PEACH SCAB AND ITS CONTROL. 41 



(ventral) surfaces are not infected. The development of the fungus was easily traced. 

 Material collected 48 hours after inoculation showed abundant germination, the tubes 

 averaging 40 to 60 n in length. No evidence of penetration, however, was observed 

 at this stage. In subsequent collections, which extended over six days, a slow devel- 

 opment of the fungus could be traced, but this appeared to be almost entirely super- 

 ficial. Frequently germ tubes were traced over stomata, but in no case were they 

 observed to enter them. In certain cases, in material collected on the third day and 

 subsequently, slender colorless hyphse were observed in such positions as strongly to 

 indicate direct penetration of the cuticle, the apical portions being in a slightly lower 

 focal plane than the spores from which they originated. However, the thinness and 

 tlie delicacy of the cuticle, which on the lower surface of the leaf is usually less than 

 1 J Ai thick, and the fact that the fungus does not penetrate farther into the leaf tissues 

 made it impossible to draw final conclusions from the material at hand. There was no 

 opportunity of extending these studies. These data, however, in conjunction with 

 the fact that penetration through stomata could not put the fungus in its characteristic 

 subcuticular position, leave no reasonable doubt that leaf infection occurs by means 

 of direct penetration of the cuticle. 



No special studies of twig and fruit infection have been under- 

 taken. However, since the nature of the surfaces of the very young 

 twigs is so similar to that of the lower surfaces of the leaves, it is 

 to be expected that leaf and twig penetration are of the same type. 



In the earliest stages of fruit infection observed in histological 

 preparations, the fungus was found closely appressed to the host 

 cells in the minute depressions surroundmg the bases of hairs. These 

 cavities obviously offer exceptionally favorable conditions for infection 

 courts, and it is entirely probable that most fruit infection occurs 

 in this fashion, the development of the fungus being chiefly super- 

 ficial, in close contact with the cells of the host. 



PERIOD OF INCUBATION. 



On fruits. — ^The time which elapses between infection and the 

 macroscopic appearance of fruit lesions has been shown to vary, not 

 only with environmental conditions, but also with the nature and 

 stage of development of the fruit. In the moculation experiments 

 this period was observed to vary between 42 and 77 days. In 

 nature, with more extreme conditions, it is highly probable that the 

 variations are even greater. 



On leaves. — In leaf infection similar variations occur, but they are 

 less marked, since the complicating factor of masking by hairs is 

 eliminated. In the inoculation experiments it was shown that the 

 period of incubation for leaf lesions might vary between 25 and 

 45 days, and probably more. Under the more extreme conditions 

 of the field, it is likely that the variations are greater. 



On twigs. — In the inoculation experiments the period of incubation 

 for twig infections of the early summer closely approximated that of 

 leaf infection. It is evident, however, that great variations in this 

 regard may occur in the case of late twig infections, many of which 

 do not become macroscopic until the following spring. 



