Oct. 21, i9i8 Angular-Leafspot of Cucumber 219 



the effectiveness of the treatments in killing the causal organism, but 

 they do indicate that no important injury ^ to the seed from these treat- 

 ments may be expected. Extensive field tests with treated seed and 

 further field trials of disinfectants with special reference to injury to the 

 seed are under way in Wisconsin, Michigan, and Indiana under the super- 

 vision of Mr. W. W. Gilbert and Dr. M. W. Gardner. 



SUMMARY 



Angular-leaf spot of cucumber was first noted in Wisconsin in 1914 

 and its bacterial nature established in 1915. The disease is the same as 

 that described by Smith and Bryan {15) and earlier reported by Burger 

 (2), Traverso {16), and Potebnia {12). 



The disease is probably world-wide in its distribution. Under favor- 

 able meteorological conditions it does a good deal of damage. Because 

 of its widespread and frequent occurrence it should be ranked among 

 the cucumber diseases of major economic importance. 



Leaf infection is stomatal. Inoculations made at different hours 

 showed that infection occurs chiefly during the day rather than the 

 night. This is probably to be explained by the fact that the stomata 

 are open during the day and closed at night. 



Fruit infection is stomatal. The disease first appears there as small, 

 localized, circular, water-soaked spots. The centers of the spots later 

 become whitened, so that they are more readily noticed. 



Rain is the most important means of dissemination, but pickers and 

 probably insects play a part in this process. 



The causal organism is sensitive to desiccation, is readily killed in 

 artificial media by freezing, is killed in liquid media by an exposure for 

 10 minutes at 50° C, and is readily killed by dilute solutions of formal- 

 dehyde, copper sulphate, or mercuric chlorid. The sensitiveness of the 

 organism to these chemicals is increasingly greater in the order mentioned. 



There is substantial evidence that the causal bacteria overwinter with 

 the seed. 



No marked variation in resistance or susceptibility has been found 

 among horticultural varieties of cucumbers. A few ornamental gourds 

 are attacked by the disease. Attacks are limited to the cucurbits, and 

 in that family no important crop plant other than the cucumber has been 

 found affected. 



Sanitary measures, such as precautions in picking and in control of 

 insects, may be helpful. Spraying with Bordeaux mixture checks the 

 disease, but is of doubtful value as a general commercial procedure in 

 regions where spraying would not otherwise be practiced. Seed treat- 

 ment offers the greatest hope of satisfactory control. 



1 la the subsequent field tests carried on by Gilbert and Gardner, the 4 per cent formalin treatment 

 caused considerable injury to cucumber seedlings, resulting in marked rolling of cotyledons and retarda- 

 tion of growth . The mercuric chlorid treatment (i: 1,000 for five minutes) has proved safe and eSective (5). 



