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



only must the temperature of treatment be considered, but also the 

 temperature surrounding plants after exposure and to a much less 

 extent that before exposure. It has been shown that high tempera- 

 tures are more injurious than low, and hi the case of each of the three 

 fumigation environments, the prefumigation, actual fumigation, and 

 postfumigation, the maximum optimum fell below 80° F. Exactly 

 how much this maximum for any particular environment fell below 

 80° F. depended on the temperature of the other two; when any two 

 were low the maximum optimum for the third approximated 80°; 

 when they were high, however, the maximum for the third was a few 

 degrees less than 80°. In one case it did not exceed 75°. These 

 conclusions differ very little from the writer's experience in orchard 

 fumigation in southern California, for which 70° is held as the maxi- 

 mum when a heavy dosage is used. This same maximum is recom- 

 mended by Sasscer and Borden (16) for greenhouse plants. An 

 interesting relation apparently exists between the maximum opti- 

 mum temperature for fumigation and the activity of plants, for Mac- 

 Dougal (10) states that temperature is one of the most widely inter- 

 locking factors concerned in the activity of protoplasm, and that the 

 temperature of greatest activity in seed plants varies from 80° to 

 100° F. The experimental work presented in this paper shows 

 increasing fumigation injury as the temperature of 80° is approached 

 or exceeded, which corresponds with the degree at which greatest 

 protoplasmic activity commences. 



It is possible to conduct fumigation at temperatures of 80° F. or 

 above without serious injury provided the prefumigation and post- 

 fumigation conditions are ideal. A high postfumigation temperature 

 increases the probability of damage, and especially is this true if the 

 fumigation temperature is also above the optimum. The greatest 

 damage follows when all three temperatures surrounding the treat- 

 ment are high. A prefumigation temperature hi shade and darkness, 

 even up to 100° F., appears to alter the results very little unless the 

 fumigation or postfumigation temperature is also high, in which case 

 the high prefumigation temperatures are more injurious than the low. 

 The temperature of 55° F. was the minimum at which experimental 

 work was conducted. The little injury evidenced at this temperature 

 showed it to be within the range of the optimum. In field work it 

 has been stated by the writer (19) that operations are safe as low as 

 38° F., although fumigation below this point is not advocated. 



In experiments 24 to 27 it was shown that a sudden increase in 

 temperature immediately preceding or during the first few minutes 

 of exposure produces very severe injury, especially if followed by 

 sharp fluctuations of temperature. This factor offers a partial 

 explanation for the severe injury hi sunshine fumigation on the sun- 

 ward side of the tree, especially toward the top, and also presents a 



