FUMIGATION OF CITRUS PLANTS. 27 



injury was not especially severe; where, however, the sudden increase 

 was over a wider range, as in experiments 24 to 27, of from 16° to 28° 

 and accompanied by fluctuation in temperature of 5° to 10°, the 

 injury was most severe. 



These data are of importance in showing that sudden and wide 

 fluctuations of temperature during the gas exposure should be avoided 

 where possible. Such fluctuations appear to be damaging to plants 

 even where the ranges of temperatures of exposure fall below 70° F., 

 which is considered within the range of optimum for field fumigation. 



GENERAL DISCUSSION OF FACTORS WHICH INFLUENCE FUMIGATION 



INJURY. 



Evidence has been presented in the foregoing experiments bearing 

 on the relation of darkness, diffused light, sunshine, temperature, 

 and moisture to foliage injury during the fumigation of citrus trees. 

 Abundant additional data have accumulated during field and labo- 

 ratory experiments to offer further corroboration of the deductions 

 made from these experiments. The effect of hydrocyanic acid on 

 fruit was not taken up in connection with this series of laboratory 

 experiments, but a very large amount of data on this subject has been 

 taken during field experimentation. 



In the fumigation of citrus trees injury to fruit and injury to foliage 

 should be considered separately. The fruit has been observed to be 

 severely injured without the foliage being burned in the least, while 

 on the other hand trees have been noted as defoliated although the 

 fruit was entirely uninjured. Several different types of injury are 

 presented in the fruit and foliage of fumigated citrus trees. These 

 types, though somewhat related in the case of either the fruit or the 

 foliage, are sufficiently distinct to be easily detected. 



Foliage injury is properly characterized by discoloration or burning, 

 and is usually accompanied by the shedding of leaves which vary in 

 appearance from those completely burned to others free from deface- 

 ment of tissue. The tender expanding leaves of very tender succu- 

 lent stems usually show the first signs of fumigation injury, and this 

 is not localized at any particular place, but sometimes occurs at the 

 edge and sometimes in the body of the leaf. These affected areas 

 are frequently confined to one surface of the leaf though more com- 

 monly the injury is equally apparent on both surfaces. As the 

 degree of injury increases the entire tender tips are affected, this at 

 first being evidenced by wilting and finally by death. The length of 

 time following fumigation before tip injury appears depends upon 

 the tenderness of the tip, the concentration of the gas, and such fac- 

 tors as temperature and sunshine conditions surrounding the fumi- 

 gation. The tender foliage of plants placed in bright sunshine imme- 

 diately after treatment with a strong dosage may start wilting or dis- 



