240 MANUAL OF TREE DISEASES 



and often become confluent, causing the leaf to curl. So far 

 as is known, there is only the one period of infection and no 

 subsequent spread to the other leaves of the tree occurs. 

 Practically all the leaves of the tree may be infected, however, 

 and where the blisters are numerous the leaves fall. 



Cause. 



Leaf-blister of oak is caused by the fungus Taphrina cceru- 

 lescens. This fungus belongs to the family Exoascacese, aU the 

 members of which are parasitic and cause leaf-curls, leaf-blisters, 

 plum-pockets, witches'-brooms and other types of over-growths. 

 The common orchard disease, peach leaf-curl, is caused by a 

 closely related form and is similar to oak leaf-blister in many 

 ways. There are no fruiting-bodies formed- by these fungi. 

 The spores are borne in asci which stand free on the surface of 

 the blistered or ciu-led area. From the production of the spores 

 which takes place as the blisters are forming, until infection 

 occurs the next season, nothing is known concerning the life 

 history of this entire group of fungi. No other stage of de- 

 velopment is suspected but it is thought that the spores simply 

 rest until the next spring and are present in some way so that 

 they can infect the unfolding leaves. In the case of peach 

 leaf-curl, cold wet weather following a warm period, at the time 

 the buds are bursting, causes epiphytotics. Similar weather 

 conditions may result in more extensive infection in the case 

 of oak leaf-blister, but no observations are recorded on this 

 point. 



The mycelium does not enter the tissue of the oak leaf. It 

 simply penetrates the cuticle of the lower epidermis and lies 

 in contact with the outer walls of the epidermal cells. The 

 enzymes of the fungus diffuse into the leaf and cause a marked 

 reaction on the part of the leaf-tissues. The lower epidermal 

 and spongy mesophyl cells increase in number and the palisade 

 and upper epidermal cells increase greatly in size, causing the 



