132 DISEASES OF ECONOMIC PLANTS 



probable, therefore, that at least one cause of crown gall 

 is the organism mentioned. 



All diseased stock should be avoided, as well as stock from 

 nurseries where the disease is known to exist. 



Bacteriose (Bacterium pruni E. F. Smith ^), — A "shot 

 hole " disease of peach leaves, which causes prema- 

 ture defoliation, is attributed by Smith and by Rorer ^ to 

 •^"^ bacteria. The disease has been found 



during several years in different states 

 and is believed to constitute one of the 

 I ' most common of the " shot-hole " dis- 



1 ,, ., ,' eases in the South and Middle West. 



yft||||nH|^P''' The fruit and twigs are also affected. 

 ^^^^^^^^^ Upon the leaf it appears as somewhat 

 ^^^^^^ angular purplish brown spots, 2-5 mm. 



Fig. 59. — Mildew upon in diameter, which may coalesce and in- 

 peach fruit. After yolvc large arcas. Upon twigs it kills 

 the bark, forming purplish black sunken 

 areas, 2-3 mm. wide, which may extend to 3-8 cm. in length, 

 and even girdle the twig and kill the shoot. Infection is 

 most frequent at a leaf scar. Upon the fruit small pur- 

 plish spots appear. Over these the skin soon cracks. The 

 Elberta is especially susceptible. No effective treatment 

 has yet been demonstrated. 



Powdery mildew, podosphaerose (Podosphcrra Oxycanthcp 

 (DC.) DeBy.). — This widespread mildew is in general ap- 

 pearance very similar to the powdery mildew of the grape- 

 cherry, lilac, and rose, etc. It sometimes becomes so abun- 



1 Smith, E. F., Sci. n. s. 17, 456, and Bacteria in Relation to Plant Dis- 

 ease, I, Figs. 11, 15, 70, 71, 72, and PI. 19. 



2 Rorer, J. B., Mycologia 1, 23, January, 1909, and Sci. n. s. 30, 224. 



