FUMIGATION OF CITRUS PLANTS. 4] 
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 peronmes at 
temperatures felons 80a. 
(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 condrtion 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. 
