EFFECT OF HCN GAS AFTER COPPER FUXGICIDES 3 

 PRACTICAL CONSIDERATIONS INVOLVED 



Copper fungicides are used to control some diseases of vegetable and orna- 

 mental crops under glass in Massachusetts. Bordeaux is frequently used to assist 

 in the control of cucumber powdery and downy mildews, caused respectively 

 by the fungi Erysiphe cichoracearum DC. and Peronopla.nnopara cubensis (B. & C.) 

 Clint., and no other commercial greenhouse crop in Massachusetts has been 

 subject to greater destruction as the result of the incompatibility of Bordeaux with 

 hydrocyanic acid gas. However, only 1-1-50 and l-V^-50 mixtures can be used 

 since the higher concentrations cause hardening and stunting of growth. Some 

 other minor uses for Bordeaux on greenhouse plants may be cited, but no diseases 

 of greenhouse crops are known which require for their control such ratios of 

 Bordeaux as 4-16-50 and 4-24-50. These mixtures are very difficult to apply, and 

 the residue resulting on the sprayed leaf would obviously impair photosynthesis. 

 A large excess of lime also reduces the concentration of the precipitated copper 

 in the residue and acts as a deterrent to its fungicidal value. 



According to Holland (7), the best ratio of milk of lime or hydrated lime to 

 copper sulfate, as judged by suspension, is approximately a 4-2-50 mixture, and 

 this might also be regarded as the lowest ratio practical from the standpoint of 

 safety to the plant. Bordeaux 4-1-50 in practical preparation is not necessarily 

 neutral, while 4-0.7-50 and 4-0.8-50 mixtures, even under laboratory preparation 

 when the strength of the lime is previously determined, may show soluble copper. 

 Practice makes little distinction with regard to the freshness of lime, neither has 

 it adopted an}- simple qualitative tests to insure against a deficiency of lime. 

 It is necessary, therefore, in practice to adhere rather closely to mixtures with a 

 lime content in sufficient excess to eliminate water soluble copper. If it is true 

 that hydrocyanic acid gas acting on neutral Bordeaux is safe, then the factor 

 of practical preparation of such a mixture becomes important. 



The allegation that 4-16-50 and 4-24-50 mixtures are safe with gas if the treated 

 plants are kept dry does not take into consideration prevailing greenhouse condi- 

 tions, notably high humidity, abundant transpiration, guttation, and high 

 temperatures. There is no assurance of dry foliage even in the absence of wetting; 

 neither are dry conditions under glass conducive to good plant growth. 



EXPERIMENTAL METHODS 



The cucumber plant was used to identify and measure the degree to which the 

 copper residue is incompatible with gas, because it is extremely sensitive to 

 injury from the combined copper and gas treatments. Potted plants were used. 

 The sprays were applied with a quart atomizer and allowed to dry completely 

 before exposure to gas. The plants were gassed in a tight greenhouse of a volume 

 of 1500 cubic feet. Controls against gas and spray were included in each treat- 

 ment. 



Milk of lime, filtered lime water, and chemical hydrated lime were used with 

 powdered copper sulfate in preparing lime Bordeaux. According to Holland, 

 Dunbar and Gilligan (7) and Holland and Gilligan (10), chemical hydrated lime 

 makes an excellent substitute for quicklime in the preparation of ordinary Bor- 

 deaux mixtures, and it is of such purity that it can be substituted pound for 

 pound for quicklime. A chemical hydrated lime having a screen test of 98 per 

 cent through 200 mesh and containing 73.72 per cent calcium oxide was used. 

 When milk of lime was used, it was previously strained through a double thickness 

 of cheesecloth. The method of mixing advocated by Holland, Dunbar and Gilligan 

 (7) was adopted; namely, pouring dilute copper into concentrated lime. Accord- 



