DISTINGUISHING CHARACTERS OF THE BLIGHT CANKER. 



Blight cankers occur most commonly and are most destructive 

 on trees 8-14 years old — trees just coming into bearing. Unless 

 promptly attended to they very frequently result in the death of 

 the entire tree. 



In young trees with smooth bark the cankers are easily de- 

 tected, even in their first stages. They appear as discolored and 

 somewhat sunken areas, the margin along the advancing front 

 being usually slightly raised or blistered. The tissue in actively 

 spreading cankers is of a darker green than the healthy bark 

 and is very watery or sappy. On damp cloudy days drops of a 

 milky, sticky fluid (Fig. 6) exude from the cankered tissues 

 thru the lenticels or pores in the bark. After a short time the 

 diseased tissue begins to turn brown and dry out. Unless in a 

 very active state of progress the margins are very distinct, marked 

 by a crack where, in drying, the diseased tissue has separated 

 from the healthy bark (Fig. 1). The older cankers are brown, 

 somewhat darker than the healthy bark. They are distinctly 

 sunken. The surface is smooth, never checked or roughened or 

 beset with pustules or pimples, except in the old cankers, where, 

 after a time, 'rot fungi gain entrance and, thriving in the already 

 dead tissues, produce their fruit bodies on the surface. The 

 progress of the spreading canker depends largely on the con- 

 tinuation of favorable weather conditions, which seem to be a 

 humid atmosphere and cloudy days. With the return of bright 

 •sunny weather, the active spread of the canker is checked ab- 

 ruptly, often to be resumed again with the return of favorable 

 •conditions. This checking and renewing of activity sometimes 

 results in large cankers 'with concentrically arranged cracks 

 within the cankered area (Fig. 7). This renewal of activity may 

 take place during the same season or the canker may partially 

 heal over to spread anew the following year (Fig. 8). A large 

 percentage of the cankers are active during but one season. 

 There are always some, however, in which the disease is peren- 

 nial, living thru the winter to become active again the following 

 spring, spreading and enlarging the original limits of the can- 

 kered area (Fig. 9). The diseased bark is usually killed to the' 

 wood, to which it clings tenaciously the first season. It grad- 

 ually decays, however, and falls out, leaving the wood bare and 



