. FUMIGATION OF CITRUS PLANTS. 9 



ferent conditions. In each of the five conditions the prefumigation 

 sun-exposed plants developed decidedly greater injury than those 

 that were in shade before treatment. The results of experiment 

 18, in which plants were exposed to a hot sunshine (83° F.) imme- 

 diately before fumigation and fumigated at a high temperature, 

 show all sun-exposed plants killed regardless of the postfumigation 

 conditions ; yet others in the shade at approximately the same pre- 

 fumigation temperature (76° F.) and placed at a temperature of 

 56° F. after the treatment had only the very tenderest foliage singed. 

 The effect of prefumigation sunshine is not so conclusively brought 

 out in the other experiments, but, with the exception of No. 3, it 

 appears to have intensified the injury in at least a few of the trees 

 treated. Experiment 3 alone shows no difference between plants 

 in the shade and those in the sunshine previous to fumigation. It is 

 noted in this case that the sunshine temperature was 60° F. and the 

 fumigation and postfumigation conditions were equally cool, all 

 three being ideal for exposure to cyanid gas. 



A comparative study of the temperature before and after treatment 

 shows that the effect of prefumigation sunshine is modified by the 

 degree of heat present at these different times. For instance, in 

 experiment 3 where the sunshine appeared not to affect the degree 

 of injury more than the shade, all fumigation conditions, the pre- 

 fumigation, postfumigation, and actual fumigation, approximated 

 60° F. On the other hand, experiment 2, which was performed 

 with the same type of plants, the same dosage, and at approximately 

 the same temperature, exhibited a decided difference in injury between 

 the prefumigation shade and sunshine-exposed plants, the injury 

 in the latter being greater than for experiment 3. It is noted that 

 the sun temperature in experiment 2 was 75° F., whereas in experi- 

 ment 3 it was 60° F., which shows that the degree of injury attribut- 

 able to sunshine increased with the increased temperature. Thus a 

 hot sunshine before fumigation is more to be avoided than a cool 

 sunshine. 



In conclusion it can be stated that the experimental evidence in 

 this paper appears to show that sunshine coming in contact with 

 citrus plants before fumigation tends to produce greater injury than 

 where plants are in the shade or darkness; that sunshine accom- 

 panied by a high temperature is more injurious than if accompanied 

 by a low temperature; that the degree of injury is modified by the 

 postfumigation conditions, greater injury developing at high tem- 

 peratures than at low temperatures; that the most critical environ- 

 ment is to subject plants exposed to a hot sunshine before fumiga- 

 tion to a hot sunshine after fumigation ; finally, that a high tempera- 

 ture during fumigation probably increases the injury of prefumiga- 

 tion sunshine-exposed plants over that taking place at a low tem- 

 perature. 



4533°— Bull. 907—20 2 



