DISSEMINATION OF THE UREDOSPORE. 53 



grammis and P. rubigo-vera which have survived the winter in the 

 north and are ready to infect the growing grain. 



The great problem for rusts in many places of the South, however, 

 is not how to live over the winter, but how to pass through the 

 extremely hot months of July, August, and September. This is 

 especially true of the cereal rusts in portions of eastern and southern 

 Texas, as volunteer grain is scarce at that time; but in northwest 

 Texas the authors noticed vigorous rust pustules of both Puccinia 

 graminis and P. rubigo-vera on volunteer wheat during September, 

 1907, so that in the higher altitudes in the Southwest the rust does 

 exist in the uredo form on volunteer grain in late summer and early 

 fall. The early-sown fall wheat can thus become infected with spores 

 from this source, as described later in this paper. 



DISSEMINATION OF THE HTtEDOSPOItE. 



METHODS OF DISSEMINATION. 



Rusts in the uredo stage have been shown to be present in parts 

 of both the North and South at almost all times of the year, and in 

 order to explain their constant menace to the crops of the country it 

 remains only to determine their means of dissemination. Rust spores 

 are extremely numerous, hundreds occurring in a single pustule. 

 They are very light, much more so than dust particles, which have 

 been known to be carried in the air for hundreds of miles and dis- 

 tributed over large territories in a few days. An example of the 

 carrying power of the air is cited by Klebalm (63, pp. 66-68) who 

 relates that dust clouds arising in northern Africa, March 9, 1901, 

 were driven over a large part of the continent of Europe in the next 

 two days. Corresponding dust showers were noticed March 9 and 

 10 in Tunis, West Tripoli, and Algiers; early March 10 in southern 

 Sicily; night of March 10-11 in the East Alps; early March 11 in 

 Maingebiet; at 4.30 in the afternoon in Hamburg; and a little after 

 midnight in the Danish Islands (Stege auf Moen). The dust was 

 composed of clay, fine .quartz particles, and other minerals, sup- 

 posedly derived from the African deserts. 



Undoubtedly, rust spores, which are much lighter than these dust 

 particles, can be carried more easily by the wind and air currents 

 over as great, if not greater, distances. Rising into the air, these 

 spores may reach the upper atmosphere and be carried hundreds of 

 miles a day in whichever direction the air currents are moving. In 

 this way innumerable rust spores may be carried from regions where 

 they are plentiful, either by reason of the presence of the secidial 

 hosts, or overwintering uredos, to regions where grain is in a receptive 

 condition. This interchange of spores between localities may take 



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