30 James G. Horsfall 



or other susccpt response occurs. If they be numerous, however, the leaf 

 sooner or later succumbs, first turning yellow around the spots. Soon 

 the entire blade is involved and finally turns brown and then falls off. 

 The uredinia do not cause evident hypertrophy. 



The telia are distinguishable from the uredinia only by their slightly 

 darker brown color 



Etiology 



Name, history, and classification of the paihogenes. The nomenclature 

 of the various rust fungi on clovers has been so muddled that it is untangled 

 with difficulty. Kern (1911:3-6) gives a good history of the situation 

 up to 1911. He referred the rust on red clover to Uromyces fallens (Desm.) 

 Kern, and this disposition seems to be a good one. He was puzzled about 

 the report of aecia by Miss Howell (1890:131) and believed that they 

 arose as a contamination of the white-clover rust. He listed the aecial 

 and pycnial stages as lacking and suggested the possibility that they 

 occurred on Euphorbia. Since the publication of Kern's paper, the aecial 

 stage has been reported independently by Hoffer (1917 : 325-326), by Davis 

 and Johnson (1917:75), and by Melhus and Diehl (1917:70), proving 

 this rust to be autoecious as are the other clover rusts. 



Prior to the paper of Kern, American plant pathologists placed all the 

 clover rusts in one or another species of Uromyces, but referred them 

 chiefly to Uromyces trifolii; some ascribing the specific name to Albertini 

 and Schweinitz, some to Winter, and some to Hedwig. 



Liro (1906) already in 1906 had pointed out correctly that the red- 

 clover rust should be differentiated from the rusts on white and alsike 

 clover. He placed the forms on alsike and white clover in Uromyces 

 trifolii-repentis (Cast.) Liro, retaining that on red clover in Uromyces 

 trifolii (Hedw. f.) Lev. 



With Uromyces fallens as the name for the rust on red clover and 

 Uromyces trifolii-repentis for the one on white and alsike clover, the 

 nomenclature of the clover rusts rested until 1924 when Davis (1924 a: 

 203-219) established a new species, Uromyces hybridi, of the alsike form 

 on the basis of suscept specificity of the three fungi and on small differences 

 in the aecial and pycnial stages which he found. Arthur (1929:350-351) 

 in his new book on the plant rusts considers the common clover rusts all 

 as Uromyces trifolii, consisting of physiologic forms U. trifolii fallens on 

 red clover, U. trifolii hybridi on alsike clover, and U. trifolii trifolii- 

 repentis on white clover. 



Pathogenicity. Miss Howell (1890 b:228) by inoculation experiments 

 first formally connected the various spore forms on the clovers. She 

 procured aecia from telial inoculations and uredinia from aecial inocula- 

 tions. Telia developed later. Liro (1906:11-15) also proved the patho- 

 genicity of these rust fungi by inoculation. 



