32 BULLETIN 395, U. S. DEPARTMENT OF AGRICULTURE. 



LIFE HISTORY 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. 



The iirst 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 lea^st, 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 hghten- 

 ing of this covering as the fruit rapidly expands prior to maturity. 

 The records upon which this statement is based are outlined as follows : 



Su-pporting records. — At Cornelia, Ga., in 1912 and 1913, the peacli 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. 



ON 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 m 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. 



