4 LEAFLET 209, U. S. DEPARTMENT OF AGRICULTURE 
Covers 
A tight cover, which will hold the vapors in and expose the plants 
to their action throughout the entire night, is essential to effective 
treatment. Excellent results have been obtained with a 62/64—thread 
muslin weighing about 4% ounces per square yard, Muslins running 
48 to 56 threads per inch have proved fairly satisfactory but not equal 
to the heavier material, and more open grades are not recommended. 
The vapor-retaining ability of the muslin can be greatly increased ifit 
is soaked with water at the beginning of the treatment. Numerous 
tests have shown that, with a water-soaked cover, the amount of para- 
dichlorbenzene required to control blue mold is reduced to about one- 
half the amount required with a dry one. Artificial wetting, however, 
presents two problems. First, often it is not convenient for growers 
to soak the covers, so that the covers are likely to be dampened rather 
than soaked, and, hence, the thorough sealing effect of soaking is not 
secured. Second, the use of water-soaked covers increases the danger 
of plant injury. Injury may occur on warm nights because of exces- 
sive vaporization of the paradichlorbenzene during the early evening, 
and the use of wet covers under such conditions greatly increases the 
danger of injury. To meet this situation, growers when making use 
of wet covers can either omit the wetting on warm nights, or else start 
the treatment very late, at 7:30 to 8 p.m. It should be noted that 
new covers cannot be thoroughly wet until the sizing has been removed 
by washing them with soap and water. The muslin cover should be 
sewed in one piece and made somewhat larger than the outside dimen- 
sions of the bed. 
Size of Crystals and Amounts to Use Per Treatment 
A grade of crystals commonly referred to as No. 6, which averages 
400 to 500 particles to a %-ounce sample, is recommended. Smaller 
size grades could be used to good advantage on very cool nights, and 
still larger grades are satisfactory in warm weather, but No. 6 has 
proved the best on the average. Occasional undesirable lots of para- 
dichlorbenzene contain much fine dust. Thesmall particles may cause 
injury either by sifting through the cotton on to the plants or by vapor- 
izing too rapidly. If the cover is not soaked with water at the begin- 
ning of the treatment, the proper dosage is 24% to 3 pounds per 100 
square yards of plant bed; with water-soaked covers, 1% pounds is 
adequate. Note, however, that these amounts are recommended in 
connection with a good grade of muslin cover as previously defined. 
Duration of Treatment 
During cool, cloudy weather, treatments can be started as early 
as 4 p.m., but on bright warm days 7:30 p. m. is early enough. In 
general, treatment should start about sundown, and the beds should be 
opened about 8 a.m. Unless disease conditions make it imperative, 
treatment on cold, windy nights should be avoided. It is dangerous 
to allow the mushn cover to remain in place during the day, even if it is 
cloudy, because exposure of a wet cover to sunshine for even a short 
period may result in severe plant injury. 
Number of Treatments 
The gas treatment will check disease development even after the 
mold has made its appearance. However, it is not safe to delay treat- 
