212 



Journal of Agricultural Research 



Vol. XV, No. 3 



of the cases that have come under the writer's notice seems worth men- 

 tioning in detail. At Princeton, Wis., a patch of cucumbers of seven 

 rows was visited on August 12, 1916. In the middle of the third row, 

 counting from the north side, there was a circular area of *diseased 

 leaves, badly shattered by the rain of August 10. West of this area of 

 shattered leaves no new infections were evident. East of this area, 

 however, there were numerous recent infections, and the number of 

 these varied nearly inversely as the distance from the original center. 

 The location of diseased leaves and the position of the spots on them 

 corresponded to observations on dissemination by pickers made in 

 other places. When passing the patch later in the day the owner was 

 seen starting to pick on the west end of the first row, so that when he 

 would come to pick the third row he would be working eastward (fig. 3). 

 Evidently he must have followed the same course when the bacterial 

 exudate was abundant enough to thoroughly contaminate his hands. 



— ®-0-e- 



FlG. 3- 



-Diagram of cucumber field to illustrate picker dissemination of angular-leafspot. ®=original 

 center, 0=new infections, arrows indicate direction the picker worked. 



BY INSECTS 



Cucumber beetles {Diahrotica vittata Fab. and D. duodecimpunctata L.) 

 have repeatedly been seen crawling over infected leaves and flying 

 about the fields when the bacterial exudate was plentiful, as early as 

 5.30 a. m. Platings from these insects were made in only a few cases. 

 That some of them would crawl through exudate and become con- 

 taminated seemed unquestionable. In one instance platings from a 

 water blank, in which had been dropped three 12-spotted beetles, yielded 

 the causal organism.^ Bees have been observed visiting the plants as 

 early as 7.30 a. m. and have been seen to occasionally brush against 

 exudate-bearing leaves. 



The carrying of the causal bacteria from one part of a field to another 

 by insects is no doubt significant, but in view of the other ways in which 

 the local spread is accomplished it is far less important than is the dis- 

 semination of the organism by the same agency from diseased to healthy 



' The organism in this case was not tested as to pathogenesis, but was identified by colony characters 

 on potato-dextrose agar. These typical colonies were more numerous than any other kind, and by transfers 

 (unintentionally delayed until lo days had passed) it was shown that the organisms were dead, as is true 

 for the angular-leafspot organism with media containing dextrose. 



