PLANTS AS DISEASE PRODUCERS 



313 



with Bacillus trachciphUus and young cucumbers where the organ- 

 ism was inoculated from young cultures, and on susceptible plants by 

 needle-pricks, showed that signs of disease rarely appeared in less than 

 three to four days, and that signs of wilt and change of color usually 

 were visible in five to seven days. In the case of the white pine bhster 

 rust, Cronartium ribicola, the period of incubation in the pine is from 

 one to six years. 



Duration of Disease. — The resistance of plants to disease is various 

 even after the fungus has obtained an entrance into the tissue of the 



Fig. 127. — Chestnut, Caslanea denlaia, killed by blight fungus, lindolhia payascaili, 

 Cold Spriiig Harbor, L. I., July, 1914. 



host. In the case of large trees like the white oak, a number of years 

 may elapse before the tree finally succumbs to such fungi, as Fomes 

 {Poly poms) applanatus. A chestnut tree, a few miles outside of 

 Philadelphia resisted the chestnut-blight disease for over four years 

 from the time of first infection before it finally succumbed. Smith 

 {loc. cit.) describes how a good-sized potato tuber was half rotted in 

 five days at ordinary autumn temperatures when inoculated with 

 Bacillus phytophthorus by means of a few needle-pricks. 



