20 MASS. EXPERIMENT STATION BULLETIN 326 



insects are seldom killed and the treatment does not constitute a satisfactory 

 control measure. 



INJURY TO PLANTS BY NAPHTHALENE FUMIGATION 



Unfortunately, naphthalene fumigation is not entirely safe to use on all 

 species of plants or under all conditions, and injury may result if certain pre- 

 cautions are not observed. 



Causes of Injury 



The usual factors which induce plant injury are (1) fumigation of intolerant 

 species, (2) failure to provide favorable atmospheric conditions (temperature, 

 relative humidity, and light), and (3) improper adjustment of the vaporizing 

 apparatus. These factors actually influence the effect of the fumigation by 

 causing either too great a concentration of the fumes or too long exposure to 

 them. 



An excessive concentration of the fumes is most injurious when the melted 

 naphthalene is permitted to boil, causing an unequal distribution and pro- 

 ducing temporary supersaturation. There is also evidence that the fumes from 

 boiled naphthalene are broken down by the excessive heat into gases which 

 are more harmful to plants than are the fumes from crystal or melted naph- 

 thalene. Poor circulation in the greenhouse resulting in air pockets is often 

 responsible for injurious concentrations of naphthalene in corners. This con- 

 dition may also be caused indirectly by the recrystallization of naphthalene 

 fumes due to low temperature. The deposit of recrystallized crystals does not 

 injure the foliage immediately, but as each crystal again volatilizes, a strong 

 concentration of naphthalene fumes is created at the place of contact and the 

 plant tissue is burned at that spot. 



Injury from too long an exposure to naphthalene fumes may result when the 

 greenhouse remains closed over night after the stoves have been shut off. 

 Possible injury from this cause should be avoided by changing the air as soon 

 as possible after the fumigation is completed, especially when treatments are 

 made on two or three successive days. 



Appearance of Injury 



Plants which have been exposed to the fumes from boiled naphthalene, or 

 to too great a concentration of fumes from vaporized naphthalene, show wilted 

 and darkened areas on the foliage and stems soon after the injury has occurred. 

 In a few hours the injured areas, which may be the size of a pinhead or may 

 include all the leaves and tender stems of the plant, turn to the characteristic 

 white color of dead but not diseased plant tissue. Usually, the new leaves and 

 tender buds are injured first; and when severe or moderate injury has occurred, 

 new growth generally breaks out below the injured part of the stem, especially 

 on plants which produce buds at the leaf axils. 



Injury from too long an exposure to naphthalene fumes is less well defined 

 but usually appears as a premature yellowing and falling of the older leaves. 

 This has been especially noticeable on woody plants such as rose, gardenia, 

 citrus, and oleander, as well as on coleus and some of the larger chrysanthemums. 

 Yellow leaves similar to those following fumigation are also caused by too 



