FUMIGATION OF CITRUS TREES. 69 



in so far as the effect of its direct action on the hydrocyanic-acid gas 

 treatment is concerned. 



There are other reasons of indirect and largely mechanical nature 

 on account of which it is necessary to consider the presence of mois- 

 ture, for ignorance of these will frequently result in much burned 

 fruit. (1) When tents become moist they become heavier. This 

 renders them more difficult to handle. Much fruit is torn off, while 

 branches and limbs are frequently broken. (2) The damp tents col- 

 lect much dirt and as they are pulled over the trees they sometimes 

 scrape the fruit with which they come in contact. Such abrased fruit 

 is frequently burned by the action of the gas. (3) As the trees 

 become damp from the dew or fog, whichever it may be, so also do 

 the tents get damp. The moisture affects the fiber of the cloth so that 

 it becomes tighter and retains the gas better than when dry. Any 

 person of any considerable experience in fumigation knows that more 

 gas is left under the tents when they are pulled off on a damp night 

 than on a dry one; therefore, the gas remaining in the tops of tall 

 peaked trees is much more concentrated than is normally the case. 

 This intense strength of gas sometimes causes pitting, especially in 

 the case of some varieties least resistant to hydrocyanic-acid gas. 

 Hence, considering the disadvantages resulting from wet tents, it is 

 evident that fumigation should not be carried on at such a time. 

 Fumigation should be stopped after the leaves and tents become 

 thoroughly damp. 



The author's experience has been that the presence of moisture on 

 trees does not reduce the efficacy of hydrocyanic-acid gas against 

 scale insects. Results have been exactly as good when the trees were 

 wet as when dry, and observation of the work accomplished by com- 

 mercial outfits has corroborated it. 



Experiments by Penny * on plants in a closed box showed that 

 moisture on the leaves absorbed the gas. This would make it appear 

 that moisture on the foliage of orange trees absorbs some gas, and 

 undoubtedly such is the case. Remembering that on dry nights all 

 the gas within tents of the character ordinarily used has escaped 

 within an hour's exposure but on damp nights frequently much gas 

 remains when the tents are removed, it is at once apparent that the 

 retardation of gas by the damp cloth easily offsets any absorption 

 which may have taken place. 



TEMPERATURE. 



Heat. — Heat probably is the factor which is responsible for more 

 injury to fruit than any other cause. Throughout the experience of 

 commercial fumigation instances of very severe injury have occurred 

 almost every year in some one of the different citrus fruit producing 



» 12th Ann. Rep. Del. Agr. Exp. Sta., 1900. 



