conspectus: recovery from disease 



19 



remain in abeyance for a longer or shorter period, according to 

 the varying fortunes of the host and the capabihties of the 

 parasite. The tomato plants inoculated with Bacterium sol- 

 anacearum (Medan iii) and photographed for Volume iii of 

 ''Bacteria in Relation to Plant Diseases," 

 (Plate 45 D), entirely outgrew the disease 

 (Figs. 10 and 11), as did also certain sugar- 

 canes (series vi) inoculated with Bacterium 

 vascularum.^ 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 of virulence on the part of the para- 

 site. This loss often occurs when bacteria 

 are grown for some time on culture-media, 

 and it occurs also in nature, but its cause 

 \s obscure; possibly it is due to oxidations. 

 Practically nothing is yet known about 

 acquired immunity on the part of the host 

 plant. 



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 

 (Pear blight, citrus canker, crown gall, etc.). 



Seeds, tubers, bulbs, grafts, or the 

 whole plant may be responsible for the 

 appearance of the disease the following 

 year in the old localities, and through the 

 agency of seedsmen, nurserymen, or whoever 

 for outbreaks in regions hitherto exempt. 



1 Smith, Erwin F.: "Bacteria in relation to plant diseases," Carnegie Inst. 

 Washington, Publ. 27, Vol. 3, p. 33, 1914. 



2 Ibid., p. 179. 



Fig. 12.— Bacterial 

 black chaff disease of 

 wheat. Kansas, end of 

 June, 1915. Black 

 stripes on glumes and 

 rachis. Kernels shriv- 

 eled and also bacterially 

 invaded. X 23'2- 



disseminates plants, 



