8 



BULLETIN 1162, U. S. DEPARTMENT OF AGRICULTURE. 



The six days from May 22 to 28 represent the term during which 

 only secidiospores caused infection. The remaining 13 days shown in 

 Table 2 are the period in which the rapid spread was due to succes- 

 sive uredospore generations. 



The direction and distance of infection in the period from June 5 

 to 7 are shown in Figure 3. The lack of infection southwest of Le 

 Mars apparently is due to the fact that no oats were grown for several 

 square miles. This area also is heavily wooded, and it may be pos- 

 sible that the trees offered some obstruction to the spread of the f 

 uredospores. In a detailed survey made of the area shown in Figure 

 3 no* other specimens of Rhamnus were present than those in the 

 hedge already described. 





























































































































































































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Fig. 3.— Sketch map of the district surrounding Hinton, Iowa, show- 

 ing the spread of uredospore infection on oats at the end of June 7, 

 by successive uredospore generations from the infected fields shown 

 in Figure 2. The squares in the diagram represent square mlies, and 

 the heavy line is the outermost boundary of uredospore infection on 

 that date. 



On June 10 the infection was traced to a maximum distance of 53 

 miles from the hedge. The spread of infection after this time was 

 too rapid and covered too much territory to be followed by the survey. 



The destructiveness of this epidemic is shown in Figure 4, which 

 represents the conditions on June 7 in the same area and drawn to 

 the same scale as Figure 2. In comparing Figures 2 and 4 it will be 

 noticed that the heaviest infection is located in the same relative place 

 in each, showing that the areas of heaviest primary infection main- 

 tained a relatively higher infection throughout the season. The oats 

 in the fields adjoining the hedge were a total loss (figs. 5 and 6) and 

 were not harvested. The yields were small on the entire 1,020 acres 

 comprising the oat acreage in this area, the kernels were light, and 

 a relatively small percentage of the crop was of marketable grade. 



From the studies made it became evident that this hedge was a 

 menace to oat production in northwestern Iowa and probably north- 

 ward into South Dakota and Minnesota. The farmers of this com- 



