34 PEACH LEAF CURL*. ITS NATURE AND TREATMENT. 



Relative to the use of fiingicides the same writer says: ''These 

 treatments are of doubtful value as far as the curl is concerned, and 

 were it not that they are useful in other ways I would not mention 

 them." It is evident that these views are the result of Dr. Cobb's 

 belief that the perennial mycelium is resppnsible for the major portion 

 of the spring infection of the tree. The writings of others to the 

 same effect could be cited, but the views of the workers already named 

 are sufficient to show that their recommendations for treatment have 

 been based upon the hypothesis that the spring infection could not be 

 prevented by treatment with fungicides, as it arose mainly from in- 

 ternal mycelium rather than from the germination of external spores. 

 That this view has held hack the preventive treatment of the disease, 

 as already claimed, can not be doubted, and that a perennial mycelium 

 is not responsible for more than a very small percentage of the spring 

 infections seems evident from the results of the present ex|3eri- 

 ments; in fact it may even be questioned if such infection takes place 

 except under exceptionally favorable conditions. Our experiments 

 have demonstrated that as high as 98 per cent of infections may be 

 prevented by a single thorough application of a suitable fungicide. 

 This is as high a percentage of control as is often obtained in the 

 treatment of fungous diseases where no perennial mycelium exists, 

 and it seems probable that the infections by this means may not com- 

 monly exceed 5 per cent of each spring's infections. Were this not 

 the case we would be forced to assume that the spray has a direct 

 effect upon the hibernating mycelium, which certainly would be 

 unusual and scarcely to be expected. 



The second mode of spring infection — that by means of spores — is 

 probably much more general and important in this disease than has 

 been supposed. That 90 to 98 per cent of the infections of the tree 

 are prevented by a single spra3^ing suggests that at least such percent- 

 age of the infections is by means of spores. 



The mycelium of Exoascus deformans as found in the peach, shows 

 great differences in the form and appearance of its hyphtv. These 

 differences depend upon the stage of development of the fungus and 

 the various functions of the mycelium. The writer recognizes three 

 types of hypha?, which ma}" be termed vegetative, distributi^'e, and 

 fruiting. 



The vegetative hyphw are found most conmionly in the leaf paren- 

 chyma, but are also met with in the leaf stalk and cortical parenchyma 

 of badly diseased and distorted branches. These hyphie may be most 

 distinctly seen, and are most highly developed, in infested leaves which 

 have not j'^et formed the hymenium of ascogenous cells, but in which 

 the parasite has been present a sufficient time to entirely alter the 

 character of the palisade tissue and cause the loss of the chloroph}'!!. 

 In the leaf blade the palisade tissue first shows the serious action of 



