Minnesota Plant Diseases. 163 



of the cluster-cup spore and this thread may cause infection. 

 They are scattered by the wind and are the chief cause for the 

 rapid spread of rusts through the fields of wheat and other 

 cereals. Towards the end of the summer, often in the same 

 cluster with the summer spores, the winter spores commence to 

 develop and continue to form until late in autumn. In many 

 cases, however, the winter spores are formed in separate clus- 

 ters. The variety of habit of these winter spores has already 

 been described. They are in general resting-spores and germi- 

 nate in the following spring, thus again commencing' the life- 

 story. It may be well to summarize at this point the order of 

 succession of these spore forms. First at the germination of 

 the winter spore on the ground or under moist conditions any- 

 where, the basidium spores are produced. Soon after on a suit- 

 able host the pycnidia appear, followed closely or accompanied 

 by the cluster-cup spore. Next in late spring commences the 

 formation of the summer 

 spores which may continue to 

 form until late in fall. From 

 mid-summer on the winter 

 spores may be produced until 

 snow flies in late autumn. 

 There are thus five kinds of 

 spores of which the winter 

 spore corresponds to the smut FlG 76 ._ cluster . cup spores from rust fun . 

 spore of the smuts. In addi- g?. s of Fig. 75 Highly magnified. 



Microphotograph by E. W. D. Holway. 



tion to these five forms a sixth 



is known but is of rare occurrence and seems to be very similar 



to the summer spores in some respects. 



Not in all rust fungus life-stories, however, do all of these 

 spore-forms occur. One or more may be missing so that nu- 

 merous combinations are conceivable and actually occur. For 

 instance, a rust fungus may possess no cluster-cup stage or no 

 summer-spore stage, or both may be missing. In fact, some 

 have apparently retained only the winter spore form, which, of 

 course, always bears the basidium spores later. 



In addition to this variety of spore-forms, rusts have fur- 

 ther complicated their life-stories by selecting different hosts 

 upon which to form their various spores. For instance, one of 



