434 Journal of Agricultural Research voi. \^, no. n 



SUMMARY 



(i) In fields where wilt had largely destroyed the cucumber crop dur- 

 ing the preceding season the disease did not appear in 1915 on cucumbers 

 in 48 beetle-proof cages. On the other hand, wilt was very prevalent in 

 those fields on all sides of the cages. In a large number of greenhouse 

 tests where one out of two plants in a pot was inoculated and wilted to 

 the ground the second plant in no case contracted the wilt. The inocu- 

 lations by means of bacterial suspensions poured on the soil around 

 potted cucumber plants showed a small but varying percentage of wilt. 

 Root injuries were found in most of these cases of root infection. Ap- 

 parently infection does not enter the uninjured root system from the soil. 



(2) In all cases seeds from diseased fruits failed to produce diseased 

 plants, and cultures from such seeds in no case gave the wilt organism, 

 but further tests should be made. 



(3) In the tests made stomatal infection did not occur. 



(4) The experiments thus far completed show that cucumber beetles 

 {Diahrotica spp.) are the most important, if not the only, summer car- 

 riers of the wilt organism {Bacillus tracheiphilus) and that at least one 

 species (D. vittata) is capable of carrying the wilt over winter and infecting 

 the spring planting of cucumbers. In the tests by the writers the squash 

 bug {Anasa tristis), the flea beetle (Crepidodera ciicutneris), the melon 

 aphis {Aphis gassy pii), and the twelve-spotted lady-beetle {Epilachna 

 borealis) have failed to transmit the disease. 



(5) In the field experiments during one season with many different 

 varieties of cucurbits, the greatest difference in resistance was shown by 

 varieties of squash, in which the percentage of infection varied from o to 

 100. The varieties of cucumber and cantaloupe, while showing some 

 difference in their susceptibility to the wilt, give much less promise of 

 control by varietal resistance. 



(6) In the spraying experiments of 1915 wilt was effectively con- 

 trolled by early treatments with a combination of B.ordeaux mixture 

 and arsenate of lead. Plots sprayed with either mixture alone showed 

 much less wilt than unsprayed plots, but control was not as complete as 

 where the two were used together. Both field observations and green- 

 house experiments indicate that the wilt control is effected through the 

 bactericidal action of Bordeaux mixture, the insecticidal action of 

 arsenate of lead, and the repellent action of both against the cucumber 

 beetles. 



(7) Inasmuch as it has been definitely proven that the striped cucum- 

 ber beetle {D. vittata), and also the twelve-spotted cucumber beetle (£>. 

 duodecimpunctata) , are the most active carriers of the bacterial wilt, it 

 becomes necessary to control the insects in order to prevent the disease. 

 This phase of the work will be actively undertaken in cooperation with 

 the Bureau of Entomology during the coming season. 



