CURRANT DISEASES 217 



the growing-season, a new generation of uredospores being pro- 

 duced every two weeks if conditions favor. Thus the fungus 

 spreads rapidly and over great distances, assuming, of course, 

 that Ribes are present. 



From the latter part of July until leaf-fall, spores of another 

 kind are formed on the currant and gooseberry leaves (Fig. 

 56). They are recognized as groups of three to twelve stout 

 threads, measuring a quarter of an inch or less in length. The 

 spores of this stage are called teliospores. The teliospores 

 differ from the uredospores in that they cannot reinfect the 

 currant and gooseberry. They germinate in a peculiar fashion : 

 a short tube is produced on which small spores, called sporidia, 

 are developed. The sporidia are extremely light and are blown 

 easily by the wind. These blow to the white pines, germinate, 

 and infect the bark of the same. The mycelium resulting 

 spreads through the bark for several inches, but there may be 

 no external evidence of the disease for several years (one to 

 six or more). Finally the bark begins to thicken, and the first 

 external evidence of the fungus itself consists of minute bodies 

 known as pycnia; these appear from June to September. 

 Closely following the development of the pycnia are the aecia. 

 Spores from the secia blow back to the currants and goose- 

 berries. 



Control. 



From the point of view of the people at large the removal of 

 the less important host would involve the currant and goose- 

 berry. But in some cases the commercial grower of bush-fruit 

 should destroy the white pine only in those cases where but a 

 very few trees stand in close proximity to the berry plantation. 

 The safety-zone may be put at five hundred feet ; that is, the 

 two hosts, white pine on the one hand, and currants and goose- 

 berries on the other, should not be allowed to grow within a 

 distance of less than five hundred feet of each other. It is safer 

 at present to destroy those Ribes plants which are persistently 



