1892.] PUBLIC DOCUMENT — No. 33. 225 



with those of this species developed in the open air. This 

 difference may, however, well be due to the different condi- 

 tions under which they have developed. 



It is interesting to find that this parasite, so destructive 

 under the given conditions, is the same with one of our 

 commonest out-of-door species in summer and fall on various 

 Co?npositce, asters, golden-rods and sunflowers, on Verbenas, 

 on Phlox, and on various other host-plants, to which it does 

 comparatively little harm. 



Professor Bailey and Dr. Fisher, as well as the writer, 

 have found that this disease may be kept in check in the 

 greenhouse by spraying the plants as often as is necessary 

 with a solution of sulphide of potassium (liver of sulphur) 

 in water, an ounce of the sulphide to four gallons of water. 

 A stronger solution injures the plants and fruits. Spraying 

 with the ammoniacal carbonate of copper has been found 

 even more effective. But Professor Bailey finds more 

 effective than either exposure to sulphur vapor. This is 

 accomplished by closing the house as tightly as possible for 

 half an hour or an hour at a time, while it is filled by the 

 vapor arising from a vessel of sulphur kept a little above 

 the melting point on a small oil stove. The vessel should 

 be porcelain lined, to protect the iron from the action of the 

 sulphur. This vapor appears to be harmless to the host- 

 plants while fatal to surface parasites like the powdery mil- 

 dews. Great care must be taken to avoid the ijmition of the 

 sulphur, since a few minutes' exposure to the fumes of burn- 

 ing sulphur would be fatal to a house full of plants ; but, 

 with reasonable care in protecting the sulphur from contact 

 with the flame and in preventing the temperature from rising 

 too high, there should be no danger of such a catastrophe. 



A house in which this disease has been troublesome should 

 be thoroughly disinfected by burning sulphur before a simi- 

 lar crop is again started. The soil should be entirely 

 removed and replaced by fresh. But that removal of the 

 earth is not alone sufficient has been clearly shown during 

 the past season. The house in which the study of this dis- 

 ease was carried on during the winter and spring of 1891 

 remained dry and unused during August, and early in Sep- 

 tember the soil was who.lly removed. New bottoms were 



