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



formed on each leaf, and many of the pedicels and fruits were com- 

 pletely covered with secidia, which caused hypertrophy and subsequent 

 falling. The leaves on the upper as well as lower branches were 

 infected. On May 22 the first uredosori appeared on the oats growing 

 within 70 feet of this hedge. 



Previous to May 22 no crown rust had been found in any other part 

 of Iowa within 40 miles of this farm surrounded by the Rhamnus 

 hedge. In addition, oat fields were inspected along the Lincoln 

 Highway from central Iowa to the western border and thence north 

 to Hinton, but no uredosori were found. The first uredospores were 

 found at Ames, Iowa, on June 4. 



The percentages of rust infection were estimated on the basis of 

 the scale in the agronomic notebook issued by the Office of Cereal 

 Investigations of the Bureau of Plant Industry. Using one side of 

 the field as a base line, perpendiculars were erected every 5 rods. 

 Rust estimates were then made at approximately 5-rod intervals on 

 these imaginary perpendiculars, and these percentages were recorded 

 at the corresponding points on the map of this area. By this method 

 an average of one rust reading was made for each 20 square rods. 

 Each field of oats in the vicinity was studied in this way (fig. 2). 



The spread of secidiospores from this hedge during the 7-day period 

 from May 22 to May 29 was determined by the apperance of uredosori 

 on the oats in adjoining fields. The possibility of increase from the 

 first uredospore generation to the second was excluded, as it requires 

 fully seven days under average field conditions for one uredo genera- 

 tion to develop. The infection on oats from May 22, when the first 

 uredo infection was found near this hedge, until May 29, therefore, 

 was caused only by secidiospores. 



The hachured area in Figure 2 represents oat fields, and the 

 intensity of the crosshatching is proportional to the degree of infec- 

 tion. A maximum infection of 85 per cent was evident on the oats 

 adjoining the hedge, and the infection decreased in degree as the 

 distance from the hedge increased. Additional oat fields were 

 inspected at increasing distances from the hedge until not a trace of 

 crown rust was found in the fields on the final date. Where only 

 part of the area within a field boundary is hachured oats were growC 

 ing in the entire field, but no infection was found in the areas which 

 have no crosshatching. The infection in these outer fields usually 

 was less than 1 per cent, and no infection was found during this 

 period in the oat fields beyond the limits of the area represented in 

 Figure 2, which covers 5 square miles. 



The secidiospores were dispersed in all directions, but chiefly toward 

 the southwest; that is, a heavier infection was present on the oats 

 southwest of the hedge, although the maximum distance to which 

 infection was found in a southwesterly direction was but little greater 

 than that in other directions. The maximum distance from the hedge 

 at which secidiospores caused infection was 452 rods, or nearly 1^ 

 miles. It is probable that most of the secidiospores were dropped 

 within 5 rods of the hedge, as the highest percentage of infection was 

 found within that distance. These data confirm an observation made 

 oy Eriksson (4), in which he suggests that the dispersal of spores from 

 an infected buckthorn is rather limited. 



iEcidiospores are mucilaginous, and it is most probable that they 

 adhere to the first surface with which they come in contact, while 



