32 



numerous on the trunks. The spots appear mostly in fall and winter, 

 from November until January. The}^ may easily be detected when 

 no larger than a pin head. They increase quite rapidly in size and at 

 the same time grow deeper, penetrating through the bark into the 

 sap wood beneath, as evidenced by the brownish discoloration. 



Nature of injury. — Almost from the first the spots are slightly 

 sunken below the surrounding health}^ bark, a fact evidently caused 

 by the death and shrinking of the tissue. When the spots have 

 attained their full dimensions, which occurs in February and later, the 

 epidermis of the bark at the edge of the discolored spot commonly 

 bursts. Still later in the season this deepens into a crack, which 

 sharply separates the diseased from the healthy tissue. When once 

 this crack has appeared the limit of the growth of the spot is reached, 

 and beyond this limit it never spreads. The mature spots vary 

 greatly in size and shape. Ordinarily they are oval, from 1 to 

 3 inches long and about half as wide; quite commonl}^ two or more 

 spots merge together, and not rarely girdle the trunk or branch. 

 Sometimes the diseased areas are from 1 to 2 feet in length and 

 completeh" girdle the attacked branch for the whole distance. On 

 small twigs a similar girdling is very common. 



When the spots are ^ix months or more old numerous pustules as 

 large as a pin head or larger burst through the dead epidermis. At first 

 these pustules are whitish from the numerous spores borne on their 

 surfaces, but as the spores fall off they become blackish. These spores 

 are curved, colorless, about one-sixteen-hundredth of an inch long 

 and one-fourth of that in width, and are borne singly on stalks as 

 long as the spores. These spores are blown about in the air and by 

 them the disease is spi'ead. A spore will germinate readily in water, 

 sending out a germ tube which later becomes branched into a myce- 

 lium. In nutrient substances this mycelium bears numerous second- 

 ary spores from the ends of short lateral branches. Under natural 

 conditions these secondary spores seldom occur. The primary spores 

 are able directly to give rise to new spots on health}^ bark, the germ 

 tubes apparently entering through a lenticel. This then completes 

 the life history of the fungus as it occurs under natural conditions. 



About the time the spores become mature the dead bark has usually 

 separated from the wood beneath, being uplifted b}^ the surrounding 

 growing bark and wood, and sooner or later falls off, leaving the 

 characteristic scars of the disease. When these scars are small and 

 few, the new wood and bark may in time completely cover them. Un- 

 fortunatel}^ the spots are usuall}^ numerous enough to partiall}' girdle 

 the tree and so seriously weaken it as to make it almost worthless. 

 Not rarely the trunks of young trees or the branches of older ones are 

 completel}" girdled, which necessaril}^ results in the death of all parts 

 above such injury. 



