RHAMNUS IN THE DISSEMINATION OF CROWN RUST. 17 



RHAMNUS PURSHIANA. 



Rhamnus purshiana DC. is a western species occurring commonly in 

 California, Oregon, Washington, Idaho, and Montana. Its natural 

 habitat is at an elevation where cultivated fields do not occur. 



Although R. purshiana has been infected with the secidial stage of 

 crown rust in the greenhouse, it probably is not a source of infection 

 in oats, as it grows at high altitudes in the mountains while oats are 

 grown in the valleys. 



RHAMNUS CALIFORNICA. 



This species and the variety tomentella are largely confined to the 

 western coast of the United States. The field observations on Rham- 

 nus californica Esch. are limited, but probably it plays little or no 

 part in the spread of crown rust to oats. It grows in a warm tem- 

 perate climate where the uredospore stage of crown rust exists 

 throughout the entire year. 



RHAMNUS CROCEA. 



Experiments (8) have show^n that Rhamnus crocea Nutt. will produce 

 secidiospores when exposed to infection with teleutospores obtained 

 from Calamagrostis canadensis. The results of experiments conducted 

 in 1922 gave a like degree of infection with teleutospores from Festuca 

 elatior and Avena sativa. These data indicate that this species might 

 be able to play an important role in the spread of crown rust to oats. 

 However, R. crocea is limited to a small area in southern California 

 and to certain islands off the coast and is unimportant because it 

 commonly does not occur within an oat-producing area. 



SUMMARY. 



Rhamnus cathartica L., a native of Asia, has long been used as an 

 ornamental shrub in this country and has escaped commonly into 

 wood lots and waste lands throughout the north-central and north- 

 eastern United States. 



This shrub commonly bears aecidia of crown rust (Puccinia coronata 

 Corda) , and was found to be the cause of severe epidemics of crown- 

 rust in Iowa, Wisconsin, Minnesota, and Illinois. 



The epidemic at Hinton, Iowa, caused by secidial infection on a 

 520-rod hedge, was so severe in 1921 that the oats in adjoining fields 

 were not harvested. 



Topography, wind, and other climatic conditions during the time 

 the secidiospores are forming probably are the most important factors 

 in the spread of crown rust. Wild or cultivated grasses growing 

 near the infected hedge apparently had little influence on the spread 

 of the rust. 



iEcidiospores from R. cathartica caused infection to a distance of 

 452 rods, or nearly 1£ miles, in seven days, or before the first uredo- 

 spores were ripe. The infection was heavy near the hedge and 

 progressively lighter as the distance from it increased. After the 

 first generation of uredospores was mature, the spread of infection 

 was very rapid. Thirteen days later oats were infected at a maximum 

 distance of 53 miles from the hedge. 



