Minnesota Plant Diseases. 317 



the leaf gradually becomes lighter yellow. The sum- 

 mer and winter spores are not so conspicuous and occur on the 

 same host. The fungus threads live permanently in the rasp- 

 berry tissues, so that the destruction of the whole infected 

 plants is necessary. 



All diseased plants should be dug out and burned. Spray- 

 ing with bordeaux has been suggested to hold the spread of the 

 disease in check. 



The cluster-cup rust of gooseberry and currant {Aecidium 

 grossidariae Schum.). The fungus causing this disease is exceed- 

 ingly abundant on wild gooseberries and currants throughout 

 the state and is also known an cultivated forms. Serious dam- 

 age, however, is seldom reported. The fungus is a rust fungus 

 and is known at present only in its cluster-cup stage. This is 

 found on the leaves of the host plant and the cups are always 

 found on bright, orange-yellow, swollen spots of the leaf. The 

 cups stand on the lower surface and when they open, release 

 the golden-orange powder of spores, which may carry the infec- 

 tion to other plants. The cups are accompanied by very 

 minute pear-shaped capsules, containing spores on the upper 

 surface of the yellow leaf-spots. The openings of these can be 

 seen with the naked eye as small black dots on the upper sur- 

 face of the leaf. These spores do not assist in spreading the 

 disease. It is not known where the summer and winter spores 

 are formed. They probably occur on some species of sedge. 



In our lack of knowledge of the complete life history of the 

 disease, the only suggested remedy has been the destruction of 

 diseased parts of the plant. 



Mint rust {Puccinia menthae Pers.). Mint rust is very 

 abundant on a great many of our wild and cultivated mints. It 

 is said to be very destructive to the latter, where these are cul- 

 tivated on a large scale. The summer and winter spores are 

 most abundant and form small, dark-brown, powdery patches, 

 chiefly on the leaves. These patches, or sori, often occur in such 

 profusion that they completely cover the under surface of the 

 leaves. The cluster-cups are formed, as usual, in the spring or 

 early summer and are of less frequent occurrence. They occur 

 on the same plants. The mycelium passes the winter in the 

 underground stems of the host. The cluster-cup stage has 



