FUMIGATION OF CITRUS PLANTS. . ok 
This important conclusion offers a possible explanation of certain. 
features which the writer has observed in connection with orchard 
fumigation, namely, the greater resistance of the blossom growth than 
that of the leaf growth appearing at the same time on the same tree; 
the greater resistance of this first leaf growth in the winter or spring 
than that which develops during the hot summer months; the greater 
resistance of the ripening orange than that of immature fruit. 
That chemicals in the cell sap other than reducing sugars can 
modify injury is strikingly brought out in the case of citrus trees 
sprayed with Bordeaux mixture or where trunks and main branches 
have been painted with Bordeaux paste a short time before fumigation 
with heavy dosages. This damage attributable to Bordeaux appli- 
cations is evidenced by burning of the foliage and fruit. Plate IIT 
shows the severity of injury that frequently follows the fumigation of 
a tree whose branches and trunk were previously Bordeaux painted. 
Since the foliage and fruit of this tree were not touched by the 
Bordeaux paste, but only the branches and trunk, it is evident that 
certain elements of the fungicide must have been taken into the cell 
sap and transported to the fruit and foliage which was so severely 
injured. Proof of this contention has been seen in the extraction 
of traces of copper from the foliage of such treated trees by Mr. H. D. 
Young,! while chemist of the Citrus Experiment Station at Whittier, 
Calif. It would thus appear probable that the injury, at least in part, 
was due to reaction of the cyanid gas on the copper for which it has a 
ereat affinity. 
It has’ been observed by the writer (20) that fruit injury from 
fumigation often occurs at places of weakness in the epidermis and 
that such a condition is sometimes the result of insect action. Plate 
IV, A, shows that insects can likewise influence injury to leaves by 
feeding. In this particular case it is of great interest to note that the 
injury is most apparent at the leaf surface opposite to that on which 
the insects rest. 
It has been pointed out by different authors that the moisture condi- 
tions surrounding growing plants influence their development and their 
susceptibility to injury from hydrocyanic-acid gas, those growing un- 
der moist conditions being less resistant to gas than those growing 
under dry ; inshort, that dry soil induces gas-resistant plants. Studies 
made by the writer in the case of field-grown citrus trees appear in 
general to support this theory. A large lemon orchard, through 
which a deep, narrow swale extended, was fumigated in November, 
1918. At the time of treatment the soil in this swale was moist and 
had been in this condition at least since the previous irrigation six 
weeks before. The soil on the upper slopes was very dry. The tents 
were pulled in a straight string which extended down the slope on 
1 From unpublished results. 
