SYCAMORE OR PLANE-TREE DISEASES 



337 



ascospores from the dead leaves on the ground or conidia from 

 the fruiting-bodies on dead twigs or 



cankers may cause the primary infec- ^£V M ■ "■" 

 tion. Internal over-wintering mycelium JH^I JBffi 

 may also grow out into the newly de- 

 veloping twigs and leaves from the in- 

 fected twigs of the previous year. After 

 a few leaves are infected, the spores 

 from the fruiting-structures along the 

 veins are thought to be largely responsi- 

 ble for the general infection of the re- 

 maining leaves of the tree. Frequent 

 rains and a humid atmosphere are con- 

 ducive to epiphytotics of this disease 

 when these conditions occur in proper 

 correlation with the development of 

 the leaves and the first fruiting-bodies 

 resulting from primary infection. The 

 twigs and branches are infected by the 

 spores produced on the leaf-lesions or 



by the mycelium growing down through Hfilffl l^^fc^ 

 the petioles into the twigs. Twigs in- 

 fected the season before are often 

 girdled and killed just after the leaves 

 expand the next spring. After the 

 leaves fall to the ground, the mycelium 

 lives saprophytically and produces per- 

 ithecia which matiu-e in the spring just 

 as the buds are biu-sting. It is probable that the spores are 

 shot out of the perithecium, as is the case with similar fungi. 



Control. 



Since this disease is universally present on sycamore and com- 

 mon on oak as well, the first steps in controlling it must begin 



Fig. 69. — Cankers on 

 limbs of sycamore. 



