42 
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 through 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 first application 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, 
due 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 thinner and more easily wettable, while, with 
increased weight, the peaches generally turn downward and become 
more subject to inoculation by sporiferous meteoric water from infected 
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 macrosoopioally 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 little 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 until 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 
