of Plant Industry, U. S. Department of Agriculture, and the 
Botanical Garden of the University of Michigan. Published by permission of the Secretary 
of Agriculture. 
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