PINE DISEASES 279 



greatly in susceptibility. The cultivated black currant is 

 most susceptible, the under sides of the leaves being often 

 completely covered with the felt-stage. Gooseberries in general 

 are more resistant than currants. 



Cause. 



The blister-rust of five-needle pines and the felt-rust of cur- 

 rants and gooseberries are caused by the fungus, Cronartium 

 ribicola. Before the stages on the two kinds of plants were 

 known to be caused by the same fungus, the stage on the pine 

 was called Peridermium strobi. The life history of this fungus 

 has been indicated above. The aeciospores formed in the 

 blisters on the pine branches infect the leaves of gooseberry 

 and currant. After a period of development in the leaf-tissue, 

 ^rediniospores are formed which infect other gooseberry and 

 currant leaves. From the same pustules the teliospores are 

 developed in long hair-like masses. These spores are not 

 disseminated but germinate and produce small, globose spores 

 ( basidiospores ) on the short germ-tubes. The basidiospores 

 are shot from their attachment and may cause infection of the 

 pine. In this way, although the pine is not infected by the 

 aeciospores produced in the blisters, the mycelium of the 

 fungus after about two months' growth in the ciu-rant or 

 gooseberry produces the kind of spore which will infect the 

 pines. 



Weather conditions in relation to the spread and severity 

 of attack of this fungus are not fully understood. Moisture 

 is necessary for the germination of the different spores and 

 from analogy to other similar diseases, it would be expected 

 that the stage on currants and gooseberries would be more 

 abundant in wet seasons. The distance over which the spores 

 are transported by the wind depends largely on prevailing 

 air currents and the topography of the region. In a dense 

 growth of underbrush in the forest the fungus would not be 



