LEAF DISEASES AND INJURIES 33 



In general the life history of these fungi is similar for all the 

 species. The spores causing primary infection in the spring 

 may come from two sources. The ascospores developed on the 

 dead leaves are forcibly ejected into the air and are borne by 

 the wind. Usually the leaves of the lower branches are most 

 heavily infected by these spores. Also, if the mycelium of 

 the fungus should be one which affects the bark-tissues of the 

 twigs, primary infection may result from spores produced in 

 fruiting-bodies pn the bark. During the spring and summer, 

 the destructive spread of the fungi is due to the abundant 

 spore-production in the fruiting-bodies on the dead areas of 

 the leaves. 



Wet seasons are particularly favorable for epiphytotics of 

 leaf-spot diseases. This is due to the fact that abundant 

 moisture is necessary to disseminate the spores and to induce 

 germination. In this way several generations of spores are 

 produced until the leaves are so generally affected that defolia- 

 tion may result. 



Control of leaf-spots. 



In the forest no direct methods of control for leaf diseases 

 are practicable. With shade and ornamental trees, however, the 

 appearance of the tree or the protection from repeated defolia- 

 tion, may warrant the expense necessary to control the fungus. 

 All leaves which fall from affected trees should be raked together 

 and burned in the autumn. If this is carefully done for some 

 distance away from the trees, much of the primary infection 

 will be avoided. In case the fungus also affects the twigs, 

 all the dead and cankered twigs should be pruned off before the 

 buds burst in the spring (see under control of leaf-blight and 

 witches'-broom of sycamore, page 337). Even if these eradica- 

 tion measures are taken, the fungus may appear, especially in 

 wet seasons, by spreading from surrounding trees, or because 

 a sufficient number of spores were available for primary infection 



