FUMIGATION OF CITKUS PLANTS. 35 



Temperature has a very direct relation to sunshine influence, and 

 the degree of injury appears to be increased or decreased as the 

 temperature of the sunshine is greater or less. Furthermore, where 

 plants are subjected to sunshine immediately before treatment, the 

 actual fumigation and postfumigation temperatures influence the 

 degree of injury. Likewise in the case of plants exposed to sunshine 

 immediately after treatment, the prefumigation and actual fumigation 

 temperatures require consideration. The writer's own experimental 

 evidence shows that the optimum temperatures of fumigation are 

 below 80° F., and that work conducted at higher temperatures is 

 performed with increased risk. The greatest injury follows the sub- 

 jection of plants to both sunshine and high temperatures both before 

 and after fumigation. The effect of this combination, sunshine and 

 temperature, has long influenced the time of starting orchard work in 

 California. The fumigation season begins in the summer and extends 

 throughout the autumn into the winter. During the warmest 

 weather work is not- started until the sun has set, but with the advent 

 of the' late autumn and cooler temperatures the first row of trees is 

 sometimes covered before sundown, and in the winter period of 

 dormancy entire rows are treated in the sunlight. During very hot 

 weather, when the atmosphere is clear and dry, injury to the first row 

 fumigated at night and the last in the morning has been of frequent 

 occurrence and in the latter case has been observed to occur even 

 when the tents were removed from the trees before sunrise. Full 

 explanation of this situation is presented in the results of experi- 

 ments 20 to 23 which show that the postfumigation sunshine influence 

 may extend up to three hours after exposure, although the maximum 

 of influence is confined to the first few minutes after removal from the 

 gas. 



In spite of the fact that sunshine has from the first been considered 

 one of the most harmful agents to plants in connection with fumiga- 

 tion, the greater desirability of daylight work has led to continued 

 attempts to substitute day practice for that at night. These efforts to 

 revolutionize accustomed practice usually were made during the 

 winter months, and frequently successfully over short periods if the 

 weather was moderate and the trees well hardened. Sooner or later, 

 however, this attempt to fumigate by day without modification of 

 dosage or exposure was followed by severe tree damage and was 

 promptly discontinued. 



It has been explained previously that cyanid injury is modified by 

 the concentration of the gas and by the length of exposure. There- 

 fore it is reasonable to assume that either of these factors can be so 

 reduced as to render fumigation safe on the hottest sunshiny day. 

 Orchard work is performed with a concentrated gas, usually as con- 

 centrated as an active tree will stand safely during cool nights. In 



