Oct. 21, i9i8 Angular-Leafspot of Cucumber 213 



fields. The evidence supporting the latter idea is observational. During 

 191 6 six experimental fields were grown near Madison in the same 

 vicinity with four privately owned commercial fields. The distances 

 between the commercial and the experimental fields varied from about 

 30 rods to K rnile. Angular-leafspot appeared in all six of the experi- 

 mental plots early in July. It appeared in only three of the others and 

 in these not until nearly the middle of August. Prior to this, in the fore- 

 part of the month, there was a period in which there was an abundance 

 of bacterial exudate in the infested fields and when insects, especially 

 the beetles, were very active. It is of interest here to note that in no 

 case did the original center of infection in a private field develop at the 

 edge, but rather in the interior of each of the three patches. It may be 

 stated with confidence that during the time in question no one except 

 Dr. M. W. Gardner and the writer visited both the experimental and 

 commercial fields, and as these visits were made when the leaves were 

 dry there seems little probability of the organisms having been trans- 

 ported by us. Comparable developments were observed at Ripon, 

 Princeton, and Pittsville, Wis. 



Closely correlated with picker dissemination and the probability of 

 spread by insects is the relation of atmospheric humidity to the disease. 

 Under conditions of high relative humidity, such as frequently prevail 

 on summer nights, the invaded areas of the leaves take on clear-cut 

 angular shapes and the bacterial exudate becomes abundant. (See PI. 

 13, A). Such nights were those of August 8 and 9, 191 6. The relative 

 humidity, as recorded by a Friez hygrograph, varied on those nights 

 from 74 and 80 per cent, respectively, at 7 p. m. to 90 per cent, where it 

 continued until 6 a. m. Observations in the early mornings showed 

 abundant signs, as described, that conditions for the progress of the 

 disease had beeti most favorable. In steam-heated greenhouses, with 

 the relative humidity varying from 45 to 60 per cent, the disease develops 

 poorly or not at all. 



OVERWINTERING 



The several ways in which the causal organisms of other bacterial dis- 

 eases have been thought to pass the winter have been kept in mind in 

 searching for evidence as to how the angular-leafspot bacterium over- 

 winters. 



SOIL 



The sensitiveness of the organisms to freezing, as elsewhere recorded, 

 renders doubtful the possibility of their living over the winter in the soil 

 or in the debris of diseased vines in northern climates. A limited amount 

 of work on this question indicates that the bacteria do not live for long 

 periods in the soil. The question can not, however, be definitely settled 

 until further study of it has been made. 



