Minnesota Plant Diseases. 291 



that locality. In the former case the seed would of course be 

 valuable, in the latter it might easily fall a prey to the first attack 

 of rust. 



It must further be pointed out that the conditions in a given 

 community must be thoroughly known, i. e. the chief kind of 

 rust and the conditions favoring its spread, as alternate host- 

 plant, etc. The problem, therefore, may divide itself into special 

 sectional problems. Again any new rust-proof varieties may not 

 always remain so, for the rusts can vary and adapt themselves 

 to new hosts and may at some future time find a way tO' invade 

 the new rust-proof variet)'. It seems therefore that the problem 

 before us is not one to be dispensed with by one discovery, but 

 that it may involve a long series of breeding experiments — in 

 other words, a continuous fight. It seems furthermore reason- 

 able, when all evidence is weighed, to hope that in such a com- 

 bat the plant breeder will ultimately emerge victorious. (See 

 also Fig. II and Chapter XL) 



In view of the importance of wheat rusts, detailed accounts 

 may be of value, and are given below for the benefit of those 

 who may be especially interested in this subject and who may 

 wish to be able to determine the chief rusts in the field or 

 laboratory. The italicized portions point to distinguishing 

 differences. The crown rust occurs chiefly on oats, more rare- 

 ly on wheat. In can be recognized microscopically by the 

 crown of prejections at the end of the winter spores. (See 

 Fig. 143.) The two remaining are the important wheat rusts. 



The orange leaf rust {commonly called "red rust"). The clus- 

 ter-cups are unknown in this country (except for a form on 

 Elymus with cluster-cup on Impatiens). The summer-spore- 

 clusters (so-called "red rust") : chiefly on the lower surface of 

 the leaf blade, often abundant ; occasionally on the leaf sheath ; 

 small, oblong, up to i m. m. long, usually arranged in rows and 

 often running together forming long lines, orange-colored when 

 fresh. The summer spores (under the microscope) : globose or 

 somewhat globose or broadly elliptical, finely spiny, orange- 

 colored. The winter-spore-clusters : accompany and follow 

 summer spores ; on the leaf blade (seldom on the stem) ; arranged 

 in lines or scattered ; oblong, dark brown or black; remain covered 

 by the host epidermis for a long time. The winter spores : club- 

 shaped, rounded or truncate or conic at the apex, slightly con- 



