56 THE EXISTS OF GB-AIXS IN THE UNITED STATES. 



the old leaves died and the rust disappeared, not being noticed again 

 until June 21. In 1908 this rust was first found in the field June 18, 

 and in 1909, June 9. P. graminis was first found on winter wheat 

 July 26, 1907, July 3, 1908, and July 5, 1909, while aecidia on bar- 

 berries were producing spores in 1907 about June 15, in 1908 about 

 June 1, and in 1909 between June 14 and 26. Generally speaking, 

 P. rubigo-vera tritici and aecidia on barberries appear at St. Paul, 

 Minn., about the middle of June, and P. graminis tritici the first 

 half of July — that is, from two to three weeks after the other two. 



Puccinia rubigo-vera is believed not to have any a?cidial stage in 

 this country. If this is so and the impossibility of direct infection 

 from the teleutospore is granted, the appearance of this rust in spring 

 must be accounted for by infection from wintering uredo, either as 

 mycelium or spore, or by infection from wind-borne spores from 

 fields farther south. Both methods are possible, and both un- 

 doubtedly may be employed. That viable uredospores of this rust 

 have not been found between April 15 and the first part of June in 

 the locality under consideration might furnish some argument that 

 infection from wintering uredos is not possible. Considerable light 

 is thrown upon this question by a study of the difference in length 

 of incubation period of rusts under varying conditions. Under the 

 cool temperatures of early spring the incubation period — that is, the 

 time from inoculation until pustules appear — is lengthened from 



7 to 10 days in warm weather to between 3 and 4 weeks and possibly 

 more in cool weather. This lengthened incubation period under cool 

 temperatures has been noticed many times by various investigators. 



In 1910, experiments on this point were performed in warm and 

 cool greenhouses at Washington, D. C. A large number of oat plants 

 were inoculated with the uredo of Puccinia graminis February 3, 1910. 

 Half of them were placed in a house where the temperatures ranged 

 between 42° arid 67° F., reaching 70° F. for an hour or two February 



8 and 14, and the other half were placed in a greenhouse where the 

 temperatures ranged between 62° and 90° F. On the plants kept in 

 the cool house pustules began to appear after a period of 18 days, 

 while on the plants kept in the warm house pustules were abundant 

 after 8 days. Puccinia graminis on wheat under similar conditions 

 began to show pustules after 16 days on plants kept in the cool 

 house, while pustules were abundant after 6 days on plants kept in 

 the warm house. Could the temperatures in the cool house have 

 been kept consistently lower than those indicated, undoubtedly the 

 incubation period would have been considerably lengthened. Christ- 

 man (32, p. 106) made similar observations in 1903 at Madison, Wis. 

 He noticed an early outbreak of uredospores of Puccinia rubigo-vera 

 on winter wheat and rye between March 20 and April 3, 1903. This 



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