FUMIGATION OF CITRUS PLANTS. 41 



SUMMARY. 



(1) It is necessary to consider the prefumigation and postfumi- 

 gation environments of fumigated plants as well as that during the 

 actual treatment. 



(2) Sunshine is the chief prefumigation factor that increases 

 injury and this influence is greater at high temperatures than at low. 

 Under darkness or diffused light, temperatures upward to at least 

 100° F. do not appear to increase injury unless the fumigation or 

 postfumigation temperatures exceed 80° F. 



(3) The environment after fumigation approximates in im- 

 portance that during the actual treatment. Of the postfumigation 

 factors both sunshine and temperature modify the degree of injury. 

 Sunshine, the more important, is most destructive to plants exposed 

 immediately after fumigation, but affects them deleteriously at least 

 two hours after the treatment. Temperatures of 80° F. or above 

 injure plants more severely than lower temperatures. 



(4) The fumigation of citrus plants is most safely performed at 

 temperatures below 80° F. 



(5) Diffused light before, during, or after fumigation exerts no 

 more deleterious influence than darkness. 



(6) Moisture on citrus plants does not increase the degree of injury. 

 An application of cool water to plants in hot sunshine immediately 

 prior to fumigation appears to reduce slightly the effect of the gas. 



(7) Sudden changes of temperature over a wide range during ex- 

 posure to hydrocyanic-acid gas tend greatly to increase plant injury. 



(8) The optimum environment for safety to plants is diffused light 

 or darkness at uniform temperatures below 80° F. before, during, and 

 after the fumigation. The lowest temperature tried, 55° F., was 

 within the range of the optimum. 



(9) Fumigation at temperatures upward of 80° F. is safest under 

 cool prefumigation and postfumigation environments. The maxi- 

 mum of injury follows high temperatures for all three environments. 



(10) The physical and chemical conditions of the soil influence 

 injury from fumigation. Trees in a wet soil tend to be more severely 

 injured than healthy trees in a dry soil. However, trees in soils 

 deficient in moisture for such protracted periods as to be severely 

 weakened are more susceptible to injury than if grown under opti- 

 mum moisture conditions. Irrigation should follow fumigation, not 

 precede it. 



(11) The physiological condition of plants is one of the most im- 

 portant factors regulating fumigation damage. A condition akin to 

 hardiness appears to be the optimum for gas resistance and is brought 

 about by dryness of the soil, cold weather, and possibly by continued, 

 very hot dry weather which exceeds the optimum for the plant. 



(12) Sunshine fumigation can be conducted with safety by proper 

 regulation of the dosage and length of exposure. 



