558 



SPECIAL PLANT PATHOLOGY 



the growth of the dry-rot fungus. Coating materials should be avoided 

 unless the woods are absolutely dry and the well-seasoned wood should 

 be painted at once as neglect on this score may cause a lot of trouble. 

 The walls on which timbers are laid should be perfectly dry. 



Sap-rot {Daedalea quercina (L.) Pers). — One of the most im- 

 portant enemies of structural oak, produces a soft, mushy decay of 

 the wood (Fig. 202, also page 76). 



/mv 



m^m. 



Fig. 202. — Dadalen quercina destroying a fence post, Nantucket, Aug. 23, 1915. 

 Xerophytic hoof-shaped fruit-body above, mesophytic bracket below in contact 

 with the grass. 



Violet {Viola spp.) 



Spot Disease {AUernaria violcB Gall. & Dorsett) (Fig. 203). — 

 The wild violets in the yard of the author have been attacked by 

 the spot disease every year for the past six years. In some years, the 

 attack is more virulent than in other years. It is also common on vio- 

 lets grown under glass, and in some districts, commercial violet growing 

 has been practically abandoned. The fungus attacks plants that are 

 making a rapid and vigorous growth. The first spots are circular, 

 greenish or yellowish white ones. They have a light colored central 

 portion surrounded by a narrow ring of discolored tissue, usually 



