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