142 



MANUAL OF TREE DISEASES 



ing midsummer the cankers enlarge at the rate of about one- 

 half inch in diameter each week. The usual shape of the canker 

 is ellipsoidal (Fig. 17). The margin of 

 the canker is usually regular and some- 

 what raised. On smooth bark, the thin 

 cork-layer is wrinkled, forming concen- 

 tric rings about the central point of 

 the canker. As the canker becomes 

 larger, the bark splits and after a time 

 falls away in shreds, leaving the wood 

 bare. 



^Vhen the fungus is working in the 

 living tissues under the rough bark, there 

 are no outward indications of its pres- 

 ence until the fruiting-structuresare pro- 

 duced in crevices of the bark. If the 

 bark is peeled from the edge of a canker, 

 the tawny mycelial fans are readily seen 

 (Fig. 18). The invaded tissues are 

 changed to a light brown in contrast to 

 the normal light colored healthy tissue 

 of the bark. The thick bark is reduced 

 to a mass of shreds which are a uniform 

 dark brown. The first layers of wood 

 under the cankered areas also become 

 brown. 



The effects of the disease on the 

 general appearance of the tree are most 

 noticeable during the summer when the 

 trees are in leaf. In localities in which 

 the disease is common, large numbers 

 of the newly affected branches and twigs 

 are girdled by the cankers during the late siunmer and the 

 brown shriveled leaves hang to the limbs. This most striking 



Fig. 17. — Endothia canker 

 of chestnut. 



