Studies on the Fire-Blight Organism, Bacillus amylovorus 35 



rate of migration in the suscept 



Many fruit growers and pathologists who have attempted to combat 

 fire blight iu orchards are of the opinion that the disease progresses at 

 a rapid rate. This is probably owing to the sudden appearance of 

 symptoms on blasted blossoms or blighted twigs. The method of migra- 

 tion has been studied by Bachmann (1913) and by Nixon (1927), both 

 of whom hold that it is largely intercellular ; some difference of opinion 

 exists, but in the main their findings are similar. The writer's tests 

 were made on the rate of migration. Waite (1896) states: "It rarely 

 extends farther than 2 or 3 inches from the point of attack in one 

 day, but occasionally reaches as much as 1 foot." The basis for this 

 statement is probably observation, as no supporting data are given. 

 Whetzel and Stewart (1909) thought that the bacteria could not pene- 

 trate more than a few inches a day, even under the most favorable 

 conditions. D. II. Jones (1909) stated that the disease "may travel 

 down the twig at the rate of from a quarter of an inch to two inches 

 a day, the rate of its progress depending largely upon the succulency 

 of the twig and the atmospheric temperature." That both temperature 

 and succulency are factors in rate of migration was proved by Brooks 

 (1926), who added humidity also as a limiting factor. He found that 

 the rate of migration at optimum temperature of 21° to 28° C. varied 

 from 0.33 to 1.25 inches a day; at optimum relative humidity (80 per 

 cent), the rate varied from 0.19 to 1 inch a day. The rate in twigs of 

 ordinary growth at 50 per cent relative humidity was as low as 0.07 

 (inch a day; and at 10° C. it was as low as 0.07 inch a day. Brooks, 

 however, did not report using optimum temperature and optimum 

 humidity in any controlled experiments. 



The writer used apple trees growing under greenhouse conditions 

 with a temperature range from about 65° to 95° F. The method 

 employed was briefly as follows: French seedling apple trees were 

 grown in the greenhouse and were tested for their susceptibility to 

 the fire-blight organism. After a rest period out of doors ;> the plants 

 were dug up, potted, and given a heavy application of sodium nitrate 

 to induce rapid growth. The plants selected for inoculation had formed 

 new growth from 20 to 25 inches in length. Inoculations were made, 

 about 1 inch from the tips of the shoots, by puncturing with a hypo- 

 dermic needle and injecting a drop of bouillon culture seventy-two 

 hours old. The same culture was used for all plants in a series. 

 Twenty-four hours after inoculation, the tips of six plants were cut off 

 at 1, 2, and 7 inches, respectively, below the point of inoculation, two 

 plants being used for each distance. Inoculation and removal of the 

 tips was performed at nine o'clock each morning. On the second 

 day — forty-eight hours after inoculation — eight tips were removed, at 



