Minnesota Plant Diseases. 



283 



by their life-histories and 

 second under the micro- 

 scope, chiefly by the shapes 

 of the winter spores and the 

 forms of the spore clusters. 

 The summer and winter 

 spores may arise from the 

 same mycelium. The winter 

 spores remain unchanged 

 throughout the winter and 

 in the spring under favor- 

 able conditions of moisture 

 and heat produce from each 

 cell a short thread or promy- 

 celium, which gives rise to 

 four little spores known as 

 sporidia. These sporidia are 

 borne by the wind to other 

 plants, where they germinate 

 and produce a parasitic my- 

 celium, from which arise the 

 cluster cups and pycnidia. 

 In the stem rust this clus- 

 ter-cup stage is formed on 

 barberry leaves, in the or- 

 ange leaf rust on certain 

 borages, as hounds-tongue 

 (in Europe), and in the 

 crown rust on the buck- 

 thorn (species of. Rhatnnus). 

 The last is very abund- 

 antly found in Minnesota 

 on the alder-leaved buck- 

 thorn (Rhamnus alnifolia). 

 These cluster cups are usual- 

 ly formed on the under leaf- 

 surface of their host and are 

 formed on yellowish spots. 



FIG. 139. Wheat rust (Puccinia graminis). 

 Stems of wheat showing opened and un- 

 opened black clusters of winter spores. 

 This is commonly known as "black rust" 

 or "stem rust." Slightly magnified. Orig- 

 inal. 



