32 BULLETIN 395, U. S. DEPARTMENT OF AGRICULTURE. 
LIFE fflSTORY OF THE CAUSAL ORGANISM IN RELATION TO PATHO- 
GENESIS. 
In order to trace clearly the detailed life history of the parasite in 
relation to pathogenesis, it is necessary first to follow carefully the 
seasonal development of the disease under field conditions. This 
will now be briefly outlined. 
SEASONAL DEVELOPMENT OF THE DISEASE. 
ON FRUIT. 
The first fruit infections of the season usually become evident 
shortly prior to the ripening period of such early varieties as the 
Carman. The early varieties usually show the first general infection, 
while the late-maturing fruits , which are usually subject to severe 
attacks of scab, frequently show little macroscopic evidence of the 
disease until several weeks later, This is probably due, in large 
measure at least, to the fact that infection is first mechanically 
hindered and later masked by the hairy thatch of the young peach, 
becoming visible on early varieties rather suddenly with the lighten- 
ing of this covering as the fruit rapidly expands prior to maturity. 
The records upon which this statement is based are outlined as follows : 
Supporting records. — At Cornelia, Ga., in 1912 and 1913, the peach trees were in 
full bloom during the last days of March. The Carmans were harvested during the 
first week of July, while the Elbertas ripened about a month later. 
In 1912 the first scattered fruit infections were noted on unsprayed Carmans, 
Belles, and Elbertas on June 15, though the lesions had probably been visible for 
several days. On June 17, 19, and 25 the disease was increasingly evident, especially 
upon the rapidly maturing unsprayed Carmans, where it was quite conspicuous by 
July 4. As was emphasized in examining over 20,000 Carman fruits in taking results 
on spraying experiments, this early infection occurred almost exclusively about the 
depressions surrounding the peduncles and upon the closely adjacent wettable sur- 
faces. On later varieties the disease became increasingly conspicuous until the 
maturity of the fruit, the lesions frequently becoming confluent over large patches. 
In 1913 the course of the disease was somewhat modified by a severe spring drought, 
which occurred in April and early May. Scattered lesions were observed upon 
Carmans and Belles on June 14, while the first infections on Elbertas were noted on 
June 25. Only occasional infections were observed until July 1, when the disease 
was conspicuous upon unsprayed Carmans. On Elbertas little infection was evident 
until the middle of July, after which the disease became increasingly abundant. 
ox TWIGS. 
Scattered twig lesions usually become evident at about the time 
the disease appears upon the fruit. Generally, however, they do 
not appear abundantly until late summer and fall. They pass the 
winter in various stages and complete their development during the 
second season. 
Supporting records. — In Georgia, in 1912, the first scattered twig infection was 
noted on unsprayed Elbertas at Fort Valley on June 12. At Cornelia it was not 
observed until July 4, though it probably had been visible for more than a week. 
