SBlight Cankeu of the Apple Tree. 



183 



the tree, often killing it to the ground. The cankers have also been ob- 

 served in the upper crotches and on the sides of the large limbs in old 

 trees. Spreading from these points of infection, the disease may gradually 

 involve the entire limb, causing its death. Old trees weakened by age and 

 neglect may suffer seriously from such attacks and the dead limbs protrud- 

 ing here and there from the green 

 foliage in old orchards are often to 

 be attributed to the ravages of this 

 canker. 



As already pointed out. it is on 

 young trees that the canker is most 

 dangerous, for unless promptly at- 

 tended to the disease is almost cer- 

 tain to result in the death of the 

 entire tree. 



Fig. 50. — Baldivin tree killed by attack 

 of blight canker in the crotches of the 

 main limbs. 



In young trees with smooth bark 

 the cankers are easily detected, even 

 in their first stages. They appear 

 as discolored and somewhat sunken 

 areas, the margin along the advanc- 

 ing 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. 51) exude from 

 the cankered tissues through the lenticels or pores in the bark. After a 

 short time the diseased tissue besins to turn brown and drv out. Unless 



Fig. 52. — T pyical canker showing the 

 crack about the m.argin where the 

 diseased bark has dried away from 

 the healthy tissue. 



in a very active state of progress the margins are very distinct, marked by 



