RHAMNUS IX THE DISSEMINATION OF CROWN RUST. 



favorable for dissemination and germination of the spores. The 

 temperature, which averaged 8.6° F. above the normal for this 

 period, was favorable to the germination of the secidiospores. The 

 first week in June usually marks the appearance of the secidiospores 

 under Iowa conditions, but their early appearance in 1921 may be 

 due to the relatively high temperature during May. Infection was 

 favored also by the high average relative humidity of 70.6 per cent 

 for this period and the rainfalls on May 16, 17, 18, 19, 22, and 30, 

 which were sufficient to moisten the foliage but not to wash off the 

 spores. The heavy rains of May 24, 25, and 26 were sufficient to 

 moisten the soil and keep a high relative humidity around the oat 

 plants. 



THE ROLE OF GRASSES IX THE DISSEMINATION OF CROWN-RUST INFECTION. 



Earlier investigations (8) have shown that crown rust of oats is 

 capable of infecting many of our native grasses in the greenhouse. 

 It has been a common belief that the aecidiospores fell from their 

 Rhamnus host to the grasses beneath and caused infection and the 

 uredospores thus formed were then carried by the wind to adjoining 

 oat fields. A careful study of the grasses in the entire infected area 

 shown in Figure 2 was made and only one, timothy (Phli um pratense), 

 was found infected. Where growing beneath the hedge and within 

 a maximun distance of 8 rods from it this host showed a few small 

 rust sori. It is evident that the wild grasses of northwestern Iowa 

 had little or no influence on the primary spread of crown rust in the 

 spring of 1921. The principal grass of this district is bluegrass 

 (Poa pratensis), which probably is not a host for crown rust of oats. 



THE RATE AND DISTANCE OF UREDOSPORE DISSEMINATION. 



As mentioned earlier, the first uredosori on oats near the Rhamnus 

 hedge appeared on May 22, and as it takes from six to seven days 

 for a generation of uredospores to mature, the infection in the period 

 from May 22 to 28 represents only the spread of recidiospores. This 

 was not a gradual spread, as all oat fields in the area of 2,980 acres 

 covered in Figure 2 were infected in some degree by May 23. This 

 area then remained constant until May 28, after which time the first 

 generation of uredospores was mature and capable of spreading infec- 

 tion. As no spore traps were set, it is impossible to ascertain whether 

 or not the secidiospores were blown over a greater area than the one 

 in which infection was found, but it is probable that they were not 

 viable when carried to a greater distance. Viable aecidiospores were 

 being produced up to June 10, but as they had not been able previ- 

 ously to produce infection at a greater distance than 4.52 rods, the 

 more extended spread of infection must have been due to uredospores 

 produced from a?cidiospores. (Table 2.) 



Table 2. — Rate of spread of crown rust to oats from a hedge of Rhamnus cathartica near 



Hinton, Iowa. 





Source of infection. 



Area. 



Period. 



Acres. 



Square 

 miles. 



May 22 to 28 



.Ecidiospore 



Uredospore 21. L20 



5 



May 29 to June 4 ' 



33 



June 5 to 7 



do 



80,640 



407.040 



126 



June 8 to 10 



...do .. 



636 









