42 BULLETIN 395, U. S. DEPARTMENT OF AGEICULTURE. 



TIME OF NATURAL INFECTION. 



Of fruit. — The inoculation experiments previously reported show 

 that infection may occur from spores applied 10 days after the calyces 

 are shed, but it should be remembered that special precautions were 

 taken to insure the passage of these spores thi'ough the difficultly 

 wettable hairy thatch of the fruit. It is evident, however, that such 

 early infection is rare in nature, since (1) the disease may be com- 

 mercially controlled when the ffi:"st apphcation of spray is made one 

 month after the petals fall and (2) the interim between this time and 

 the general macroscopic appearance of the disease coincides satisfac- 

 torily with the period of incubation of the fungus, as worked out in 

 the inoculation experiments. The chief reasons for this delay appear 

 to be (1) the hindrance of access of spores to the surface of the fruit, 

 duo to the difficultly wettable hairy thatch of the peach and (2) the 

 diminution of infection by means of water-borne spores from sub- 

 tending twigs, due to the upturned position of many fruits during the 

 earlier stages of their development. As the fruits increase in size, the 

 hairy covering becomes thhiner and more easily wettable, while, with 

 increased weight, the peaches generally turn downward and become 

 more subject to inoculation by sporiferous meteoric water frominfected 

 subtending twigs. Abundant infection usually begins to occur about 

 five to seven weeks after the petals fall and may continue until the 

 fruit matures, but on account of the long period of incubation of the 

 fungus the later infections do not become macroscopioally evident. 



Of twigs and leaves. — From the results of the inoculation work it is 

 evident that twig and leaf infection may occur whenever viable spores 

 come in contact with tender young twigs or the under surfaces of 

 leaves under conditions favorable for the development of the fungus. 

 Under field conditions in Georgia, however, as is shown by the data 

 given concerning the seasonal development and the period of incuba- 

 tion of the fungus, very httle infection occurs on twigs and leaves 

 until several weeks after the petals fall. Subsequently, such infec- 

 tions occur abundantly, with favorable conditions, throughout the 

 season. Important reasons for the fact that the bulk of the twig and 

 leaf infection occurs relatively late are (1) that in the South much of 

 the new wood of the peach tree is formed relatively late and (2) that 

 the conidia borne upon fruit lesions may constitute an important 

 source for infections which, on account of the long period of incubation 

 of the fungus, do not become visible imtil late in the season. 



SOURCES OF NATURAL INFECTION. 



Primary. — From the data presented in the preceding topics it is 

 apparent that primary infection results from conidia from over- 

 wintered twig lesions. Practically all the fruit infections, especially 

 in the case of early and midseason varieties, come from this source, as 



