EFFECT OF HYDROCYANIC ACID GAS ON 



CUCUMBER PLANTS PREVIOUSLY 



SPRAYED WITH COPPER FUNGICIDES 



By E. F. Guba, Assistant Research Professor of Botany, and E. B. Holland, 

 Research Professor of Chemistry 



INTRODUCTION 



The extensive greenhouse area devoted to the forcing of vegetables in Massachu- 

 setts has for many years necessitated study of control measures for insect pests 

 and fungous diseases peculiar to these crops. Such studies have encouraged the 

 use of fungicides and insecticides, and the intermingled application of these 

 materials presents a problem of the utmost importance to the growers. One of 

 these problems relates to the conflict existing between copper fungicides and 

 hydrocyanic acid gas. 



The understanding and solution of this problem has necessitated consideration 

 of copper fungicides of different reaction, concentration, and composition and of 

 diiTerent sources of hydrocyanic acid gas. The classification of the compatibility 

 of copper lungicides with gas on cucumber plants and confirmatory knowledge 

 obtained from chemical analyses of the spray residues would appear to contribute 

 to our knowledge of the problem and suggest recommendations for a safe pro- 

 cedure. 



HISTORICAL REVIEW 



It has been noted b}' Guba (3, 4) that Bordeaux mixture and hydrocyanic 

 acid gas are incompatible when gas follows spray in the greenhouse, and that 

 when Bordeaux is used to control diseases tobacco fumigants should be substi- 

 tuted for the gas to avoid injury. Published information suggested that the gas 

 (HCN) combined with copper to form cupric cyanide Cu(CN)2, an unstable 

 compound giving off cyanogen and leaving insoluble cuprous cyanide, Cu(CN). 



Butler and Jenkins (1) have reported that as the ratio of lime to copper sulfate 

 is increased in Bordeaux from 4-1-50 to 4-4-50 injury increases, and then decreases 

 as the ratio is increased to 4-24-50. They claim that neutral Bordeaux, as rep- 

 resented by the formula 4-0.8-50, followed by gas is safe and that mixtures with 

 high lime ratios, e.g. 4-16-50 and 4-24-50, are safe if the plants are kept dry 

 after gas. According to these investigators, injury with certain ratios of lime to 

 copper sulfate and none with others is correlated with the allegation that (a) in- 

 soluble cuprous cyanide is formed with a 4-0.8-50 mixture; (b) no cupric cyanide 

 is formed with a 4-0.8-50 mixture but increasing amounts with mixtures up to 

 4-4-50, negligible amounts with 4-16-50 and none with 4-24-50; (c) the soluble 

 double salt is formed in increasing amounts with 4-4-50 to 4-24-50 mixtures. 



Quayle (13) was unable to confirm the safety of Bordeaux 4-0.8-50 with gas 

 on lemon, but reported less injury than with Bordeaux 4-4-50, and that it was 

 similarly graduated but more marked on increasing the interval between spray 

 and gas. A spray of zinc sulfate and lime was safe, but it is not as effective as 

 Bordeaux in controlling the brown rot disease caused by the fungus Pythiacystis 

 citrophthora Sm. & Sm. It is, therefore, obvious, on the basis of our present 

 state of knowledge of the problem, that further investigation is extremely desirable. 



