166 THE BOOK OF WHEAT 



the infection. While it has been generally held that seed from 

 rusted wheat will not transmit rust to succeeding crops, 1 the 

 observations of Bolley in 1904 and 1905 proved that seed from 

 badly rusted wheat plants was quite uniformly infected in- 

 ternally, there being spore beds beneath the bran layer, con- 

 taining both uredospores and teleutospores that subsequently 

 germinated. 2 This demands a new line of investigation, for 

 it has not yet been demonstrated whether or not the internally 

 infected seeds will transmit the infection to the plants grown 

 from them. Variations in the spore forms and in the com- 

 plicated life cycle of rusts give them great strength in self- 

 perpetuation, the different methods of which present several 

 chances of escaping threatening destructions. Many wild 

 grasses also serve as hosts for the wheat rusts. They may be 

 infected from wheat and wheat from them, which is another 

 resource that aids rust in maintaining itself. 



Distribution of Rusts. Rusts being true parasites able to 

 live only in the tissue of some host, their distribution is co- 

 extensive with that of their native hosts and that of the wheat 

 crop, with one single restriction. Only in countries where no 

 dews set and no rains fall are rusts absent, for moisture is es- 

 sential to their first growth which causes the infection of the 

 host plant. In irrigated regions where dew and rain are lack- 

 ing, wheat grows without being rusted. The leaf rust is most 

 regular in its occurrence and is also most widely and univer- 

 sally distributed. It is the most common rust of Australia and 

 India, and in the United States it is most abundant in the At- 

 lantic and southern states. The stem rust is irregular in its 

 occurrence, usually missing one or two years in five or six, es- 

 pecially in some localities. In the United States the severe 

 attacks occur most frequently in the central states, and in 

 parts of Texas and California. This rust is very common in 

 northern Europe, and in some seasons it is also quite abundant 

 in Australia and Tasmania. It seems to be comparatively un- 

 important in India. 3 



Conditions Favorable to Rust Development. There must first 

 be such a wind from infected districts as will bring plenty of 



1 TL S. Dept. Agr., Farm. Bui. 219 (1905), p. 8; Minn. Press 

 Bui. 24. 



2 N. D. Bui. 68 (1906), p. 646. 



3 Carleton, Cereal rusts of U. S., pp. 21-22. 56-57. 



