150 TIMBERS AND THEIR USES 



fungi have a greater importance ; when the 

 leaves of any tree are damaged the wood pro- 

 duced during the year is small in quantity and 

 poor in quality. 



Among the commonest leaf parasites are the 

 Mildews, Erysiphce, which usually take the form 

 of an ash-grey growth on the leaf surface. Oaks, 

 Beeches and Birches are frequently attacked by 

 mildew, but any tree is liable to the fungus. A 

 strong, healthy tree in a normal season may be 

 but little damaged by mildew, but in a damp 

 summer or if the host plant be weakly the 

 injury caused by these fungi may be consider- 

 able. 



Some leaves, especially those of Sycamore and 

 Maple, are liable to attack by the Tar Spot 

 fungus, Rhytisma, so called on account of the 

 shining black patches of fungus growth which 

 appear on the leaves. The vegetative stage of 

 the parasite only is reached while the leaves are 

 living on the tree ; after leaf fall the spores are 

 formed. 



The Rusts, Uredince, are the most injurious of 

 all the leaf fungi. They are so called because 

 they produce rust-coloured spots upon the plants 

 they infest. Their life histories are often com- 

 pUcated and the reader who would go further 

 into the matter is urged to refer to some of the 

 recognized textbooks on the subject. Two hosts 



