292 Minnesota Plant Diseases. 



stricted near the middle, two-celled, brown, smooth, have a 

 short stalk or none; accompanied by sterile threads (paraphyses). 



The stem rust {commonly called "black rust"). The cluster- 

 cup grows on barberry. The summer spore clusters: chiefly on 

 the leaf sheath, occasionally on the stem, seldom on the leaf blade, 

 scattered or in rows, elongated or linear, {2-'^ m. m. long), often 

 running together to long lines ; bounded by the fissured epider- 

 mis of host, becoming powdery, brown when fresh, yellow when 

 dry. The summer spores: usually ellipsoidal or ovate oblong, 

 finely spiny, yellow brown (when fresh). The winter spore clus- 

 ters (so-called "black rust") : chiefly on the stem or on the leaf 

 sheath (less common on the leaf blade); scattered or in 

 rows, elongated, often running together into long lines, i. c. m. 

 or longer; soon exposed (naked) by the breaking of the host epi- 

 dermis, black and powdery. The winter spores : oblong or club- 

 shaped, two-celled, smooth, chestnut brown; apex rounded or 

 long-conical and much thickened; base attenuate; have brown 

 stalks, often as long as the spore; no sterile threads. 



Rust of corn (Puccinia sorghi Schw.). This very common 

 rust occurs on species of sorghum and on corn. The cluster-cup 

 stage has recently been discovered; summer and winter spores 

 are well known. The two latter spores occur in usually small, 

 sometimes considerably elongated, groups or sori of red- or red- 

 dish-brown color. The winter spores are two-celled. The clus- 

 ter-cup is found on a species of Oxalis. This rust is usually not 

 in sufficient abundance to cause any serious loss. 



Clover leaf rust \Uromyces trifolii (Hedw.) Lev.]. The 

 clover rust occurs on several kinds of clover, notably on red and 

 white. Cluster-cups are formed, but not commonly, in the 

 spring, on the petioles and blades of the leaves, and summer and 

 winter spores are found later appearing as red and brownish 

 powdery spots, usually on the under surface of the leaf. The win- 

 ter spores are single-celled. The fungus thrives in damp cold sum- 

 mers and is usually not abundant or dangerous in the sprino-; 

 but it may increase during the summer, especially if conditions are 

 favorable. The early red clover crop is therefore usually unaf- 

 fected, but later crops may be damaged. 



The plowing under of later crops when badly infected has 

 been recommended, as has also the burning over of the fields 

 to prevent a recurrence of the disease in the following season. 



