1909.] PUBLIC DOCUMENT — No 31. 73 



Fumigation Dosage for Forcing Crops. 



BY H. T. FERNALD. 



For four years experiments have been continued on the resist- 

 ance of different crops raised under glass to hydrocyanic acid 

 gas. These experiments have been completed for the tomato 

 and cucumber, and it seems desirable to summarize at this time 

 the results obtained bv W. V. Tower and C. W. Hooker, who 

 conducted the exj^eriments. 



The insects most often present in greenhouses in Massachu- 

 setts are the white fly, thrips and plant lice. The latter may be 

 neglected here, for any treatment effective for the first two 

 named will also destroy the plant lice. Dr. Morrill has shown ^ 

 that the white fly can be destroyed by fumigation with from 

 .007 to .01 gram of potassium cyanide per cubic foot for three 

 hours' time after sunset, and Dr. Hinds' experiments- indicate 

 that this strength will, in all probability, destroy thrips also. 

 How much the plants themselves can stand, however, has been 

 unknown with any certainty, varying results having been at- 

 tributed to differences in the conditions of the experiments. 



What these differences are, how much influence they have, 

 and what strength of fumigation the plants can withstand with- 

 out injury under all these conditions were accordingly the ques- 

 tions to be determined. For this j^urpose tomatoes were selected 

 for the first crop to be tested, the more common greenhouse 

 varieties grown in Massachusetts being used. 



The plants were raised in accordance with the methods of the 

 most successful growers in the State, and experiments were 

 made with them at ages of six, seven, eight, nine and eleven 

 weeks, the last fumigations being given while the crop was 

 ripening. No effect of the treatment upon the fruit could be 



1 Technical Bulletin No. 1, Hatch Experiment Station, Massachusetts, p. 50, 1903. 



2 Bulletin No. 67, Hatch Experiment Station, Massachusetts, p. 11, 1900. 



