1 62 Minnesota Plant Diseases. 



the cluster-cup stage or rather just preceding it one very often 

 finds another accessory spore-form in which small spore-cases 

 of pear-shaped structure are produced, sunken into the oppo- 

 site — usually upper — side of the leaf from that on which the 

 cluster cup occurs. Inside of these pear-shaped cases the spores 

 are produced on long threads from which they are pinched ofif 

 just as in very similar structures found in many of the acces- 

 sory spore-forms of the sac fungi. These spores are often ac- 

 companied by the production of sugary, sweet fluids which are 

 probably attractive to insects and thus aid in spore distribution. 

 The exact use of these spores is not yet known for they have 

 not been proven to be able to cause infection of a host plant 

 though they will germinate under certain conditions. They 

 have been supposed to be unused male sexual elements and re- 

 cent research points to a confirmation of such a supposition. 

 Sometimes this pycnidium spore accompanies other spore- 

 forms, e. g., the summer or even the winter spores. The cluster- 

 cups are produced almost universally in spring so they are the 

 first rust spores (excepting the pycnidia and basidium-spores) 

 which one finds after the resumption of growth by flower- 

 ing plants, after the winter has 

 passed. In early summer or 

 even late spring and from this 

 time throughout the summer 

 season and far into the au- 

 tumn are found what are 

 known as the summer spores 

 or red rust spores. These 

 are like the cluster-cup spores 



Fig. 75. — Cluster-cups of ash-leaf rust fun- ;„ „ . ,i 



gus, on ash twig. The cups are long '" some rcspccts ; they are or- 



cylindrical. Highly magnified. Micro- ancrp-rprl nr -iroll/^iir i^ ,-^1,-x^ 

 photograph by E. W. D. Holway. ^"S^ '^^^ '^^ yCUOW m COlOr 



and are often provided with 

 external warts or spines. They are, however, not formed in 

 closed cup cases and are not formed in chains. They arise 

 singly on short stalks in dense clusters from which they are 

 shed as a red-rust powder. They may be formed continuously 

 for long periods from the same cluster, and are capable of im- 

 mediate germination under favorable conditions. They ger- 

 minate by sending out a fine thread in a similar manner to that 



