1915J 



SMITH BACTERIAL DISEASES OF PLANTS 



387 



anacearum (Medan hi) and photographed for Volume in of 

 'Bacteria in Eelation to Plant Diseases' (plate 45 D), en- 

 tirely outgrew the disease, as did also certain sugar-canes 



(series vi) inoculated with Bacterium vascularum. 1 Also, I 

 have seen tomato plants recover only to develop a second and 

 fatal attack of the vascular brown rot three months after the 

 first attack, during which period they had made an extensive 

 healthy-looking growth. 2 



Recovery from disease may depend on loss 



virulence 



on the part of the parasite. This often occurs when bacteria 

 are grown for some time on culture-media, and it occurs also 

 in nature, but its cause is obscure. 



AGENTS OF TRANSMISSION 



These may be organic or inorganic. In many cases the 

 plant itself harbors the parasite indefinitely, carrying it over 

 from year to year on some portion of its growth. 



Seeds, tubers, bulbs, grafts, or the whole plant may be re- 

 sponsible for the appearance of the disease the following year 



in the old localities, and 



through the 



agency of seedsmen, 



nurserymen, or whoever disseminates plants, for outbreaks 



in regions hitherto exempt. 



There is good reason to believe that the black rot of cabbage 

 and Stewart's disease of sweet corn have been disseminated 

 broadcast in the United States in recent years by ignorant and 

 unscrupulous seedsmen. Both diseases are transmitted to 

 seedling plants from the seed. The yellow disease of hya- 

 cinths is carried in the bulb. Potato tubers from diseased 

 fields may infect healthy fields. Apple grafts have transmit- 

 ted crown-gall. Slightly infected trunks and limbs of trees 

 (hold-over pear blight, walnut blight, canker of the plum) may 

 infect shoots, leaves, blossoms, or fruits the following season. 

 The soil around the infected plant may serve for years as a 

 source of infection to other species (crown-gall), or to other 

 individuals of the same kind (various leaf-spots). Occasion- 

 ally, however, a parasite seems to die out of certain soils {Bac- 



1 Smith, E. F. Bacteria in relation to plant diseases. Carnegie Inst. Wash- 

 ington, Publ. 27 s : p. 33. 1914. 



2 Ibid. p. 



179. 



