42 PEACH LEAF CURL*. ITS NATURE AND TREATMENT. 



the base of the leaf petioU' gradually deercnise in numbers as they 

 recede from the leaves, and they appear to be wholly lost at a short 

 distance from the point of entrance into the shoot. As a rule, little or 

 no mycelium has been found extendino- more than 1 or 2 inches beyond 

 a point where external macroscopic evidence of disease exists. 



The preceding- facts lead to the belief that where mycelial infection 

 of foliage takes place from the ])ranch it is usually done in the spring 

 from hyphffi arising from spore-infected leaves of the same season, 

 and that this occurs only in comparatively few instances or in bad 

 cases of disease. They also indicate that living perennial mycelium 

 which succeeds in accomplishing spring infection, is comparatively 

 rare. Badly infested and swollen l)ranches are apt to die and dry out, 

 thus affording no living tissue for the support of the infesting myce- 

 lium. Such branches, even if living until the following spring, are 

 not apt to produce much growth, and frequently produce none what- 

 ever. Furthermore, the badly swollen mycelium-infested branches 

 are comparatively few, and it is believed that the infested winter buds 

 of these branches very rarely exceed 2 to 3 per cent of the total num- 

 ber of buds upon the tree. Most branches appear to suffer from the 

 disease only in an indirect manner, that is, by the fall of affected 

 foliage. It seems probable to the writer, therefore, that the swollen 

 branches, in which the swelling is apparent to the eye, constitute the 

 true and almost exclusive home of the perennial mycelium, and there- 

 fore supply the only possible source of spring infection by the win- 

 tering hyphw, and consequently the only source of infection not 

 controllable by sprays. This is in harmony with the results of wide- 

 spread orchard treatment. All but 2 to 3 per cent of infections have 

 been prevented by a single spraying. (See the results of work on 

 half -sprayed trees.) That such spraying did not. prevent the spread 

 of the mycelium in the inner tissues of the host is shown by the fact 

 that when it is delayed until the leaves have fairly started and have 

 become infested, the treatment is ineffective and the disease will con- 

 tinue to develop and both foliage and crop may be lost. It is not the 

 checking of the spread of the mycelium from the branch to the new 

 leaves, therefore, that results from spraying, but the prevention of the 

 early spore infections from without; and as all but 2 to 3 per cent of 

 the year's infections may be thus prevented, all of such infections 

 must be considered as arising from spores. 



The limitation of the perennial mycelium of Exoascm deformmu to 

 the swollen Ijranches or branch parts, as here held, is in harmony with 

 observed facts respecting other species of Exoascece. It is not under- 

 stood, for instance, that trees developing witches' brooms are infested 

 in all their branches, but that the branch-infesting mycelium is limited 

 in its distribution to those centers which develop the abnormal multi- 

 plication of shoots, the swellings and other external manifestations 

 of disease. (See Pis. I, V, and VI. and descriptions, in connection with 



