RHAMNUS IN THE DISSEMINATION OF CROWN RUST. 



13 



crown rust and whether or not it is instrumental in starting the 

 initial infection on oats and other grasses. The situation has been 

 studied in detail un Iowa, and many observations have been made 

 throughout the region in which this shrub grows. The principal 

 studies were carried on near Indianola, as this town is located about 

 in the center of the area in which R. lanceolata occurs in Iowa. 

 These studies were further substantiated by observations at Bloom- 

 field and Osceola. 



Each spring since 1917 field observations have been made on R. 

 lanceolata in the vicinity of Indianola to ascertain its relation to a 

 yearly infection of crown rust on oats. iEcidial infection was found 

 on these shrubs every year. However, the maximum infection, in 

 1919, showed only 10 per cent of the leaves bearing secidia. When 

 the berries were infected the secidial cups covered the entire fruit and 

 often the pedicel. These shrubs, which varied from mere seedlings 





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Fig. 8.— Sketch map of an oat field and adjacent shrubs of Rhamnus 

 lanceolata near Indianola, Iowa, showing the spread of uredospores 

 from secidiospores, causing an infection of 15 per cent on the oats 

 nearest the shrubs, with gradually diminishing infection to less than 1 

 per cent at the distant side of the field. 



to trees 15 feet high, bore secidia on the topmost leaves as well as on 

 those near the ground. 



Observations also were made in a locality 3£ miles southwest of 

 Indianola, w^here, along the banks of a small stream, hundreds of 

 seedlings as well as mature plants were found. This stream flows 

 along a 90-acre field which was sown to oats in 1919. Shrubs bear- 

 ing secidia grew within 100 feet of this field. An infection of 15 per 

 cent occurred on the oats nearest the shrubs, with a gradually dimin- 

 ishing infection at greater distances until only a trace or less than 1 

 per cent w T as found in the most distant portions of the field. (Fig. 8.) 

 This field had a southeastern exposure toward the bordering stream. 

 The area showing the heaviest infection also was most favored by 

 heavy dews, which fall earlier in the evening and remain longer in 

 the morning, thus providing more favorable conditions for infection 



