Oct. 21. i9i8 Angular-Leafspot of Cucumber 203 



PRELIMINARY CONSIDERATIONS 



The chief purpose of this paper is to present evidence bearing on the 

 phases of the problem which are of direct economic significance. Certain 

 other parts of the work which has been done have yielded results worthy 

 of record; and, since some of these results are pertinent to the questions 

 of dissemination, overwintering, and control, they may appropriately be 

 presented before passing to the consideration of the latter points. 



DESICCATION 



Many questions in regard to the dissemination and overwintering of 

 the causal organism of angular-leaf spot depend on its sensitiveness to 

 desiccation. The organism has been shown by repeated tests to be 

 relatively sensitive to drying on glass. With a 3-mm. platinum loop 

 drops were transferred to carefully cleaned cover glasses from 36-hour 

 cultures in beef bouillon and in cucumber-leaf decoction from the leaf 

 exudate and from a suspension in distilled water of the organisms from 

 freshly invaded tissue. None of these showed viable organisms after 

 four days' drying at room temperature. Smith and Bryan (15, p. 470) 

 found that the organisms from a young bouillon culture when dried on 

 glass were viable after 2 1 days. The variance in these results may possi- 

 bly be due to some slight difference in methods, which may have made a 

 difference in the time for which the bacteria were exposed to complete 

 desiccation. 



Freshly invaded fruit and leaf tissue dried in diffuse light at room 

 temperature showed viable organisms after 3 and 10 days, but none 

 were alive after 32 days. 



Short periods of drying, four to five days, resulted in the death of all 

 organisms on seed which had been disinfected with mercuric chlorid, 

 washed thoroughly, and then wet with a young bouillon culture of 

 Bacterium lachrymans. The fact, however, that the bacteria do survive 

 long periods of desiccation on or in the seed is shown by the evidence to 

 be presented under the discussion of overwintering. 



The results of one test on culture media are here pertinent because 

 they show that there are conditions under which the organisms may 

 withstand long periods of drying. On February 2, 191 6, six tubes of 

 potato-dextrose agar, in each of which had been suspended approximately 

 0.5 gm. of powdered calcium carbonate, were slanted and inoculated. 

 The purpose was to see if the life of the cultures might be prolonged by 

 neutralizing with the carbonate the acid resulting from the growth of 

 the organisms. In a dextrose-containing medium the bacteria ordi- 

 narily make a rapid growth for a short time and then all die, so that the 

 tubes become sterile, usually within 10 days. The tubes in this test 

 were set away at room conditions and, because of the low relative hu- 

 midity of the laboratory air, rapidly dried out. Before they dried com- 



