FUMIGATION OF CITRUS TREES. 67 



shedding of leaves is more apparent than real. These weakened 

 leaves normally would not be held on the trees much longer. The 

 fumigation merely hastens their removal and is usually followed by a 

 fresh invigorated growth superior in all respects to the old. 



GREATER SUSCEPTIBILITY TO INJURY OF SOME VARIETIES 



THAN OTHERS. 



The lemon tree is much more resistant to injury from fumigation 

 than the orange and seldom suffers any appreciable damage when 

 treated under normal conditions. Some varieties of oranges are 

 more easily injured than others. Of the varieties of commercial 

 importance in California the Navel and Valencia are the least sus- 

 ceptible to injury from the gas treatment. The seedling is almost 

 equally hardy. Next comes the Mediterranean Sweet while the 

 Homosassa and St. Michael can seldom be treated with schedule 

 No. 1 without some injury resulting. Fortunately the Navel and 

 Valencia comprise the bulk of the oranges grown in this State. 



THE DISTRIBUTION OF GAS WITHIN A TENT. 



Hydrocyanic-acid gas, being lighter than air, has a tendency to 

 rise toward the top of the tent. The column of gas rises straight up 

 from the generating vessel until broken up by coming in contact with 

 the leaves and branches of the tree. The greater density of gas 

 toward the top of a tent is indicated by the greater amount of injured 

 fruit there than elsewhere. Only infrequently is fruit at the bottom 

 of the tree pitted. Dr. Morrill has given records of the difference in 

 destruction to the citrus white fly at different heights in a tree. 1 

 Similar results have been observed against the scale insects in Cal- 

 ifornia. The insects at the top of a tree may all be destroyed while 

 some on branches close to the ground will escape. Hence, in the case 

 of the purple scale, when the infestation is generally toward the bot- 

 tom of the tree the necessity of a strong gas is evident. 



FUMIGATION FOR PHYSIOLOGICAL EFFECTS. 



There are people in California who believe that citrus trees should 

 be treated with hydrocyanic-acid gas whether they are infested with 

 scale or not; that the treatment invigorates the tree, producing a 

 heavier crop and superior fruit than would otherwise result. 



A tree infested with scale, on being relieved of its burden responds 

 to the treatment. This response is not due to the physiological action 

 of the gas on the tree itself but rather to the destruction of the large 

 number of insects which have been constantly sapping the plant 

 juices. The removal of this heavy drain allows the tree to resume 

 its normal activity, which it does by first producing invigorated 

 growth. 



1 Bui. 76, Bur. Ent, U. S. Dept. Agr., p. 51, 1908. 



