FUMIGATION OF CITRUS PLANTS. 31 



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 III 

 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, 1 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 

 great 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 ; in short, 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 



i From unpublished results. 



