INSECTICIDES AND FUNGICIDES. 29 



II. DISINFECTANTS. 



1. Corrosive Sublimate. 



This fungicide (known also as mercuric chloride or niercurj' bichloride) 

 is used only as a disinfectant. Its toxicity to foliage and its solubility 

 prevent its use as a spra}^ Its only use in Massachusetts on the farm or 

 in the orchard is for disinfection of seed potatoes and of wounds on trees 

 produced by pruning, canker removal, etc. Corrosive sublimate is a 

 white, dry crystalline salt which may be secured in the market in the 

 powdered form or as tablets. The tablets, which are commonly purchased 

 at drug stores, are of such a size that one tablet produces a 1-1,000 solution 

 when dissolved in a pint of water. 



Formula. — Corrosive sublimate for all purposes is used at a dilution 

 of 1-1,000. This dilution may be secured by dissolving 2 ounces of the 

 salt in 15 gallons of water. 



2. Formaldehyde. 



Formaldehj^le is a toxic gas extensively used as a disinfectant since 

 1S88. Its abihty to kill fungi and bacteria is dependent on its reducing 

 power, that is, on its power to remove oxygen from matter Avith which it 

 comes in contact. The formaldehyde (formalin) which is sold on the 

 market is a solution of the gas in water. According to the United States 

 standard of purity for interstate commerce, 37 per cent of the weight must 

 be formaldehyde gas. Although commonlj^ spoken of as a 40 per cent 

 solution, analyses of samples in recent years have shown it to be frequently 

 much lower, even down to 32 per cent. Also, a white sediment (parafor- 

 maldehyde) is frequently deposited in the bottom of containers. Since 

 the formation of paraformaldehyde lowers the percentage of formalde- 

 hyde, the solution should be warmed until the white sediment has dis- 

 appeared before it is used. Commercial formaldehyde also contains 5-10 

 per cent or more of wood alcohol, but this does not impair its fungicidal 

 value. The fumes are very irritating to the nose and eyes, but it is a 

 safer disinfectant than corrosive sublimate. 



Uses and Formulas. — The use of formaldehyde (formalin), at a dilu- 

 tion of 1 part in 240 (1 pint to 30 gallons), for disinfection of seed potatoes 

 against scab has now been almost discontinued in favor of corrosive sub- 

 limate because the latter is also effective against black scurf. 



For disinfection of grain seed against smut, a dilution of 1-240 is recom- 

 mended except where the spray method is used. In the latter case equal 

 parts of commercial formaldehyde and water are used. 



For onion smut the 1-128 formula has been recommended most exten- 

 sively. 



