WEST VIRGINIA GEOLOGICAL SURVEY. 



77 



take up separately in this report the many species or even the 

 many classes of parasitic and saprophytic fungi that live in trees 

 and timbers and discuss their appearance, their work, etc. In- 

 stead, a few of them will be grouped according to their favorite 

 feeding places and some of those that have been collected in West 

 Virginia will be mentioned incidentally in that connection or list- 

 ed near the end of this chapter. 



Fungi That Injure the Leaves of Trees. 



The most important of the fungi that are parasitic on the 

 leaves of trees are the mildews, the rusts, and the leaf-spot fungi. 



The whitish mycelium of mildews frequently covers the 

 whole surface of leaves and tender twigs giving them a frosty 

 appearance during the summer and fall. The food of the mil- 

 dews is drawn from their hosts through short branches of the 

 mycelium which penetrate the cell walls of the interior leaf tis- 

 sues. Late in the year small, black fruiting bodies (perithecia) 

 appear. These hold on to the leaf surfaces during the winter, 

 and in the spring burst open and scatter their spores. The dam- 

 age done by mildews to large trees is not often great ; but to see'l- 

 lings it may be more serious. In nurseries or in seedling planta- 

 tions this fungus can be controlled by the use of powdered sul- 

 phur or some liquid fungicide such as Bordeaux mixture. 



The leaves of deciduous trees suffer but little from rusts. 

 Ash, poplar, birch, and willow leaves are occasionally affected, 

 and one stage of the cedar rust is found on service, hawthorn, 

 and other trees belonging to the rose family. The familiar ''ce- 

 dar-apples" often found growing on red cedar trees in West Vir- 

 ginia are caused by this cedar rust which is injurious both to the 

 cedar and, in another stage, to the hardwood trees last mentioned 

 above. A rust {Peridermium tsugae) sometimes occurs here on 

 the leaves of hemlock but does only slight damage. The European 

 currant rust {Cronartium ribicola) has been introduced from 

 European nurseries, in one of its stages, on white pine seedlings. 

 This disease is very destructive to young trees and due care 

 should be exercised by purchasers not to pocure infected stock. 



Rusts can sometimes be controlled by fungicides when it is 



