CONTROL OF PEACH BACTERIAL SPOT. 3 



July 15 in the case of the fruit. At these times the leaves are begin- 

 ning to become ragged and shot holed and the affected fruits are 

 showing conspicuous cracks. 



As far as the peach is concerned, the direct killing of twigs and 

 branches is rare, and this phase of the disease is not in itself to be 

 considered as very serious. The twig lesions are of importance, 

 however, in that it is in these that the causal organism passes the 

 winter. Orchards in which direct damage to the fruit causes much 

 loss are also rather rare. It is the injury to the leaves, sometimes 

 amounting to complete defoliation and often to 25 to 50 per cent 

 defoliation, and the consequent damage to tree and fruit through the 

 curtailment of the supply of plant food, which make this an important 



disease. 



CAUSE OF THE DISEASE. 



The disease is of bacterial origin and is caused by Bacterium pruni 

 Erw. F. Smith. 1 



Smith 2 in 1908 first successfully inoculated the foliage of the 

 peach with the organism which he had obtained in pure cultures 

 from the plum. 



Rorer 3 in 1909 reported successful inoculations on peach leaves 

 with pure cultures of the organism from the same source. He also 

 obtained the organism from twigs and described the disease on the 

 fruit. 



Many successful cross inoculations were reported upon by Rolfs i 

 in 1915, confirming the work of Smith and Rorer and establishing 

 beyond doubt that the disease on foliage, fruit, and twigs is caused 

 by the same organism. 



Bacterium pruni is described as a short rod, occurring singly, in 

 pairs, or sometimes in chains, motile by polar flagella, from 0.001 

 mm. to 0.0018 mm. long and about half as wide. Warm, moist 

 weather conditions, with the trees in a weakened state, due to previous 

 injury by freezing, lack of pruning, lack of fertilization, etc., are most 

 favorable for infection and subsequent growth of the causal organism. 



VARIETAL SUSCEPTIBILITY. 



Practically all varieties are attacked, at least to some extent, by 

 this disease. The Elberta, the leading commercial peach, is very 

 susceptible. It is very difficult to estimate the relative suscepti- 



i Smith, Erwin F. Observations on a hitherto unreported bacterial disease, the cause of which enters 

 the plant through ordinary stomata. In Science, n. s., v. 17, no. 429, p. 456-457. 1903. 



2 Smith, Erwin F. Occurrence of Bacterium pruni in peach foliage. In Science, n. s., v. 30, no. 763, 

 p. 224. 1909. Abstract of paper read at the meeting of the Society of American Bacteriologists, December, 

 1908. 



3 Rorer, J. B. A bacterial disease of the peach. In Mycologia, v. 1, no. 1, p. 23-27. 1909. 



4 Rolfs, F. M. A bacterial disease of stone fruits. N. Y. (Cornell) Agr. Exp. Sta. Mem. 8, p. 381-436, 

 fig. 59-70. 1915. Literature cited, p. 435-436. 



