UNITED STATES DEPARTMENT OF AGRICULTURE 



m BULLETIN No. 907 mm 



*jf ^j^jMi* Contribution from the Bureau of Entomology 



j&^&SU L. O. HOWARD, Chief 



— a©->-Y — 



Washington, D. C. 



PROFESSIONAL PAPER 



October 20, 1920 



FUMIGATIOxN OF CITRUS PLANTS WITH HYDRO- 

 CYANIC ACID: CONDITIONS INFLUENCING 



INJURY. 



By R. S. Woglum, 1 Entomologist, Tropical and Subtropical Fruit Insect Investigations. 



CONTENTS. 



Introduction 1 



The effect of hydrocyanic acid on plants 2 



Details of experiments 3 



The effect on plant injury of temperature, 



light, and moisture before fumigation 4 



The effect on plant injury of temperature, 



light, and moisture after fumigation 10 



The effect on plant injury of temperature, 



light, and moisture during fumigation 22 



Page 

 General discussion of factors which influence 



fumigation injury . . .~ 27 



The concentration of the gas 28 



The length of exposure 29 



The physiological condition of the plant 30 



Atmospheric and light conditions 33 



Summary 41 



Literature cited 42 



INTRODUCTION. 



The important factors long known to modify damage to the fruit 

 and foliage of citrus trees under orchard conditions of fumigation 

 with hydrocyanic acid include temperature, moisture, light, and 

 physiological condition of the plant. Of these, light appears to 

 have more completely influenced the application of this gas than any 

 other factor and early confined fumigation to a night practice. 

 Coquillett OS), 2 the originator of orchard fumigation with hydro- 

 cyanic acid, found early in his studies that citrus trees were less 

 liable to injury by this gas when treated at night than in daytime, 

 and explained that this result was due to decomposition of the gas 

 by light and heat into other gases more injurious to the plants. 

 Gossard (7), working with citrus trees in Florida, stated that "mid- 

 day fumigation can hardly be practiced." More recently Fernald, 

 Tower, and Hooker (5), experimenting with cucumbers and tomatoes 

 under glass, concluded that for such tender plants day fumigation, 

 even in cloudy weather, is unsafe. 



Literature treating of the causes of fumigation injury is confined 

 almost exclusively to the consideration of conditions, physiological 



Resigned September 11, 1920. 

 4533°— Bull. 907—20 1 



2 Reference is made by number (italic) to "Literature cited," p. 42. 



1 



