202 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



BLACK STEM RUST OF WHEAT AND OTHER CEREALS. 



Reseaich on tliis widespread and destructive disease has been con • 

 tiniiedj in cooperation princii)ally with the Minnesota Agricidtural 

 Experiment Station. 



kpiui:mioix)gy studiks. 



Epidemiolooy studies were made to ascertain the source of initial 

 infection in tlie sprin<r and to determine the conditions under which 

 rust develops and spreads most rapidl}'. The three possible sources 

 of rust in the spring are : (1) Ked spores (urediniosporcs which over- 

 winter in the 5s^orth), (2) red spores which overwinter in the South 

 and from which infection spreads gradually northward, and (3) yel- 

 low or cluster-cup spores from the common barberr3\ Extensive in- 

 vestigations show that while the red spores overwinter abundantly as 

 far north as Oklahoma there is no proof that any overAvinter nor- 

 mally in the Xorthern States. There is, however, some circumstantial 

 evivlence of loi al overAvintering near Madison, Wis., in 1920. 



The common barberry became infected as far south as Wichita, Kans., 

 and southern AVest Virginia. Very little infection was found in Ken- 

 tucky and Missouri, and none at all south of these States. The fact 

 that it rusts very little south of about the latitude of 39° N. is due to 

 loss of viability before spring by the black spores (teliospores) in the 

 South. In the Northern States the common barberry becomes heavily 

 infected and is a most important factor in spreading the black stem 

 rust. The development of local epidemics in the upper Mississippi 

 Valley in 1919 and 1920 has been shown to be due largely to early 

 spring infections of the common barberry and the spread of the rust 

 from them to adjacent grasses and grain fields. Such infection be- 

 gins very early in May, and if conditions for rust development are 

 favorable infection may become widespread by the middle or latter 

 part of June. This is early enough to permit severe injury to spring 

 wheat, though usually too late to cause much damage to winter 

 wheat. 



Important results have been obtained in the study of the possible 

 effect of native barberries and the closel}^ related forms sometimes 

 called Mahonia. Only one species of native barberry {Berheris 

 canadensis) rusts abundantly enough to be dangerous. This species 

 causes wheat rust in West Virginia and some portions of adjacent 

 Virginia. The Mahonia of the Ivocky Mountains {M. repens or 

 B. refcns)^ which is found in the mountainous portions of the eradi- 

 cation area in Colorado, Wyoming, and Montana, apparently does 

 not rust when growing wild and has not yet been infected in the 

 greenhouse tests. The taller species of the Pacific Northwest, known 

 as the Oregon grape {M. aguifolimn or B. aquifolium), which is 

 sometimes cultivated as an ornamental, has been infected by artificial 

 inoculation in the greenhouse and should not be planted in the 

 eradication area. 



BIOLOGIC FORjrS. 



Steady progress has been made in the study of biclogic specializa- 

 tion. These biologic forms can be distinguished from each other 

 only by their action on varieties of wheat, some being able to attack 

 certain wheat varieties and unable to infect others. At least five can 



