40 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 



protoplasm passes into a state of immobility. Since the maximum 

 daily temperature of the Tulare citrus belt cluring the summer fre- 

 quently exceeds 100° F., the mean maximum for the hottest months 

 seldom falling more than 1° to 3° below this temperature, a reason for 

 reduced activity is presented. 2 This condition is doubtless promoted 

 by the usual practice of withholding irrigation until after fumigation. 

 In the more equable, damper climate of the coastal region, where the 

 temperature very seldom attains a maximum of 100° F., but rather 

 approaches the optimum for protoplasmic activity, dormancy during 

 the- summer is less noticeable and the physiologically active plants 

 are more subject to fumigation injury. 



MOISTURE. 



Moisture, even when present in excessive amounts, appears to 

 have no influence on injury to citrus trees either before, during, or 

 after the treatment, under conditions of shade or darkness, and this 

 conclusion agrees with the work of Gossard (7), Morrill (13), and 

 others. Under conditions of exposure to hot sunshine before fumi- 

 gation the application of cool water appears to reduce the degree of 

 injury slightly. These results on the relation of moisture to the 

 fumigation of citrus trees do not necessarily apply to tender green- 

 house plants, for moisture on such plants with thin cuticles has been 

 shown by various authors (11, 18) to produce increased injury. 

 Clayton (1), however, in a recent paper states that some species are 

 made more susceptible to injury by wetting the leaves while other 

 species are not visibly affected. He places the tomato in the latter 

 class although Moore found that wetting tomatoes as well as various 

 other plants with thin cuticle increased their susceptibility to injury. 



The influence of soil moisture has been referred to in previous dis- 

 cussions and may be passed with the statement that soil moisture in 

 sufficient quantity to make plants physiologically very active and 

 tender renders plants more susceptible to fumigation injury than 

 where present only in such quantities as place the plants in a re- 

 sistant or hardened condition. 



Hydrocyanic-acid gas has a great affinity for water, and under con- . 

 ditions of excessive moisture sufficient gas might be absorbed to so 

 materially reduce the concentration that less injury would be pro- 

 duced than otherwise, and, furthermore, its insecticidal value would 

 be lessened. Where fumigation is conducted in gas-tight con- 

 tainers this condition can not be ignored, and the necessity of atten- 

 tion thereto has been clearly shown by Penny (14), and Sasscer and 

 Borden (16). In orchard work under cloth tents the absorption 

 of gas by moisture is offset by the greater gas-holding prop- 

 erties of the moist canvas. 



2 Taken from published records of the U. S. Weather Bureau, which are lower than orchard sunshine 

 temperatures. 



