2 BULLETIN 1162, U. S. DEPARTMENT OF AGRICULTURE. 



Although crown rust is not of economic importance in Sweden, he 

 suggested that R. cathartica might cause it to become so and recom- 

 mended that this shrub be destroyed. 



In America the literature on the role of Rhamnus in spreading 

 crown rust has been confined to a few scattered observations. These 

 observations, supplemented by recent cultural studies by Melhus, 

 Dietz, and Willey (8), emphasized the necessity for a detailed study 

 of this problem, including such phases as the distribution of the genus 

 Rhamnus, the prevalence of secidial infection, the dissemination of 

 the secidiospores, and the subsequent spread of uredospores. It is 







Fig. 1.— Sketch map of Iowa, showing the distribution of four species of Rhamnus, two native and 



two introduced. 



the purpose in this bulletin to present data dealing with these phases 

 of the problem. 



RELATION OF RHAMNUS SPECIES TO THE EPIDEMIOLOGY OF 

 CROWN RUST. 



The part that Rhamnus species play in starting infections of 

 crown rust has been studied by the writer during the past five years. 

 Detailed studies have been made in Iowa throughout the season, from 

 before any crown rust was present in the spring until the oats were 

 harvested. These data have been substantiated by studies in the 

 adjoining States of Minnesota, Wisconsin, Illinois, and Missouri and 

 by periodical observations throughout the main sections of the United 

 States in which species of Rhamnus occur. The distribution of native 

 and introduced species of Rhamnus, or buckthorn, in Iowa is shown 

 in Figure 1. These data are based on records obtained by the writer 

 and his colleagues and by many other workers. 



